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The first (N=N)2- complex of a rare-earth metal with an end-on dinitrogen bridge, {K-(crypt)}2{[(R2N)3Sc]2[μ-η1:η1-N2]} (crypt = 2.2.2-cryptand, R = SiMe3), has been isolated from the reduction of Sc(NR2)3 under dinitrogen at -35 °C and characterized by X-ray crystallography. The structure differs from the characteristic side-on structures previously observed for over 40 crystallographically characterized rare-earth metal (N=N)2- complexes of formula [A2Ln(THF)x]2[μ-η2:η2- N2] (Ln = Sc, Y and lanthanides; x = 0, 1; A = anionic ligand such as amide, cyclopentadienide and aryloxide). The 1.221(3) Å N-N distance and the 1644 cm-1 Raman stretch are consistent with the presence of an (N=N)2-bridge. The observed paramagnetism of the complex by Evans method measurements is consistent with DFT calculations that suggest a triplet (3A2) ground state in D3 symmetry involving two degenerate Sc-N2-Sc bonding orbitals. Upon brief exposure of the orange Sc3+ bridging dinitrogen complex to UV-light, photolysis to form the monomeric Sc2+ complex, [K(crypt)][Sc(NR2)3], was observed. Conversion of the Sc2+ complex to the Sc3+ dinitrogen complex was not observed with this crypt system, but it did occur with the 18-crown-6 (crown) analog which formed {K(crown)}2{[(R2N)3Sc]2[μ-η1:η1 N2]}. This suggests the importance of the alkali metal chelating agent in the reversibility of dinitrogen binding in this scandium system. [ABSTRACT FROM AUTHOR]

First-principles theoretical studies enable an electronic and magnetic characterization of the recently synthesized Ni9Te6(PEt3)8C60 ionic material consisting of Ni9Te6(PEt3)8 superatoms and C60. The PEt3 ligands are shown to create an internal coulomb well that lifts the quantum states of the Ni9Te6 cluster, lowering its ionization potential to 3.39 eV thus creating a superalkali motif. The metallic core has a spin magnetic moment of 5.3 μB in agreement with experiment. The clusters are marked by low magnetic anisotropy energy (MAE) of 2.72 meV and a larger intra-exchange coupling exceeding 0.2 eV, indicating that the observed paramagnetic behavior around 10K is due to superparamagnetic relaxations. The magnetic motifs separated by C60 experience a weak superexchange that stabilizes a ferromagnetic ground state as observed around 2 K. The calculated MAE is sensitive to the charged state that could account for the observed change in magnetic transition temperature with size of the ligands or anion. [ABSTRACT FROM AUTHOR]

Paramagnetic endohedral fullerenes are ideal candidates for quantum information processing and high-density data storage due to their protected spins with particularly high stability. Herein, we report a solid spin system based on a paramagnetic metallofullerene Y2@C79N through incarcerating it into the cage-shaped pores of a metal-organic framework (MOF-177). In this kind of guest and host complex, the Y2@C79N molecules inside the pores of MOF crystal show axisymmetric paramagnetic property. It was found that the pores of MOF-177 crystal play an important role in dispersing the Y2@C79N molecules as well as in steering their electron spin. The group of arranged Y2@C79N molecules and their electron spins in MOF crystals are potential quantum bits for quantum information science and data storage. Moreover, this kind of solid spin system can be used as a probe for nanoscale nuclear magnetic resonance or for motion imaging of a single biomolecule. [ABSTRACT FROM AUTHOR]

Spiro-biphenalenyl (SBP) boron radicals constitute an important family of molecules for the preparation of functional organic materials. The building blocks of several SBP-based crystals are p-dimers of these radicals, in which two phenalenyl (PLY) rings face each other and the other two PLYs point away from the superimposed PLYs. The dimers of ethyl-SBP and butyl-SBP undergo a spin transition between a diamagnetic and a paramagnetic state upon heating, while other dimers exhibit paramagnetism at all temperatures. Here, we present a computational study aimed at establishing the driving forces of the spin-transition undergone by ethyl-SBP at ~140 K. The ground state of the p-dimers below 140 K is a singlet state in which the SBP unpaired electrons are partially localized in the superimposed PLYs. Above 140 K, the unpaired electrons are localized in the nonsuperimposed PLYs. These high-temperature structures are exclusively governed by the ground triplet state because the open-shell singlet with the unpaired electrons localized in the nonsuperimposed PLYs does not feature any minimum in the potential energy surface of the system. Furthermore, we show that the electrostatic component of the interaction energy between SBP radicals in the p-dimers is more attractive in the triplet than in the singlet, thereby partially counteracting the bonding and dispersion components, which favor the singlet. This electrostatic stabilization of the triplet state is a key driving force of the spin transition of ethyl-SBP and a key factor explaining the paramagnetic response of the p-dimers of other SBP-based crystals. [ABSTRACT FROM AUTHOR]

Particle and domain sizes strongly influence the properties of materials. Here we present an NMR approach based on paramagnetic relaxation enhancement (PRE) relayed by spin diffusion (SD), which allows us to determine lengths in the nm−μm range. We demonstrate the method on multicomponent organic polymer mixtures by selectively doping one component with a paramagnetic center in order to measure the domain size in a second component. Using this approach we determine domain sizes in ethyl cellulose/hydroxypropyl cellulose film coatings in pharmaceutical controlled release formulations. Here we measure particle sizes ranging from around 50 to 200 nm. [ABSTRACT FROM AUTHOR]

Lanthanide(lll) complexes of a cross-bridged cyclam derivative containing two picolinate pendant arms are kinetically inert in very harsh conditions such as 2 M HC1, with no dissociation being observed for at least 5 months. Importantly, the [Ln(dota)]-complexes, which are recognized to be extremely inert, dissociate under these conditions with lifetimes in the range ca. 1 min to 12 h depending upon the Ln3+ ion. X-ray diffraction studies reveal octadentate binding of the ligand to the metal ion in the [Eu(cb-tedpa)]+ complex, while ¹H and 13C NMR experiments in D2O point to the presence of a single diastereoisomer in solution with a very rigid structure. The structure of the complexes in the solid state is retained in solution, as demonstrated by the analysis of the Yb3+-induced paramagnetic shifts. [ABSTRACT FROM AUTHOR]

Bent-core mesogens containing 6-oxoverdazyl radical as the angular central unit exhibit rich polymorphism that includes isotropic—isotropic transition, re-entant isotropic (lre), and a novel 3D tetragonal (Tet) phases. Surprisingly, the paramagnetic Tet phase interacts linearly with applied electric field and exhibits photoinduced ambipolar charge transport (µ ≈ 10-3 cm2 V-1 s-1). Magnetic analysis showed gradual increase of antiferromagnetic interactions upon cooling. [ABSTRACT FROM AUTHOR]

1-Phenyl-3-trifluoromethyl-l,4-dihydrobenzo[e][1,2,4]triazin-4-yl is the first example of a hydrazyl radical that shows a reversible sharp spin transition fully completed within 5(1) K. The nominally first-order transition takes place at ca. 58(2) K and proceeds via subtle changes of intra- and interstack interactions between two similar structural phases. The low-temperature phase (5—60 K) is diamagnetic and has a singlet ground state (2Jexp= -166.8 cm-1, gsolid = 2.0042, ρ = 0.2%) stemming from a multicenter two-electron interaction. The high-temperature phase (60—300 K) is paramagnetic as a result of noninteracting S = 1/2 spins arising from weakly bound dimers. [ABSTRACT FROM AUTHOR]

The study of inorganic crystalline materials by solid-state NMR spectroscopy is often complicated by the low sensitivity of heavy nuclei. However, these materials often contain or can be prepared with paramagnetic dopants without significantly affecting the structure of the crystalline host. Dynamic nuclear polarization (DNP) is generally capable of enhancing NMR signals by transferring the magnetization of unpaired electrons to the nuclei. Therefore, the NMR sensitivity in these paramagnetically doped crystals might be increased by DNP. In this paper we demonstrate the possibility of efficient DNP transfer in polycrystalline samples of [Co(en)3Cl3]2-NaCl-6H20 (en = ethylenediamine, C2H8N2) doped with Cr(lll) in varying concentrations between 0.1 and 3 mol %. We demonstrate that 1H, 13C, and 59Co can be polarized by irradiation of Cr(lll) with 140 GHz microwaves at a magnetic field of 5 T. We further explain our findings on the basis of electron paramagnetic resonance spectroscopy of the Cr(IIl) site and analysis of its temperature-dependent zero-field splitting, as well as the dependence of the DNP enhancement factor on the external magnetic field and microwave power. This first demonstration of DNP transfer from one paramagnetic metal ion to its diamagnetic host metal ion will pave the way for future applications of DNP in paramagnetically doped materials or metalloproteins. [ABSTRACT FROM AUTHOR]

The endohedral fullerene once erroneously identified as Sc3@C82 was recently shown to be Sc3C2@Ih-C80, the first example of an open-shell cluster metallofullerene. We herein report that benzyl bromide (1) reacts with Sc3C2@Ih-C80 via a regioselective radical addition that affords only one isomer of the adduct Sc3C2@Ih-C80 (CH2C6H5) (2) in high yield. An X-ray crystallographic study of 2 demonstrated that the benzyl moiety is singly bonded to the fullerene cage, which eliminates the paramagnetism of the endohedral in agreement with the ESR results. Interestingly, X-ray results further reveal that the 3-fold disordered Sc3C2 cluster adopts two different configurations inside the cage. These configurations represent the so-called “planar” form and the computationally predicted, but not crystallographically characterized, “trifoliate” form. It is noteworthy that this is the first crystallographic observation of the “trifoliate” form for the Sc3C2 cluster. In contrast, crystallographic investigation of a Sc3C2@Ih-C80/Ni(OEP) cocrystal, in which the endohedral persists in an open-shell structure with paramagnetism, indicates that only the former form occurs in pristine Sc3C2@Ih-C80. These results demonstrate that the cluster configuration in EMFs is highly sensitive to the electronic structure, which is tunable by exohedral modification. In addition, the electrochemical behavior of Sc3C2@Ih-C80 has been markedly changed by the radical addition, but the absorption spectra of the pristine and the derivative are both featureless. These results suggest that the unpaired electron of Sc3C2@Ih-C80 is buried in the Sc3C2 cluster and does not affect the electronic configuration of the cage. [ABSTRACT FROM AUTHOR]

The increasing interest in the functional versatility of glycan epitopes in cellular glycoconjugates calls for developing sensitive methods to define carbohydrate conformation in solution and to characterize protein-carbohydrate interactions. Measurements of pseudocontact shifts in the presence of a paramagnetic cation can provide such information. In this work, the energetically privileged conformation of a disaccharide (lactose as test case) was experimentally inferred by using a synthetic carbohydrate conjugate bearing a lanthanide binding tag. In addition, the binding of lactose to a biomedically relevant receptor (the human adhesion/growthregulatory lectin ga]ectin-3) and its consequences in structural terms were defined, using Dy3+, Tb3+, and Tm3+. The described approach, complementing the previously tested protein tagging as a way to exploit paramagnetism, enables to detect binding, even weak interactions, and to characterize in detail topological aspects useful for physiological ligands and mimetics in drug design. [ABSTRACT FROM AUTHOR]

An ensemble-based approach is presented to explore the conformational space sampled by a multidomain protein showing moderate interdomain dynamics in terms of translational and rotational motions. The strategy was applied on a complex of calmodulin (CaM) with the IQ-recognition motif from the voltage-gated calcium channel Cav1.2 (IQ), which adopts three different interdomain orientations in the crystal. The N60D mutant of calmodulin was used to collect pseudocontact shifts and paramagnetically induced residual dipolar couplings for six different lanthanide ions. Then, starting from the crystal structure, pools of conformations were generated by free MD. We found the three crystal conformations in solution, but four additional MD-derived conformations had to be included into the ensemble to fulfill all the paramagnetic data and cross-validate optimally against unused paramagnetic data. Alternative approaches led to similar ensembles. Our "ensemble" approach is a simple and efficient tool to probe and describe the interdomain dynamics and represents a general method that can be used to provide a proper ensemble description of multidomain proteins. [ABSTRACT FROM AUTHOR]

We report an organo-paramagnetic switch consisting of a linked bis(viologen) dication diradical that can be cycled reversibly between diamagnetic and paramagnetic states via noncovalent guest–host chemistry with cucurbit[7]uril (CB[7]) in room-temperature water. Computations suggest that the nature of the interaction between the viologen cation radical units is that of a pi dimer (pimer). Molecules with switchable magnetic properties have possible applications in spintronics, data storage devices, chemical sensors, building blocks for materials with switchable bulk magnetic properties, as well as magnetic resonance probes for biological applications. [ABSTRACT FROM AUTHOR]

The reaction of (ArL)Co(py) with tBuN3 afforded the isolable three-coordinate Co-imido complex (ArL)Co(NtBu) which is paramagnetic at room temperature. Variable-temperature (VT) 1H NMR spectroscopy VT crystallography and magnetic susceptibility measurements revealed that (ArL)Co(NtBu) undergoes a thermally induced spin crossover from an S = 0 ground state to a quintet (S = 2) state. The reaction of (ArL)Co(py) with mesityl azide yielded an isolable S = 1 terminal imido complex that was converted into the metallacycloindoline (ArL)Co(k2-NHC6H2-2,4-Me2-6-CH2) via benzylic C-H activation. [ABSTRACT FROM AUTHOR]

Protein-protein interactions play critical roles in cellular function and oligomerization of membrane proteins is a commonly observed phenomenon. Determining the oligomerization state and defining the intermolecular interface in the bilayer is generally a difficult task. Here, we use site-specific spin labeling to demonstrate that relaxation enhancements induced by covalently attached paramagnetic tag can provide distance restraints defining the intermonomer interface in oligomers formed by a seven-helical transmembrane protein Anabaena Sensory Rhodopsin (ASR). We combine these measurements with visible CD spectroscopy and cross-linking experiments to demonstrate that ASR forms tight trimers in both detergents and lipids. [ABSTRACT FROM AUTHOR]

Pseudocontact shifts (PCSs) arise in paramagnetic systems in which the susceptibility tensor is anisotropic. PCSs depend upon the distance from the paramagnetic center and the position relative to the susceptibility tensor, and they can be used as structural restraints in protein structure determination. We show that the use of 1H-detected solid-state correlations provides facile and rapid detection and assignment of site-specific PCSs, including resolved 1H PCSs, in a large metalloprotein, Co2+-substituted superoxide dismutase (Co2+-SOD). With only 3 mg of sample and a small set of experiments, several hundred PCSs were measured and assigned, and these PCSs were subsequently used in combination with 1H-1H distance and dihedral angle restraints to determine the protein backbone geometry with a precision paralleling those of state-of-the-art liquid-state determinations of diamagnetic proteins, including a well-defined active site. [ABSTRACT FROM AUTHOR]

An endofulleropyrrolidine, with H2 as a guest, has been reduced to a paramagnetic endofulleride radical anion. The magnetic interaction between the electron delocalized on the fullerene cage and the guest H2 has been probed by pulsed END OR The experimental hyperfine couplings between the electron and the H2 guest were measured, and their values agree very well with DFT calculations. This agreement provides clear evidence of magnetic communication between the electron density of the fullerene host cage and H2 guest. The ortho-H2/para- H2 interconversion is revealed by temperature-dependent ENDOR measurements at low temperature. The conversion of the paramagnetic ortho-H2 to the diamagnetic para-H2 causes the ENDOR signal to decrease as the temperature is lowered due to the spin catalysis by the paramagnetic fullerene cage of the radical anion fulleride. [ABSTRACT FROM AUTHOR]

We describe the design, synthesis, and biological characterization of manganese oxocluster-based "single molecule magnets (SMMs)". We demonstrate that polymeric micellar nanoparticles can serve as a carrier and help to stabilize delicate SMM molecules from breaking down easily and thus prevent their property loss. Concentrating thousands of Mn-clusters per micelle provided a high ionic and per-particle relaxivity allowing sensitive MR imaging in vivo. This reports one of the earliest examples of in vivo imaging of a rationally designed polymeric micelle that features SMM. [ABSTRACT FROM AUTHOR]

We present the synthesis of nonspherical magnetic microparticles with multiple functionalities, shapes, and chemistries. Particle synthesis was performed in two steps: polymeric microparticles functionalized homogenously with carboxyl groups were generated using stop-flow lithography, and then in situ coprecipitation was used to grow magnetic nanoparticles at these carboxyl sites. With successive growth of magnetic nanoparticles, we obtained polymeric particles with saturation magnetizations of up to 42 emu/g microparticle. The growth in the magnetic nanoparticle mean size and polydispersity was determined from the magnetization curves obtained following each growth cycle; nanoparticle sizes were limited by the physical constraint of the effective mesh within the hosting gel microparticle. Particles with spatially segregated domains of varying magnetic properties (e.g., Janus particles, particles with step changes in magnetite concentration, etc.) can be synthesized readily using this approach. [ABSTRACT FROM AUTHOR]

Gd3+ ions, once bound to the monolayer of organic molecules coating the surface of gold nanoparticles, produce a paramagnetic relaxation enhancement (PRE) that broadens and eventually cancels the signals of the nuclear spins located nearby (within 1.6 nm distance). In the case of nanoparticles coated with mixed monolayers, the signals arising from the different coating molecules experience different PRE, depending on their distance from the binding site. As a consequence, observation of the signal broadening patterns provides direct information on the monolayer organization. [ABSTRACT FROM AUTHOR]

Combining DNA and superparamagnetic beads in a rotating magnetic field produces multiparticle aggregates that are visually striking, enabling label-free optical detection and quantification of DNA at levels in the picogram per microliter range. DNA in biological samples can be quantified directly by simple analysis of optical images of microfluidic wells placed on a magnetic stirrer without prior DNA purification. Aggregation results from DNA/bead interactions driven either by the presence of a chaotrope (a nonspecific trigger for aggregation) or by hybridization with oligonucleotides on functionalized beads (sequence-specific). This paper demonstrates quantification of DNA with sensitivity comparable to that of the best currently available fluorometric assays. The robustness and sensitivity of the method enable a wide range of applications, illustrated here by counting eukaryotic cells. Using widely available and inexpensive benchtop hardware, the approach provides a highly accessible low-tech microscale alternative to more expensive DNA detection and cell counting techniques. [ABSTRACT FROM AUTHOR]

This paper describes the use of magnetic levitation (MagLev) to measure the association of proteins and ligands. The method starts with diamagnetic gel beads that are functionalized covalently with small molecules (putative ligands). Binding of protein to the ligands within the bead causes a change in the density of the bead. When these beads are suspended in a paramagnetic aqueous buffer and placed between the poles of two NbFeB magnets with like poles facing, the changes in the density of the bead on binding of protein result in changes in the levitation height of the bead that can be used to quantify the amount of protein bound. This paper uses a reaction-diffusion model to examine the physical principles that determine the values of rate and equilibrium constants measured by this system, using the well-defined model system of carbonic anhydrase and aryl sulfonamides. By tuning the experimental protocol, the method is capable of quantifying either the concentration of protein in a solution, or the binding affinities of a protein to several resin-bound small molecules simultaneously. Since this method requires no electricity and only a single piece of inexpensive equipment, it may find use in situations where portability and low cost are important, such as in bioanalysis in resource-limited settings, point-of-care diagnosis, veterinary medicine, and plant pathology. It still has several practical disadvantages. Most notably, the method requires relatively long assay times and cannot be applied to large proteins (>70 kDa), including antibodies. The design and synthesis of beads with improved characteristics (e.g., larger pore size) has the potential to resolve these problems. [ABSTRACT FROM AUTHOR]

A combination of spectroscopic and electrochemical methods--XANES, EXAFS, X-ray, 1H NMR, EPR, Mössbauer, and cyclic voltammetry--demonstrate that the most efficient Pd catalysts for the asymmetric rearrangement of allylic trifluoroacetimidates unexpectedly possess in the activated oxidized form a PdIII center bound to a ferrocene core which remains unchanged (FeII) during the oxidative activation. These are the first recognized PdIII complexes acting as enantioselective catalysts. [ABSTRACT FROM AUTHOR]

A natural bond orbital (NBO) analysis of unpaired electron spin density in metalloproteins is presented, which allows a fast and robust calculation of paramagnetic NMR parameters. Approximately 90% of the unpaired electron spin density occupies metal-ligand NBOs, allowing the majority of the density to be modeled by only a few NBOs that reflect the chemical bonding environment. We show that the paramagnetic relaxation rate of protons can be calculated accurately using only the metal-ligand NBOs and that these rates are in good agreement with corresponding rates measured experimentally. This holds, in particular, for protons of ligand residues where the point-dipole approximation breaks down. To describe the paramagnetic relaxation of heavy nuclei, also the electron spin density in the local orbitals must be taken into account. Geometric distance restraints for 15N can be derived from the paramagnetic relaxation enhancement and the Fermi contact shift when local NBOs are included in the analysis. Thus, the NBO approach allows us to include experimental paramagnetic NMR parameters of 15N nuclei as restraints in a structure optimization protocol. We performed a molecular dynamics simulation and structure determination of oxidized rubredoxin using the experimentally obtained paramagnetic NMR parameters of 15N. The corresponding structures obtained are in good agreement with the crystal structure of rubredoxin. Thus, the NBO approach allows an accurate description of the geometric structure and the dynamics of metalloproteins, when NMR parameters are available of nuclei in the immediate vicinity of the metal-site. [ABSTRACT FROM AUTHOR]

Various layered boronitrides (LaN)n(TM2B2) (TM = transition metal; n = 2, 3) have been prepared using a high-pressure synthesis technique in which an inverse α-PbO-type TM2B2 layer is separated by two or three rock salt-type LaN layers and these layers are connected through linear (BN) units. The electronic states of the distinguishing (BN) unit and intermediate rock salt-type LaN layer are discussed on the basis of density functional theory calculations. Bulk superconductivity has been found in LaNiBN (Tc ≈ 4.1 K), CaNiBN (Tc ≈ 2.2 K), and LaPtBN (Tc ≈ 6.7 K), where the Fermi level EF is located in the bands composed of the TM(d)-B(2p) antibonding state and the main TM(d) band resides well below EF. The non-superconductive TM-based compounds exhibit Pauli paramagnetic behavior, in which the highly itinerant nature of the electrons caused by strong TM(d)-B(2p) covalent bonding suppresses the long-range magnetic ordering. [ABSTRACT FROM AUTHOR]

1,3,5-Triphenyl-6-oxoverdazyl radicals 1[n] in which each phenyl group is substituted with three alkylsulfanyl groups (n = 6 8 10) exhibit a monotropic columnar rectangular (Colr) phase below 60°C. Detailed analysis of 1[n] revealed a broad absorption band in the visible region with maxima at 540 and 610 nm and redox potentials E1/20/+1 = +0.99 V and E1/20/-1 = -0.45 V vs SCE. Photovoltaic studies of 1[8] demonstrated a hole mobility of 1.52 × 10-3 cm² V-1 s-1 in the mesophase with an activation energy of 0.06 ± 0.01 eV. Magnetization studies of 1[8] revealed nearly ideal paramagnetic behavior in either the solid or fluid phase above 200 K and weak antiferromagnetic interactions at low temperatures. [ABSTRACT FROM AUTHOR]

Oxomanganese(V) species have been implicated in a variety of biological and synthetic processes, including their role as a key reactive center within the oxygen-evolving complex in photosynthesis. Nearly all mononuclear MnV-oxo complexes have tetragonal symmetry, producing low-spin species. A new high-spin MnV-oxo complex that was prepared from a well-characterized oxomanganese(III) complex having trigonal symmetry is now reported. Oxidation experiments with [FeCp2]+ were monitored with optical and electron paramagnetic resonance (EPR) spectroscopies and support a high-spin oxomanganese(V) complex formulation. The parallel-mode EPR spectrum has a distinctive S = 1 signal at g = 4.01 with a six-line hyperfine pattern having Az = 113 MHz. The presence of an oxo ligand was supported by resonance Raman spectroscopy, which revealed O-isotope-sensitive peaks at 737 and 754 cm-1 assigned as a Fermi doublet centered at 746 cm-1(Δ18O = 31 cm-1). Mn Kβ X-ray emission spectra showed Kβ' and Kβ1,3 bands at 6475.92 and 6490.50 eV, respectively, which are characteristic of a high-spin MnV center. [ABSTRACT FROM AUTHOR]

The use of a magic-angle turning and phase-adjusted spinning sideband NMR experiment to resolve and quantify the individual local environments in the high field 7Li and 31P NMR spectra of paramagnetic lithium-ion battery materials is demonstrated. The use of short radio frequency pulses provides an excitation bandwidth that is sufficient to cover shift anisotropy of >1 MHz in breadth, allowing isotropic and anisotropic components to be resolved. [ABSTRACT FROM AUTHOR]

This Communication describes the synthesis of highly monodispersed 12 nm nickel nanocubes. The cubic shape was achieved by using trioctylphosphine and hexadecylamine surfactants under a reducing hydrogen atmosphere to favor thermodynamic growth and the stabilization of {100} facets. Varying the metal precursor to trioctylphosphine ratio was found to alter the nanoparticle size and shape from 5 nm spherical nanoparticles to 12 nm nanocubes. High-resolution transmission electron microscopy showed that the nanocubes are protected from further oxidation by a 1 nm NiO shell. Synchrotron-based X-ray diffraction techniques showed the nickel nanocubes order into [100] aligned arrays. Magnetic studies showed the nickel nanocubes have over 4 times enhancement in magnetic saturation compared to spherical superparamagnetic nickel nanoparticles. [ABSTRACT FROM AUTHOR]

Significant variance in the magnitude of reported exchange coupling parameters (both experimental and computed) for paramagnetic transition metal-ethynylbenzene complexes suggests that nuances of the magnetostructural relationship in this class of compounds remain to be understood and controlled, toward maximizing the stability of high-spin ground states. We report the preparation, electrochemical behavior, magnetic properties, and results of computational investigations of a series of iron ethynylbenzene complexes with coordination environments suitable for metallodendrimer assembly: [(dmpe)2FeCl(C2Ph)] (OTf) (1), [(dmpe)4-Fe2Cl2(μ-p-DEB)] (BArF4)2 (2), [(dmpe)6Fe3Cl3(TEB)] (3), [(dmpe)6-Fe3Cl3(μ3-TEB)](OTf)3 (4), and [(dmpe)4Fe2Cl2(μ-m-DEB)](BArF4)2 (s) [dmpe = 1,2-bis(dimethylphosphino)ethane; p-H2DEB = 1,4-diethynylbenzene; BArF4= tetrakis[3,5-bis(trifluoromethyl)phenyl]borate; H3TEB = 1,3,5-triethynylbenzene; m-H2DEB= 1,3-diethynylbenzene]. As expected, the ligand topology drives the antiferromagnetic coupling in 2 (J = -134 cm-1 using the Ĥ = -2JŜ1 · S2 convention) and the ferromagnetic coupling in 4 and S (J = +37 cm-1, J' = +5 cm-1 for 4; J = +11 cm-1 for 5); the coupling is comparable to but deviates significantly from values reported for related Cp*-containing species (Cp* = η5-C5Me5). The origins of these differences are explored computationally: a density functional theory (DFT) approach for treating the coupling of three spin centers as a linear combination of single-determinantal descriptions is developed and described, and the results of these computations can be generalized to other paramagnetic systems. Unrestricted B3LYP hybrid DFT calculations performed on rotamers of 4 and S and related complexes, as well as Cp* analogues, provide J values that correlate with the experimental values. We find that geometric considerations dominate the magnetism of the Cp* complexes, while topology and alkynyl ligand electronics combine more subtly to drive the magnetism of the new complexes reported here. These calculations imply that substantial magnetic exchange parameters, with accompanying well-isolated high-spin ground states, are achievable for ethynylbenzene-bridged paramagnetic metallodendrimers. [ABSTRACT FROM AUTHOR]

Calmodulin (CaM) is the universal calcium sensor in eukaryotes, regulating the function of numerous. proteins. Crystallography and NMR show that free CaM- 4Ca2+ exists in an extended conformation with significant interdomain separation, but clamps down upon target peptides to form a highly compact structure. NMR has revealed sub- stantial interdomain motions in CaM-4Ca2+, enabled by a flexible linker. In one instance, CaM-4Ca2+ has been crystallized in a compact configuration; however, no direct evidence for transient interdomain contacts has been observed in solution, and little is known about how large-scale interdomain motions contribute to biological function. Here, we use paramagnetic relaxation enhancement (PRE) to characterize transient compact states of free CaM that are too sparsely populated to observe by traditional NMR methods. We show that unbound CaM samples a range of compact structures, populated at 5-10%, and that Ca2+ dramatically alters the distribution of these configurations in favor of states resembling the peptide-bound structure. In the absence of Ca2+, the target peptide binds only to the C-terminal domain, and the distribution of compact states is similar with and without peptide. These data suggest an alternative pathway of CaM action in which CaM remains associated with its kinase targets even in the resting state. Only CaM-4Ca2+, however, shows an innate propensity to form the physiologically active compact structures, suggesting that Ca2+ activates CaM not only through local structural changes within each domain but also through more global remodeling of interdomain interactions. Thus, these findings illustrate the subtle interplay between conformational selection and induced fit. [ABSTRACT FROM AUTHOR]

Molecular dynamics (MD) simulations have been widely used to analyze dynamic conformational equilibria of folded proteins, especially in relation to NMR observables. However, this approach found little use in the studies of disordered proteins, where the sampling of vast conformational space presents a serious problem. In this paper, we demonstrate that the latest advances in computation technology make it possible to overcome this limitation. The experimentally validated (calibrated) MD models allow for new insights into structure/dynamics of disordered proteins. As a test system, we have chosen denatured ubiquitin in solution with 8 M urea at pH 2. High-temperature MD simulations in implicit solvent have been carried out for the wild-type ubiquitin as well as MTSL-tagged Q2C, D32C, and R74C mutants. To recalibrate the MD data (500 K) in relation to the experimental conditions (278 K, 8 M urea), the time axes of the MD trajectories were rescaled. The scaling factor was adjusted such as to maximize the agreement between the simulated and experimental 15N relaxation rates. The resulting effective length of the trajectories, 311 μs, ensures good convergence properties of the MD model. The constructed MD model was validated against the array of experimental data, including additional 15N relaxation parameters, multiple sets of paramagnetic relaxation enhancements (PREs), and the radius of gyration. In each case, a near-quantitative agreement has been obtained, suggesting that the model is successful. Of note, the MD-based approach rigorously predicts the quantities that are inherently dynamic, i.e., dependent on the motional correlation times. This cannot be accomplished, other than in empirical fashion, on the basis of static structural models (conformational ensembles). The MD model was further used to investigate the relative translational motion of the MTSL label and the individual HN atoms. The derived segmental diffusion coefficients proved to be nearly uniform along the peptide chain, averaging to D = 0.49-0.55 x 10-6 cm²/s. This result was verified by direct analysis of the experimental PRE data using the recently proposed Ullman-Podkorytov model. In this model, MTSL and HN moieties are treated as two tethered spheres undergoing mutual diffusion in a harmonic potential. The fitting of the experimental data involving D as a single adjustable parameter leads to D = 0.45 x 10-6 cm²/s, in good agreement with the MD-based analyses. This result can be compared with the range of estimates obtained from the resonance energy transfer experiments, D = 0.2-6.0 x 10-6 cm²/s. [ABSTRACT FROM AUTHOR]

The magnetic behavior for Mn:CdSe (0.6%) quantum dots (QDs) exhibits size-dependent magnetic exchange mediated by the concentration of intrinsic carriers, which arise from surface states. High temperature paramagnetic behavior that can be fit to a Brillouin function with weak low temperature antiferromagnetic (AFM) coupling is observed for the large Mn:CdSe (5.0 and 5.8 nm) QDs. The 2.8 and 4.0 nm Mn:CdSe QDs display a size-independent blocking temperature (TB) at 12 K, decreasing coercivity with increasing size, and a lowering of the activation barrier for spin relaxation as the QD is increased in size. The magnetic behavior is inconsistent with classical domain theory behavior for a superparamagnet (SPM) but can be accounted for in a carrier-mediated RKKY model. Fitting the susceptibility data reveals a Pauli-paramagnetic (PPM) component that is believed to arise from the presence of carriers. The carrier density is observed to scale with the surface to volume ratio in the QDs, indicating the carriers arise from surface states that are weakly localized resulting in the onset of long-distance carrier-mediated RKKY exchange inducing overall ferrimagnetism in the Mn:CdSe QDs when the carrier concentration is above a critical threshold. [ABSTRACT FROM AUTHOR]

The Rieske protein component of the cytochrome bccomplex contains a [2Fe-2S] cluster ligated by two cysteines and two histidines. We report here the PKa values of each of the imidazole rings of the two ligating histidines (His134 and His154) in the oxidized and reduced states of the Rieske protein from Therm us thermophilus (TtRp) as determined by NMR spectroscopy. Knowledge of these PKa values is of critical interest because of their pertinence to the mechanism of electron and proton transfer in the bifurcated O-cycle. Although we earlier had observed the pH dependence of a 15N NMR signal from each of the two ligand histidines in oxidized TtRp (Lin, I. J.; Chen, Y.; Fee, J. A.; Song, J.; Westler, W. M.; Markley, J. L. J. Am. Chem. Soc. 2006, 128, 10672-10673), the strong paramagnetism of the [2Fe-2S] cluster prevented the assignment of these signals by conventional methods. Our approach here was to take advantage of the unique histidine-leucine (Hisi 34-Leul 35) sequence and to use residue-selective labeling to establish a key sequence-specific assignment, which was then extended. Analysis of the pH dependence of assigned 13C, 13Cα, and 15Nϵ2 signals from the two histidine cluster ligands led to unambiguous assignment of the PKa values of oxidized and reduced TtRp. The results showed that the pKa of His134 changes from 9.1 in oxidized to ∼12.3 in reduced TtRp, whereas the pKa of His154 changes from 7.4 in oxidized to ∼12.6 in reduced TtRp. This establishes His154, which is close to the quinone when the Rieske protein is in the cytochrome b site, as the residue experiencing the remarkable redox-dependent pKa shift. Secondary structural analysis of oxidized and reduced TtRp based upon our extensive chemical shift assignments rules out a large conformational change between the oxidized and reduced states. Therefore, TtAp likely translocates between the cytochrome b and cytochrome c sites by passive diffusion. Our results are most consistent with a mechanism involving the coupled transfer of an electron and transfer of the proton across the hydrogen bond between the hydroquinone and His154 at the cytochrome b site. [ABSTRACT FROM AUTHOR]

The detailed, quantitative characterization of unfolded proteins is a largely unresolved task due to the enormous experimental and theoretical difficulties in describing the highly dimensional space of their conformational ensembles. Recently, residual dipolar coupling (RDC) and paramagnetic relaxation enhancement (PRE) data have provided large numbers of experimental parameters on unfolded states. To obtain a minimal model of the unfolded state according to such data we have developed new modules for the use of steric alignment RDCs and PREs as constraints in ensemble structure calculations by the program XPLOR-NIH. As an example, ensemble calculations were carried out on urea-denatured ubiquitin using a total of 419 previously obtained RDCs and 253 newly determined PREs from eight cysteine mutants coupled to MTSL. The results show that only a small number of about 10 conformers is necessary to fully reproduce the experimental RDCs, PREs and average radius of gyration. Cα contacts determined on a large set (400) of 10-conformer ensembles show significant (10-20%) populations of conformations that are similar to ubiquitin's A-state, i.e. corresponding to an intact native first β-hairpin and α-helix as well as non-native a-helical conformations in the C-terminal half. Thus, methanol/acid (A-state) and urea denaturation lead to similar low energy states of the protein ensemble, presumably due to the weakening of the hydrophobic core. Similar contacts are obtained in calculations using solely RDCs or PREs. The sampling statistics of the Cα contacts in the ensembles follow a simple binomial distribution. It follows that the present RDC, PRE, and computational methods allow the statistically significant detection of subconformations in the unfolded ensemble at population levels of a few percent. [ABSTRACT FROM AUTHOR]

Oxygen and Ni(ll) are ideal paramagnetic species for NMR studies of immersion depth since they establish prominent concentration gradients across the membrane-water interface of either bilayers or micelles. Corresponding gradients of paramagnetic shifts and relaxation rates are observed by NMR for membrane embedded amphiphiles. Specifically, upon dissolution of oxygen at a partial pressure of 20 bar or more, [sup13]C NMR spectra of membrane embedded amphiphiles reveal chemical shift perturbations which depend sensitively on average immersion depth in the membrane. Similarly, depth-dependent enhancements of spin-lattice relaxation rates can be detected by [sup1]H NMR. Generally, such paramagnetic effects depend both on steric or accessibility factors and on the local concentration of the paramagnet. The steric terms can be factored out by combining paramagnetic rates arising from O[sub2] and Ni, in the form of a ratio, R[sub1p](O[sub2])/ R[sub1p](Ni). The natural logarithm of this ratio is shown to depend linearly on immersion depth in a micelle. The analysis is verified using molecular dynamics simulations of dodecylphosphocholine in a detergent micelle, while thorough consideration of the paramagnetic rate data also allows for the determination of the orientation of imipramine in the micelle. Thus, a complete picture of topology arises from this approach which is readily applicable to studies of drugs and amphiphiles in fast-tumbling bicelles, small unilamellar vesicles, and micelles. [ABSTRACT FROM AUTHOR]

A strategy for the accurate determination of protein solution structures starting from X-ray data and a minimal set of NMR data is proposed and successfully applied to two complexes of calmodulin (CaM) with target peptides not previously described, Its implementation in the present case is based on the use of lanthanide ions as substitutes for calcium in one of the four calcium binding sites of CaM and the collection of pseudocontact shift (pcs) and residual dipolar coupling (rdc) restraints induced by the paramagnetic metals. Starting from the crystal structures, new structural models are calculated that are in excellent agreement with the paramagnetic restraints and differ significantly from the starting crystal structures. In particular, in both complexes, a change in orientation of the first helix of the N-terminal CaM domain and of the whole C-terminal domain is observed. The simultaneous use of paramagnetic pcs and rdc restraints has the following crucial advantages: (i) it allows one to assess the possible presence of interdomain conformational freedom, which cannot be detected if the rdc values are derived from external orienting media; (ii) in the absence of significant conformational freedom, the global orientation tensor can be independently and precisely determined from pcs values, which are less sensitive than rdc values to the presence of local structural inaccuracies, and therefore (iii) the relative rearrangement of a domain or a secondary structure element with respect to the metal-bearing domain can be detected. [ABSTRACT FROM AUTHOR]

The thermal reaction of La@C82(C2v) with 3-triphenylmethyl-5-oxazolidinone (1) in toluene affords benzyl monoadducts La@C82(C2v)(CH2C6H5) (2a―2d). The same monoadducts are also obtained by the photoirradiation of La@C82(C2v) in toluene without the existence of 1. These reactions are applicable to paramagnetic metallofullerenes, such as La@C82(Cs) and Ce@C82(C2v). The photoirradiation of La@C82(C2v) in 1 ,2-dichlorobenzene in the presence of a,a,2,4-tetrachlorotoluene also affords the monoadducts La@C82(C2v)(CHClC6H3Cl2) (3a-3d). The monoadducts are fully characterized by spectroscopic analyses. Single-crystal X-ray structure analysis for 3d reveals the unique structure. Theoretical calculations show that the cage carbons having high spin densities are selectively attacked by radical species to form the monoadducts linked by a carbon-carbon single bond. The thermal reaction of La@C82(C2v) with 1 in benzene affords metallofulleropyrrolidine La@C82(C2v)(C2H4NCPh3) (5), unlike the reaction in toluene. [ABSTRACT FROM AUTHOR]

Abstract: A major challenge in magnetic nanoparticle synthesis and (bio)functionalization concerns the precise characterization of the nanoparticle surface ligands. We report the first analytical NMR investigation of organic ligands stably anchored on the surface of superparamagnetic nanoparticles (MNP5) through the development of a new experimental application of high-resolution magic-angle spinning (HRMAS). The conceptual advance here is that the HRMAS technique, already being used for MAS NMR analysis of gels and semisolid matrixes, enables the fine-structure-resolved characterization of even complex organic molecules bound to paramagnetic nanocrystals, such as nanosized iron oxides, by strongly decreasing the effects of paramagnetic disturbances. This method led to detail-rich, well-resolved 1H NMR spectra, often with highly structured first-order couplings, essential in the interpretation of the data. This HRMAS application was first evaluated and optimized using simple ligands widely used as surfactants in MNP synthesis and conjugation. Next, the methodology was assessed through the structure determination of complex molecular architectures, such as those involved in MNP3 and MNP4. The comparison with conventional probes evidences that HRMAS makes it possible to work with considerably higher concentrations, thus avoiding the loss of structural information. Consistent 2D homonuclear 1H-1H and 1H-13C heteronuclear single-quantum coherence correlation spectra were also obtained, providing reliable elements on proton signal assignments and carbon characterization and opening the way to 13C NMR determination. Notably, combining the experimental evidence from HRMAS 1H NMR and diffusion-ordered spectroscopy performed on the hybrid nanoparticle dispersion confirmed that the ligands were tightly bound to the particle surface when they were dispersed in a ligand-free solvent, while they rapidly exchanged when an excess of free ligand was present in solution. In addition to HRMAS NMR, matrix-assisted laser desorption ionization time-of-flight MS analysis of modified MNPs proved very valuable in ligand mass identification, thus giving a sound support to NMR characterization achievements. [ABSTRACT FROM AUTHOR]

A novel solid-state NMR technique for identifying the asymmetric insertion depths of membrane proteins in lipid bilayers is introduced. By applying Mn2+ ions on the outer but not the inner leaflet of lipid bilayers, the sidedness of protein residues in the lipid bilayer can be determined through paramagnetic relaxation enhancement (PRE) effects. Protein-free lipid membranes with one-side Mn2+-bound surfaces exhibit significant residual 31P and lipid headgroup 13C intensities, in contrast to two-side Mn2+-bound membranes, where lipid headgroup signals are mostly suppressed. Applying this method to a cell-penetrating peptide, penetratin, we found that at low peptide concentrations, penetratin is distributed in both leaflets of the bilayer, in contrast to the prediction of the electroporation model, which predicts that penetratin binds to only the outer lipid leaflet at low peptide concentrations to cause an electric field that drives subsequent peptide translocation. The invalidation of the electroporation model suggests an alternative mechanism for intracellular import of penetratin, which may involve guanidinium-phosphate complexation between the peptide and the lipids. [ABSTRACT FROM AUTHOR]

The transverse ¹H relaxivities of aqueous colloidal solutions of dextran coated Dy2O3 nanoparticles of different sizes were investigated at magnetic field strengths (B) between 7 and 17.6 T. The particle size with the maximum relaxivity (r2) appears to vary between 70 nm at 7 T (r2 ≈190 s-1 mM-1) and 60 nm at 17.6 T (r2 ≈ 675 s-1 mM-1). A small difference between r2 and r2* was observed, which was ascribed to the effect of the dextran coating. The value of r2 is proportional to B² up to 12 T after which it saturates. Independent magnetization measurements on these particles at room temperature at magnetic field strengths up to 30 T, however, show a typical paramagnetic behavior with a magnetization of the particle that is proportional to the field strength. The saturation in the curve of r2 as a function of B² was tentatively explained by the presence of an extremely fast relaxing component of the signal at high field strengths, which is not observable on the NMR time scale. The results of this Study can be exploited for the rational design of MRI contrast agents, based on lanthanide oxide particles, with high efficiencies at magnetic field strengths of more than 1.5 T. [ABSTRACT FROM AUTHOR]

A mesoporous solid with crystalline walls and an ordered pore structure exhibiting a bimodal pore size distribution (3.3 and 11 nm diameter pores) has been synthesized. Previous attempts to synthesize solids with large ordered mesopores by hard templating focused on the preparation of templates with thick walls (the thick walls become the pores in the target materials), something that has proved difficult to achieve. Here the large pores (11 nm) do not depend on the synthesis of a template with thick walls but instead on controlling the microporous bridging between the two sets of mesopores in the KIT-6 template. Such control determines the relative proportion of the two pore sizes. The wall thickness of the 3D cubic NiO mesopore has also been varied. Preliminary magnetic characterization indicates the freezing of uncompensated moments or blocking of superparamagnetism. [ABSTRACT FROM AUTHOR]

We have measured the bimolecular contribution (relaxivity) R1 (M-1 s-1) to the spin-lattice relaxation rate for the protons of H2 and H2@C60 dissolved in organic solvents in the presence of paramagnet nitroxide radicals. It is found that the relaxation effect of the paramagnets is enhanced 5-fold in H2@C60 compared to H2 under the same conditions. 13C relaxivity in C60 induced by nitroxide has also been measured. The resulting value of R1 for 13C is substantially smaller relative to the 1H relaxation in H2@C60 than expected solely on the basis of the smaller magnetic moment of 13C. The observed values of R1 have been analyzed quantitatively using an outer-sphere model for bimolecular spin relaxation to extract an encounter distance, d, as the dependent variable. The resulting values of d for H2 and 13C60 are similar to the sum of the van der Waals radii for the radical and the corresponding molecule. The value of d for 1H2@C60 is substantially smaller than the corresponding van der Waals estimates, corresponding to larger than expected values of R1. A possible explanation for the enhanced relaxivity is a contribution from hyperfine coupling. Based on the results reported here, it seems that not only is the hydrogen molecule in H2@C60 not insulated from magnetic contact with the outside world but also the interaction with paramagnets is even stronger than expected based on distance alone. [ABSTRACT FROM AUTHOR]

Paramagnetic rare-earth elements have been examined as NMR structural probes in polyoxoanionic solids, which have a variety of applications as luminescent materials that are usually disordered and therefore intractable by traditional structural methods. Thirteen Keggin and Wells-Dawson polyoxotungstates containing substitutions with lanthanides of different effective magnetic moments have been examined by 31P magic angle spinning NMR spectroscopy. The electron-nuclear dipolar interaction dominating the spinning sideband envelopes is determined by the lanthanide's magnetic moment and was found to be a sensitive probe of the nature of the polyoxoanion, of the positional isomerism, and of the ion stoichiometry. Electron-nuclear dipolar anisotropies computed based on the point-dipole approximation are generally in good agreement with the experimental results. The choice of a specific lanthanide as a structural probe can be tailored to the desired distance range between the phosphorus atoms and the paramagnetic centers to be probed. This approach is expected to be particularly useful in the paramagnetic polyoxoanionic materials lacking long-range order. [ABSTRACT FROM AUTHOR]

Transient 1:1 precursor complexes for intermolecular self-exchange between various organic electron donors (D) and their paramagnetic cation radicals (D+̇), as well as between different electron acceptors (A) paired with their anion radicals (A-̇), are spectrally (UV-NIR) observed and structurally (X-ray) identified as the cofacial (π-stacked) associates [D, D+̇] and [A-̇, A], respectively. Mulliken-Hush (two-state) analysis of their diagnostic intervalence bands affords the electronic coupling elements (HDA), which together with the Marcus reorganization energies (λ) from the NIR spectral data are confirmed by molecular-orbital computations. The HDA values are found to be a sensitive function of the bulky substituents surrounding the redox centers. As a result, the steric modulation of the donor/acceptor separation (rDA) leads to distinctive electron-transfer rates between sterically hindered donors/acceptors and their more open (unsubstituted) parents. The latter is discussed in the context of a continuous series of outer- and inner-sphere mechanisms for organic electron-transfer processes in a manner originally formulated by Taube and co-workers for inorganic (coordination) donor/acceptor dyads—with conciliatory attention paid to traditional organic versus inorganic concepts. [ABSTRACT FROM AUTHOR]

The N-terminal SH3 domain of the Drosophila modular protein Drk undergoes slow exchange between a folded (Fexch) and highly populated unfolded (Uexch) state under nondenaturing buffer conditions, enabling both Fexch and Uexch states to be simultaneously monitored. The addition of dissolved oxygen, equilibrated to a partial pressure of either 30 atm or 60 atm, provides the means to study solvent exposure with atomic resolution via 13C NMR paramagnetic shifts in ¹H, 13C HSQC (heteronuclear single quantum coherence) spectra. Absolute differences in these paramagnetic shifts between the Fexch and Uexch states allow the discrimination of regions of the protein which undergo change in solvent exposure upon unfolding. Contact with dissolved oxygen for both the Fexch and Uexch states could also be assessed through 13C paramagnetic shifts which were normalized based on the corresponding paramagnetic shifts seen in the free amino acids. In the Fexch state, the 13C nuclei belonging to the hydrophobic core of the protein exhibited very weak normalized paramagnetic shifts while those with greater solvent accessible surface area exhibited significantly larger normalized shifts. The Uexch state displayed less varied 13C paramagnetic shifts although distinct regions of protection from solvent exposure could be identified by a lack of such shifts. These regions, which included Phe9, Thr12, Ala13, Lys21, Thr22, Ile24, Ile27, and Arg38, overlapped with those found to have residual nativelike and non-native structures in previous studies and in some cases provided novel information. Thus, the paramagnetic shifts from dissolved oxygen are highly useful in the study of a transient structure or clustering in disordered systems, where conventional NMR measurements (couplings, chemical shift deviations from random coil values, and NOEs) may give little information. [ABSTRACT FROM AUTHOR]

In this proof of principle work, a technique is introduced to study hydrated surfaces using hyperpolarized (hp) 83Kr NMR spectroscopy. The longitudinal (T1) relaxation of hp-83Kr is shown to be extremely sensitive to the presence of adsorbed water on hydrophilic borosilicate and hydrophobic siliconized glass surfaces. The krypton surface relaxation is found to be largely independent of the total gas pressure applied to the studied materials, and the presented technique is therefore fairly robust. However, the relaxational properties of hp-83Kr can be ‘tuned’ by adjusting the composition of the optical pumping gas mixture. This effect may be important for practical applications such as hp-83Kr MR imaging and can be achieved without sacrificing signal intensity. Complementary information to that of hp-83Kr surface relaxation data can be obtained from hp-129Xe relaxation measurements that are sensitive to the presence of paramagnetic surface sites. In contrast to the signal decay of hp-129Xe, the longitudinal relaxation of 83Kr is largely unaffected by paramagnetic impurities, and in some materials, 83Kr and 129Xe show comparable T1 times that are caused by two completely different relaxation mechanisms. Finally, the relaxation times of 83Kr in contact with bovine lung surfactant coated glass pores that are similar in size to mammalian alveoli are presented. The results suggest that in vivo MR studies may be feasible and could provide valuable information about changes in pulmonary surface chemistry. [ABSTRACT FROM AUTHOR]

Four new phenylcyanamido-bridge dinuclear ruthenium complexes [{Ru(tpy)(thd)}2(μ-L)] with tpy = 2,2′:6′,2″-terpyridine, thd = 2,2,6,6-tetramethyl-3,5-heptanedione and L = dcbp = 4,4′-dicyanami- dobiphenyl; bcpa = bis(4-cyanamidophenyl)acetylene; bcpda = bis(4-cyanamidophenyl)diacetylene; bcpea = 9,10,-bis(4-cyanamidophenylethyriyl)anthracene have been prepared and fully characterized. The mixed valent Ru(II)Ru(III) and homovalent paramagnetic Ru(III)Ru(III) forms of all the complexes were electrochemically generated and studied by UV-vis-NIR and EPR spectroscopy. Electronic communication was quantified by the electronic coupling parameter Vab extracted from intervalence measurements in the near IR area, and magnetic communication was quantified in terms of the exchange coupling constant J, accessible from the intensity of the EPA signal when varying the temperature. Exponential decays for both electronic and magnetic coupling versus intermetallic distance were obtained and discussed. [ABSTRACT FROM AUTHOR]

The electronic structure of a family of bis(imino)pyridine iron dihalide, monohalide, and neutral ligand compounds has been investigated by spectroscopic and computational methods. The metrical parameters combined with MSssbauer spectroscopic and magnetic data for (iPrPDI)FeCI2 (iPrPDI = 2,6-(2,6iPr2C6H3NCMe)2C5H3N) established a high-spin ferrous center ligated by a neutral bis(imino)pyridine ligand. Comparing these data to those for the single electron reduction product, (iPrPDI)FeCI, again demonstrated a high-spin ferrous ion, but in this case the SFe = 2 metal center is antiferromagnetically coupled to a ligand-centered radical (SL = ½), accounting for the experimentally observed S = 1/3 ground state. Continued reduction to (iPrPDI)FeLn (L = N2, n = 1,2; CO, n = 2; 4-(N,N-dimethylamino)pyridine, n = 1) resulted in a doubly reduced bis(imino)pyridine diradical, preserving the ferrous ion. Both the computational and the experimental data for the N,N-(dimethylamino)pyridine compound demonstrate nearly isoenergetic singlet (SL = 0) and triplet (SL = 1) forms of the bis(imino)pyridine dianion. In both spin states, the iron is intermediate spin (SFe = 1) ferrous. Experimentally, the compound has a spin singlet ground state (S = 0) due to antiferromagnetic coupling of iron and the ligand triplet state. Mixing of the singlet diradical excited state with the triplet ground state of the ligand via spin-orbit coupling results in temperature-independent paramagnetism and accounts for the large dispersion in ¹H NMR chemical shifts observed for the in-plane protons on the chelate. Overall, these studies establish that reduction of (iPrPDI)FeCI2 with alkali metal or borohydride reagents results in sequential electron transfers to the conjugated π-system of the ligand rather than to the metal center. [ABSTRACT FROM AUTHOR]

A general protocol for the structural characterization of paramagnetic molecular solids using solid-state NMR is provided and illustrated by the characterization of a high-spin FeII catalyst precursor. We show how good NMR performance can be obtained on a molecular powder sample at natural abundance by using very fast (>30 kHz) magic angle spinning (MAS), even though the individual NMR resonances have highly anisotropic shifts and very short relaxation times. The results include the optimization of broadband heteronuclear (proton-carbon) recoupling sequences for polarization transfer; the observation of single or multiple quantum correlation spectra between coupled spins as a tool for removing the inhomogeneous bulk magnetic susceptibility (BMS) broadening; and the combination of NMR experiments and density functional theory calculations, to yield assignments. [ABSTRACT FROM AUTHOR]