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O- and S-heterocyclic carbenes (OHCs, SHCs) are shown experimentally and computationally to be stronger pi acceptors than NHCs and lack, of course, substituents at the heteroatoms. These different electronic and steric characteristics make OHCs and SHCs interesting ligands for coordination chemistry. Convenient synthetic routes are presented to access their iridium(I), iridium(III), and coinage-metal(I) (Cu, Ag, Au) complexes in good yields by means of dissociation of olefins, deprotonation of precursor salts, and transmetalation from a silver carbene complex Molecular structures and detailed bonding analyses of these complexes are presented.

Some examples of atropoisomeric pseudorotaxanes in which the isomerism arises by the different conformations adopted by the wheel are reported here. Upon threading hexahexyloxycalix[6]arene 1 with ammonium axles 2+ or 3+, bearing biphenyl or trifluoromethylbenzyl moieties, respectively, two atropoisomeric pseudorotaxanes were formed in which the calix[6]-wheel 1 adopts the 1,2,3-alternate and cone conformations. The interconversion between them cannot be obtained by simple rotation around the ArCH2Ar bonds of the calixarene wheel, which is blocked by the presence of the axle inside its cavity. Therefore, it can only be obtained through a mechanism of de-threading/re-threading of the axle. In all the examined cases, the 1,2,3-alternate and cone atropoisomers are, respectively, the kinetic and the thermodynamic ones.

A completely unsymmetrical porous organic cage was synthesized from a C2v symmetrical building block that was identified by a computational screen. The cage was formed through a 12-fold imine condensation of a tritopic C2v symmetric trialdehyde with a di-topic C2 symmetric diamine in a [4+6] reaction. The cage was rigid and microporous, as predicted by the simulations, with an apparent Brunauer-Emmett-Teller surface area of 578 m2 g-1. The reduced symmetry of the tritopic building block relative to its topicity meant there were 36 possible structural isomers of the cage. Experimental characterization suggests a single isomer with 12 unique imine environments, but techniques such as NMR could not conclusively identify the isomer. Computational structural and electronic analysis of the possible isomers was used to identify the most likely candidates, and hence to construct a 3-dimensional model of the amorphous solid. The rational design of unsymmetrical cages using building blocks with reduced symmetry offers new possibilities in controlling the degree of crystallinity, porosity, and solubility, of self-assembled materials.

Nuclear magnetic shieldings have been calculated at the density functional theory (DFT) level for stacks of benzene, hexadehydro[12]annulene, dodecadehydro[18]annulene, and hexabenzocoronene. The magnetic shieldings due to the ring currents in the adjacent molecules have been estimated by calculating nucleus independent molecular shieldings for the monomer in the atomic positions of neighbor molecules. The calculations show that the independent shielding model works reasonably well for the H-1 NMR shieldings of benzene and hexadehydro[12]annulene, whereas for the larger molecules and for the C-13 NMR shieldings the interaction between the molecules leads to shielding effects that are at least of the same size as the ring current contributions from the adjacent molecules. A better agreement is obtained when the nearest neighbors are also considered at full quantum mechanical (QM) level. The calculations suggest that the nearest solvent molecules must be included in the quantum mechanical system, at least when estimating solvent shifts at the molecular mechanics (MM) level. Current density calculations show that the stacking does not significantly affect the ring current strengths of the individual molecules, whereas the shape of the ring current for a single molecule differs from that of the stacked molecules.

The periplasmic binding protein (PBP) FepB plays a key role in transporting the catecholate siderophore ferric enterobactin from the outer to the inner membrane in Gram-negative bacteria. The solution structures of the 34-kDa apo- and holo-FepB from Escherichia coli, solved by NMR, represent the first solution structures determined for the type III class of PBPs. Unlike type I and II PBPs, which undergo large "Venus flytrap" conformational changes upon ligand binding, both forms of FepB maintain similar overall folds; however, binding of the ligand is accompanied by significant loop movements. Reverse methyl cross-saturation experiments corroborated chemical shift perturbation results and uniquely defined the binding pocket for gallium enterobactin (GaEnt). NMR relaxation experiments indicated that a flexible loop (residues 225-250) adopted a more rigid and extended conformation upon ligand binding, which positioned residues for optimal interactions with the ligand and the cytoplasmic membrane ABC transporter (FepCD), respectively. In conclusion, this work highlights the pivotal role that structural dynamics plays in ligand binding and transporter interactions in type III PBPs.

Hybrid materials, with co-networks of organic and inorganic components, are increasing in popularity due to their tailorable degradation rates and mechanical properties. To increase mechanical stability, particularly in water, covalent bonding must occur between the components. This can be introduced using crosslinking agents such as 3-glycidoxypropyl trimethoxysilane (GPTMS). Attachment of GPTMS to polymers in aqueous conditions is hypothesized to occur by opening of the epoxide ring by nucleophiles on the polymer chain. Despite side reactions that occur between the epoxide ring of GPTMS and water, a range of NMR techniques showed that the carboxylic acid group of poly(γ-glutamic acid) reacted with GPTMS. This result was used to identify the amino acids in gelatin that reacted most rapidly with the GPTMS epoxide ring, confirming that covalent bonding occurred in gelatin–silica hybrid materials.

The General AMBER Force Field (GAFF) has been extended to describe a series of selenium and tellurium diphenyl dichalcogenides. These compounds, besides being eco-friendly catalysts for numerous oxidations in organic chemistry, display peroxidase activity, i.e., can reduce hydrogen peroxide and harmful organic hydroperoxides to water/alcohols and as such are very promising antioxidant drugs. The novel GAFF parameters are tested in MD simulations in different solvents and the (77)Se NMR chemical shift of diphenyl diselenide is computed using structures extracted from MD snapshots and found in nice agreement with the measured value in CDCl3. The whole computational protocol is described in detail and integrated with in-house code to allow easy derivation of the force field parameters for analogous compounds as well as for Se/Te organocompounds in general.

A quench-precipitated sample of highly syndiotactic polypropylene (s-PP) (with 86% fully syndiotactic pentads), which shows an X-ray spectrum near that calculated for an ideal orthorhombic C-centered structural model of packing as proposed by Corradini et al. (with chains in a helical (TTGG) n conformation), has been analyzed by high-resolution solid-state 13 C-NMR spectroscopy. The 13 C-NMR CP MAS spectra of this sample, in addition to the resonances already reported in the literature for s-PP (in the B-centered structure), exhibit additional peaks in the regions of the methyl (at 18.9 and 22.4 ppm) and methylene (at 44.9 ppm) groups. It is established that these additional resonances belong indeed to nuclei in well-defined conformational environments fixed in rigid structures placed at the interface of the crystallites and within the strained or disordered area of the crystallites themselves. In particular, the peak at 44.9 ppm in assigned to CH 2 carbon atoms in a TX.YG conformational environment, whereas the resonance at 18.9 ppm is assigned to CH 3 in trans-planar portions of the chain (γ-gauche effect). This indicates local disorder in the C-centered crystalline s-PP sample, due to the presence of conformationally disordered chains. Models of s-PP chains comprising defective but energetically feasible portions of chains in a trans-planar conformation (disturbing the (TTGG) n dominating conformation) are imagined as packed according to a C-pseudocentered structure. In order to keep the methyl groups in a crystalline register and the chains well interdigitated, the defects should be clustered on planes which cross 3D ordered portions in the crystals. The chemical shifts of the carbon atoms in the regions comprising the defect for model chains have been calculated on an ab initio level through the IGLO method, and the results are compared with the experiments. The calculations provide a good explanation of the peculiarities of the experimental 13 C-NMR spectra of the s-PP sample under study. Thus, detailed information on chain defects can be obtained by combining X-ray scattering, solid-state NMR, and computer simulations.

The shaping of a calix(7)ar- ene macrocycle into cone-like structure 3, through exhaustive alkylation of doubly bridged calix(7)arene derivative 2 with bulky groups, has been investi- gated. Conformational details about the structure adopted by calix(7)arene derivative 3 in solution have been ob- tained by using chemical shift surface maps, as previously reported by our group. Thus, chemical shift contour plots indicated that 3 adopted a cone- shaped structure in solution analogous to that adopted by the known p-tert-bu- tylcalix(7)arene heptacarboxylic acid derivative 4. Interestingly, the X-ray structure of derivative 3 showed a high degree of similarity to the theoretical structure, which confirmed the validity of the contour plots method. The pre- organized calix(7)arene host 3 showed interesting recognition abilities toward both organic and alkali cations. In fact, an unprecedented endo-cavity com- plexation of linear and branched alkyl ammonium cations with a larger cal- ix(7)arene host was evidenced. A com- parable affinity for branched tBuNH3 + and linear nBuNH3 + guests was ob

This work is funded by the EPSRC A series of eight 1-halo-8-(alkylchalcogeno)naphthalene derivatives ( 1 – 8 ; halogen=Br, I; alkylchalcogen=SEt, SPh, SePh, TePh) containing a halogen and a chalcogen atom occupying the peri positions have been prepared and fully characterised by using X-ray crystallography, multinuclear NMR spectroscopy, IR spectroscopy and MS. Naphthalene distortion due to non-covalent substituent interactions was studied as a function of the bulk of the interacting chalcogen atoms and the size and nature of the alkyl group attached to them. X-ray data for 1 , 2 , 4 and 5 – 8 were compared. Molecular structures were analysed in terms of naphthalene ring torsions, peri-atom displacement, splay angle magnitude, X⋅⋅⋅E interactions, aromatic ring orientations and quasi-linear X⋅⋅⋅EC arrangements. A general increase in the X⋅⋅⋅E distance was observed for molecules that contain bulkier atoms at the peri positions. The I⋅⋅⋅S distance of 4 is comparable with the I⋅⋅⋅Te distance of 8 , and is ascribed to a stronger lone pair–lone pair repulsion due to the presence of an axial S(naphthyl) ring conformation. Density functional theory (B3LYP) calculations performed on 5 – 8 revealed Wiberg bond index values of 0.05–0.08, which indicate minor interactions taking place between the non-bonded atoms in these compounds. Postprint

Detailed information about unfolded states is required to understand how proteins fold. Knowledge about folding intermediates formed subsequently is essential to get a grip on pathological aggregation phenomena. During folding of apoflavodoxin, which adopts the widely prevalent alpha-beta parallel topology, most molecules fold via an off-pathway folding intermediate with helical properties. To better understand why this species is formed, guanidine hydrochloride-unfolded apoflavodoxin is characterized at the residue level using heteronuclear NMR spectroscopy. In 6.0 M denaturant, the protein behaves as a random coil. In contrast, at 3.4 M denaturant, secondary shifts and (1)H-(15)N relaxation rates report four transiently ordered regions in unfolded apoflavodoxin. These regions have restricted flexibility on the (sub)nanosecond time scale. Secondary shifts show that three of these regions form alpha-helices, which are populated about 10% of the time, as confirmed by far-UV CD data. One region of unfolded apoflavodoxin adopts non-native structure. Of the alpha-helices observed, two are present in native apoflavodoxin as well. A substantial part of the third helix becomes beta-strand while forming native protein. Chemical shift changes due to amino acid residue replacement show that the latter alpha-helix has hydrophobic interactions with all other ordered regions in unfolded apoflavodoxin. Remarkably, these ordered segments dock non-natively, which causes strong competition with on-pathway folding. Thus, rather than directing productive folding, conformational preorganization in the unfolded state of an alpha-beta parallel-type protein promotes off-pathway species formation.