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The homonuclear radio-frequency driven recoupling (RFDR) experiment is commonly used in solid-state NMR spectroscopy to gain insight into the structure of biological samples due to its ease of implementation, stability towards fluctuations/missetting of radio-frequency (rf) field strength, and in general low rf requirements. A theoretical operator-based Floquet description is presented to appreciate the effect of having a temporal displacement of the π-pulses in the RFDR experiment. From this description, we demonstrate improved transfer efficiency for the RFDR experiment by generating an adiabatic passage through the zero-quantum recoupling condition. We have compared the performances of RFDR and the improved sequence to mediate efficient 13CO to 13Cα polarization transfer for uniformly 13C,15N-labeled glycine and for the fibril forming peptide SNNFGAILSS (one-letter amino acid codes) uniformly 13C,15N-labeled at the FGAIL residues. Using numerically optimized sweeps, we get experimental gains of approximately 20% for glycine where numerical simulations predict an improvement of 25% relative to the standard implementation. For the fibril forming peptide, using the same sweep parameters as found for glycine, we have gains in the order of 10%-20% depending on the spectral regions of interest. [ABSTRACT FROM AUTHOR]

A novel strategy for heteronuclear dipolar decoupling in magic-angle spinning solid-state nuclear magnetic resonance (NMR) spectroscopy is presented, which eliminates residual static high-order terms in the effective Hamiltonian originating from interactions between oscillating dipolar and anisotropic shielding tensors. The method, called refocused continuous-wave (rCW) decoupling, is systematically established by interleaving continuous wave decoupling with appropriately inserted rotor-synchronized high-power π refocusing pulses of alternating phases. The effect of the refocusing pulses in eliminating residual effects from dipolar coupling in heteronuclear spin systems is rationalized by effective Hamiltonian calculations to third order. In some variants the π pulse refocusing is supplemented by insertion of rotor-synchronized π/2 purging pulses to further reduce the residual dipolar coupling effects. Five different rCW decoupling sequences are presented and their performance is compared to state-of-the-art decoupling methods. The rCW decoupling sequences benefit from extreme broadbandedness, tolerance towards rf inhomogeneity, and improved potential for decoupling at relatively low average rf field strengths. In numerical simulations, the rCW schemes clearly reveal superior characteristics relative to the best decoupling schemes presented so far, which we to some extent also are capable of demonstrating experimentally. A major advantage of the rCW decoupling methods is that they are easy to set up and optimize experimentally. [ABSTRACT FROM AUTHOR]

Strongly enhanced spin polarization in the form of longitudinal spin order can be generated on target molecules by using parahydrogen in a catalyzed hydrogenation reaction. An optimal control algorithm was used to generate radiofrequency pulse sequences which convert the arising longitudinal two-spin order into single-spin Zeeman order with high efficiency and distribute it evenly between three coupled spins within the same molecule. The pulses are designed to be very robust towards variations in the B0 and B1 fields. Furthermore, this strategy is applied to enhance the NMR signal in an ultrafast gradient assisted single excitation two-dimensional spectroscopy experiment. [ABSTRACT FROM AUTHOR]

The past decade has demonstrated increasing interests in using optimal control based methods within coherent quantum controllable systems. The versatility of such methods has been demonstrated with particular elegance within nuclear magnetic resonance (NMR) where natural separation between coherent and dissipative spin dynamics processes has enabled coherent quantum control over long periods of time to shape the experiment to almost ideal adoption to the spin system and external manipulations. This has led to new design principles as well as powerful new experimental methods within magnetic resonance imaging, liquid-state and solid-state NMR spectroscopy. For this development to continue and expand, it is crucially important to constantly improve the underlying numerical algorithms to provide numerical solutions which are optimally compatible with implementation on current instrumentation and at same time are numerically stable and offer fast monotonic convergence toward the target. Addressing such aims, we here present a smoothing monotonically convergent algorithm for pulse sequence design in magnetic resonance which with improved optimization stability lead to smooth pulse sequence easier to implement experimentally and potentially understand within the analytical framework of modern NMR spectroscopy. [ABSTRACT FROM AUTHOR]

Optimal control theory has recently been introduced to nuclear magnetic resonance (NMR) spectroscopy as a means to systematically design and optimize pulse sequences for liquid- and solid-state applications. This has so far primarily involved numerical optimization using gradient-based methods, which allow for the optimization of a large number of pulse sequence parameters in a concerted way to maximize the efficiency of transfer between given spin states or shape the nuclear spin Hamiltonian to a particular form, both within a given period of time. Using such tools, a variety of new pulse sequences with improved performance have been developed, and the NMR spin engineers have been challenged to consider alternative routes for analytical experiment design to meet similar performance. In addition, it has lead to increasing demands to the numerical procedures used in the optimization process in terms of computational speed and fast convergence. With the latter aspect in mind, here we introduce an alternative approach to numerical experiment design based on the Krotov formulation of optimal control theory. For practical reasons, the overall radio frequency power delivered to the sample should be minimized to facilitate experimental implementation and avoid excessive sample heating. The presented algorithm makes explicit use of this requirement and iteratively solves the stationary conditions making sure that the maximum of the objective is reached. It is shown that this method is faster per iteration and takes different paths within a control space than gradient-based methods. In the present work, the Krotov approach is demonstrated by the optimization of NMR and dynamic nuclear polarization experiments for various spin systems and using different constraints with respect to radio frequency and microwave power consumption. [ABSTRACT FROM AUTHOR]

The efficiency of dipole-dipole coupling driven coherence transfer experiments in solid-state nuclear magnetic resonance (NMR) spectroscopy of powder samples is limited by dispersion of the orientation of the internuclear vectors relative to the external magnetic field. Here we introduce general design principles and resulting pulse sequences that approach full polarization transfer efficiency for all crystallite orientations in a powder in magic-angle-spinning experiments. The methods compensate for the defocusing of coherence due to orientation dependent dipolar coupling interactions and inhomogeneous radio-frequency fields. The compensation scheme is very simple to implement as a scaffold (comb) of compensating pulses in which the pulse sequence to be improved may be inserted. The degree of compensation can be adjusted and should be balanced as a compromise between efficiency and length of the overall pulse sequence. We show by numerical and experimental data that the presented compensation protocol significantly improves the efficiency of known dipolar recoupling solid-state NMR experiments. [ABSTRACT FROM AUTHOR]

A novel type of solid-state nuclear magnetic resonance experiment for efficient transfer of coherence between different nuclear spin I=1/2 species under magic-angle spinning conditions is introduced. The method combines the attractive features of γ-encoded dipolar recoupling [Nielsen et al., J. Chem. Phys. 101, 1805 (1995)] with coherence transfer mediated by a longitudinal spin-order operator in a transferred echo experiment. Using two-channel rotary resonance recoupling with different phase and amplitude modulation schemes, the transferred echo sequence can be tuned to achieve dipolar recoupling and coherence transfer over a well-defined range of chemical shifts while keeping the ratio between the rf field strength and the sample spinning frequency relatively low. The method, referred to as gamma-encoded transfer echo, is described analytically, by numerical simulations for various different spin systems, and experimentally by [sup 15]N to [sup 13]C coherence transfers in a powder sample of [sup 13]C, [sup 15]N-labeled glycine. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Composite spin-state-selective rotations are introduced. They are devised to remedy the following problem: When determining scalar plus residual dipolar couplings by conventional NMR experiments in partially aligned molecules, a considerable variation in the effective coupling constant often entails systematic errors compromising the accuracy of the measurements. The concept of composite pulses is extended in order to design spin-state-selective composite rotations (S[sup 3]CR) robust to variation in effective coupling constants. With the S[sup 3]CR approach the broadband performance of spin-state-selective excitation improves significantly as is demonstrated both numerically and experimentally. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

1H-detected magic-angle spinning NMR experiments facilitate structural biology of solid proteins, which requires using deuterated proteins. However, often amide protons cannot be back-exchanged sufficiently, because of a possible lack of solvent exposure. For such systems, using 2H excitation instead of 1H excitation can be beneficial because of the larger abundance and shorter longitudinal relaxation time, T1, of deuterium. A new structure determination approach, 'quadruple-resonance NMR spectroscopy', is presented which relies on an efficient 2H-excitation and 2H-13C cross-polarization (CP) step, combined with 1H detection. We show that by using 2H-excited experiments better sensitivity is possible on an SH3 sample recrystallized from 30 % H2O. For a membrane protein, the ABC transporter ArtMP in native lipid bilayers, different sets of signals can be observed from different initial polarization pathways, which can be evaluated further to extract structural properties. [ABSTRACT FROM AUTHOR]

We present mobile, low-field 17O NMR as a means for monitoring oxygen in liquids. Whereas oxygen is one of the most important elements, oxygen NMR is limited by a poor sensitivity related to low natural abundance and gyro-magnetic ratio of the NMR active 17O isotope. Here, we demonstrate 17O NMR detection at a Larmor frequency of 8.74 MHz in a 1.5-T Halbach neodymium magnet with a home-built digital NMR instrument suitable for large-scale production and in-line monitoring applications. The proposed 17O NMR sensor may be applied for direct, noninvasive measurements of water content in, for example, oil, manure, or food in automated quality or process control. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: Efficient sampling of signals is a key issue for multiple-dimensional NMR experiments to establish the best ratio between experiment time and spectral quality. Focussing on the most widely used sampling strategy using standard rectangular sampling and data analysis by Fourier transformation, a central question is concerned with determining the optimal maximum sampling time in the individual dimensions. The spectral resolution depends directly on this choice, as do the overall experiment times when addressing the indirect dimensions. We present a theoretical, numerical, and experimental analysis of the sampling space problem and propose approaches to efficient sampling for typical cases. [Copyright &y& Elsevier]

Abstract: The structure of the chlorosome baseplate protein CsmA from Chlorobium tepidum in a 1:1 chloroform:methanol solution was determined using liquid-state NMR spectroscopy. The data reveal that the 59-residue protein is predominantly α-helical with a long helical domain extending from residues V6 to L36, containing a putative bacteriochlorophyll a binding domain, and a short helix in the C-terminal part extending from residues M41 to G49. These elements are compatible with a model of CsmA having the long N-terminal α-helical stretch immersed into the lipid monolayer confining the chlorosome and the short C-terminal helix protruding outwards, thus available for interaction with the Fenna–Matthews–Olson antenna protein. [Copyright &y& Elsevier]

Abstract: The structure of a synthetic peptide corresponding to the fifth membrane-spanning segment (M5) in Na+,K+-ATPase in sodium dodecyl sulfate (SDS) micelles was determined using liquid-state nuclear magnetic resonance (NMR) spectroscopy. The spectra reveal that this peptide is substantially less α-helical than the corresponding M5 peptide of Ca2+-ATPase. A well-defined α-helix is shown in the C-terminal half of the peptide. Apart from a short helical stretch at the N-terminus, the N-terminal half contains a non-helical region with two proline residues and sequence similarity to a non-structured transmembrane element of the Ca2+-ATPase. Furthermore, this region spans the residues implicated in Na+ and K+ transport, where they are likely to offer the flexibility needed to coordinate Na+ as well as K+ during active transport. [Copyright &y& Elsevier]

The fact that membrane proteins are notoriously difficult to analyse using standard protocols for atomic-resolution structure determination methods have motivated adaptation of these techniques to membrane protein studies as well as development of new technologies. With this motivation, liquid-state nuclear magnetic resonance (NMR) has recently been used with success for studies of peptides and membrane proteins in detergent micelles, and solid-state NMR has undergone a tremendous evolution towards characterization of membrane proteins in native membrane and oriented phospholipid bilayers. In this mini-review, we describe some of the technological challenges behind these efforts and provide examples on their use in membrane biology. [ABSTRACT FROM AUTHOR]

Two recent X-ray structures have tremendously increased the understanding of the sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) and related proteins. Both structures show the fifth transmembrane span (M5) as a single continuous α-helix. The inherent structural and dynamic features of this span (Lys758–Glu785) were studied in isolation in sodium dodecyl sulfate (SDS) micelles using liquid-state nuclear magnetic resonance (NMR) spectroscopy. We find that a flexible region (Ile765–Asn768) is interrupting the α-helix. The location of the flexible region near the Ca2+ binding residues Asn768 and Glu771 suggests that together with a similar region in M6 it has a hinge function that may be important for cooperative Ca2+ binding and occlusion. [Copyright &y& Elsevier]

A new approach to heteronuclear dipolar recoupling in solid-state NMR which combines symmetry-based, γ-encoded dipolar recoupling with off-resonance rf irradiation is described. By irradiating the two spin species with offsets of equal magnitude but opposite sign, the dipolar scaling factor may be increased while using lower rf field strengths than under on-resonance conditions. Larger scaling makes the experiment less susceptible to relaxation, while released rf field requirements facilitate combination with fast magic-angle spinning while maintaining efficient 1H decoupling. Furthermore, homonuclear dipolar couplings may be suppressed by adjusting the off-resonance irradiation to the Lee–Goldburg condition. [Copyright &y& Elsevier]

The quadrupole Carr-Purcell-Meiboom-Gill NMR experiment using magic-angle spinning (QCPMG-MAS) is analysed as a means of determining quadrupolar coupling and anisotropic chemical shielding tensors for half-integer I > 1/2 quadrupolar nuclei with large quadrupole coupling constants C . This is accomplished by numerical simulations and 87Rb NMR experiments wih Rb SO and Rb CrO using different magnetic fields. It is demonstrated that (i) QCPMG-MAS experiments typically provide a sensitivity gain by more than an order of magnitude relative to quadrupolar-echo MAS experiments, (ii) non-secular secondorder terms do not affect the spin evolution appreciably, and (iii) the effect of finite RF pulses needs to be considered when 2omega2Q/omega0omega RF > 0.1, where omegaQ = 2piC Q/4I2I1I - 1, omegaRF is the RF amplitude, and omega0 the Larmor frequency. Using numerical simulations and iterative fitting the magnitudes and relative orientation of 87Rb quadrupolar coupling and chemical shielding tensors for Rb2SO4 and Rb2CrO4 have been determined. [ABSTRACT FROM AUTHOR]

Abstract: A novel procedure for reconstruction of 2D separated-local-field (SLF) NMR spectra from projections of 1D NMR data is presented. The technique, dubbed SLF projection reconstruction from one-dimensional spectra (SLF-PRODI), is particularly useful for uniaxially oriented membrane protein samples and represents a fast and robust alternative to the popular PISEMA experiment which correlates 1H–15N dipole–dipole couplings with 15N chemical shifts. The different 1D projections in the SLF-PRODI experiment are obtained from 1D spectra recorded under influence of homonuclear decoupling sequences with different scaling factors for the heteronuclear dipolar couplings. We demonstrate experimentally and numerically that as few as 2–4 1D projections will normally be sufficient to reconstruct a 2D SLF-PRODI spectrum with a quality resembling typical PISEMA spectra, leading to significant reduction of the acquisition time. [Copyright &y& Elsevier]

A mobile, low-field nuclear magnetic resonance (NMR) sensor for onboard, inline detection of catalytic fines in fuel oil in the shipping industry is presented as an alternative to onshore laboratory measurements. Catalytic fines (called cat fines) are aluminosilicate zeolite catalysts utilized in the oil cracking process at refineries. When present in fuel oil, cat fines cause abrasive wear of engine parts and may ultimately lead to engine breakdown with large economical consequences, thereby motivating methods for inline measurements. Here, we report on a robust, mobile, and low-cost 27Al NMR sensor for continuous online measurement of the level of catalytic fines in fuel oil onboard ships. The sensor enables accurate measurements of aluminum (catalytic fines) in ppm concentrations in good agreement with commercial laboratory reference measurements. [ABSTRACT FROM AUTHOR]

Abstract: We present a novel sampling strategy, interleaving acquisition of multiple NMR spectra by exploiting initial polarization subsequently from 1H and 2H spins, taking advantage of their different T 1 relaxation times. Different 1H- and 2H-polarization based spectra are in this way simultaneously recorded improving either information content or sensitivity by adding spectra. The so-called Relaxation-optimized Acquisition of Proton Interleaved with Deuterium (RAPID) 1H→ 13C/2H→ 13C CP/MAS multiple-acquisition method is demonstrated by 1D and 2D experiments using a uniformly 2H, 15N,13C-labeled α-spectrin SH3 domain sample with all or 30% back-exchanged labile 2H to 1H. It is demonstrated how 1D 13C CP/MAS or 2D 13C–13C correlation spectra initialized with polarization from either 1H or 2H may be recorded simultaneously with flexibility to be added or used individually for spectral editing. It is also shown how 2D 13C–13C correlation spectra may be recorded interleaved with 2H–13C correlation spectra to obtain 13C–13C correlations along with information about dynamics from 2H sideband patterns. [Copyright &y& Elsevier]

Abstract: The folding and stabilization of α-helical transmembrane proteins are still not well understood. Following cofactor binding to a membrane protein provides a convenient method to monitor the formation of appropriate native structures. We have analyzed the assembly and stability of the transmembrane cytochrome b 559′, which can be efficiently assembled in vitro from a heme-binding PsbF homo-dimer by combining free heme with the apo-cytochrome b 559′. Unfolding of the protein dissolved in the mild detergent dodecyl maltoside may be induced by addition of SDS, which at high concentrations leads to dimer dissociation. Surprisingly, absorption spectroscopy reveals that heme binding and cytochrome formation at pH 8.0 are optimal at intermediate SDS concentrations. Stopped-flow kinetics revealed that genuine conformational changes are involved in heme binding at these SDS concentrations. GPS (Global Protein folding State mapping) NMR measurements showed that optimal heme binding is intimately related to a change in the degree of histidine protonation. In the absence of SDS, the pH curve for heme binding is bell-shaped with an optimum at around pH 6–7. At alkaline pH values, the negative electrostatic potential of SDS lowers the local pH sufficiently to restore efficient heme binding, provided the amount of SDS needed for this does not denature the protein. Accordingly, the higher the pH value above 6–7, the more SDS is needed to improve heme binding, and this competes with the inherent tendency of SDS to dissociate cytochrome b 559′. Our work highlights that, in addition to its denaturing properties, SDS can affect protein functions by lowering the local pH. [Copyright &y& Elsevier]

Aggregation of the Amyloid β peptide into amyloid fibrils is closely related to development of Alzheimer's disease. Many small aromatic compounds have been found to act as inhibitors of fibril formation, and have inspired the search for new drug candidates. However, the detailed mechanisms of inhibition are largely unknown. In this study, we have examined in detail the binding of the fibril-formation inhibitor Congo Red (CR) to monomeric Aβ1−40 using a combination of 1D, 2D, saturation transfer difference, and diffusion NMR, as well as dynamic light scattering experiments. Our results show that CR binds to the fibril forming stretches of Aβ1−40 monomers, and that complex formation occurs in two steps: An initial 1:1 CR:Aβ1−40 complex is formed by a relatively strong interaction (Kd ≈ 5 μM), and a 2:1 complex is formed by binding another CR molecule in a subsequent weaker binding step (Kd ≈ 300 μM). The size of these complexes is comparable to that of Aβ1−40 alone. The existence of two different complexes might explain the contradictory reports regarding the inhibitory effects of CR on the fibril-formation process. [ABSTRACT FROM AUTHOR]

Abstract: Aimed at selective detection of 23Na with specific quadrupolar couplings for in vitro NMR and MRI, we present a series of quadrupolar binomial pulse sequences offering high specificity with respect to the quadrupolar couplings of the excited species. It is demonstrated that pulse sequences with an increasing number of elements, e.g., 11, 121, 1331, 14641, and 15101051, with the units representing flip angles smaller than the 90° pulses typically encountered in binomial spin-1/2 solvent suppression experiments, and different phase combinations may provide a high degree of flexibility with respect to quadrupolar coupling selectivity and robustness towards rf inhomogeneity. This may facilitate efficient separation of, for example, intra and extracellular 23Na in tissues with efficient control of the excitation (or suppression) of central as well as satellite transitions through on- and off-resonance irradiation. The pulse sequences are described in terms of their analogy to binomial liquid-state NMR solvent suppression experiments and demonstrated numerically and experimentally through NMR and MRI experiments on a 7T horizontal small-bore animal magnet system. [ABSTRACT FROM AUTHOR]

Abstract: Many small cationic peptides, which are unstructured in aqueous solution, have antimicrobial properties. These properties are assumed to be linked to their ability to permeabilize bacterial membranes, accompanied by the transition to an α-helical folding state. Here we show that there is no direct link between folding of the antimicrobial peptide Novicidin (Nc) and its membrane permeabilization. N-terminal acylation with C8–C16 alkyl chains and the inclusion of anionic lipids both increase Nc''s ability to form α-helical structure in the presence of vesicles. Nevertheless, both acylation and anionic lipids reduce the extent of permeabilization of these vesicles and lead to slower permeabilization kinetics. Furthermore, acylation significantly decreases antimicrobial activity. Although acyl chains of increasing length also increase the tendency of the peptides to aggregate in solution, this cannot rationalize our results since permeabilization and antimicrobial activities are observed well below concentrations where aggregation occurs. This suggests that significant induction of α-helical structure is not a prerequisite for membrane perturbation in this class of antimicrobial peptides. Our data suggests that for Nc, induction of α-helical structure may inhibit rather than facilitate membrane disruption, and that a more peripheral interaction may be the most efficient permeabilization mechanism. Furthermore, acylation leads to a deeper embedding in the membrane, which could lead to an anti-permeabilizing “plugging” effect. [Copyright &y& Elsevier]

We present a new method to obtain information about the conformational dynamics of membrane- proteins using solid-state NMR experiments of oriented samples. By measuring the orientation-dependent 1H-15N dipole-dipole coupling, 15N anisotropic chemical shift, and 2H quadrupole coupling parameters for a single residue, it is possible to obtain information about the local dynamics of each residue in the protein. This may be interpreted on an individual basis or through models extended to study conformational motion of membrane-protein segments. The method is demonstrated for the antimicrobial peptaibol alamethicin for which combined analysis of anisotropic interactions for the Aib8 residue provides detailed information about helix-tilt angle, wobbling, and oscillatory rotation around the helix axis in the membrane bound state. This information is in very good agreement with coarse-grained MD simulations of the peptide in lipid bilayers. [ABSTRACT FROM AUTHOR]

The article demonstrates a new method that may significantly improve the resolution in solid-state nuclear magnetic resonance of oriented membrane proteins by anisotropic differential linebroadening. It demonstrates that significant improvement in the resolution of spectra for inhomogeneously disordered oriented samples may be obtained by recording spectra with homonuclear proton decoupling instead of conventional heteronuclear proton decoupling.

The article focuses on improved excitation schemes for multiple-quantum magic-angle spinning for quadrupolar nuclei designed using optimal control (OC) theory. OC theory is an extremely powerful method for optimizing systems with many degrees of freedom. Addressing nuclear magnetic resonance (NMR), recent work has demonstrated the capabilities of OC to direct the nuclear spin coherences between specific initial and desired target states with improved efficiency. This has led to a variety of new liquid-state NMR experiments providing higher sensitivity or robustness toward, for example, resonance offset. The result of the OC iteration is a pulse waveform containing a cascade of short pulses of variable phase and amplitude.