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The ordering properties of an anisotropic liquid crystal can be studied by recording C13 NMR spectra at different temperatures for a number of rigid solutes. The traditional difficulty in analyzing C13 data comes from the scarcity of experimental information about the carbon shielding tensors, and from their limited transferability among different solutes. We show that these obstacles can be overcome by computing high level ab initio shielding tensors, also including the solvent effects by the polarizable continuum model. The reliability of this combined approach is carefully verified, and the order parameters of several solutes are obtained by reanalyzing previously published spectra. The quality of the results is shown to be comparable to deuterium NMR without the need of isotopic substitution.

NMR amide proton chemical shifts of the calcium complex with the bicyclic nonapeptide BCP2, cyclo-(Glu1–Ala2–Pro3–Gly4–Lys5–Ala6–Pro7–Gly8)-cyclo-(1γ→5e)Gly9 were calculated by means of Hartree–Fock and DFT methods within the GIAO framework. Calculated results yielded a good agreement with experimental data: computed chemical shifts well reproduced the strong changes induced by the presence of the metal. The nature of the metal–peptide interaction, as well as other non-local effects on magnetic shielding, were discussed. Further, different hybrid methods have been tested to set up an affordable approach to study magnetic properties in metal–protein complexes.

The conceptual and numerical problems involved in the computation of reliable NMR chemical shifts for molecules in condensed phases are analyzed with reference to a number of case studies ranging from aromatic compounds in low-polarity solvents to carbonyl and amidic models in aqueous solution and to large polypeptides. The results show that an integrated tool including the most recent density functionals, mixed discrete-continuum solvent models, hybrid QM/MM approaches and, when needed, averaging from molecular dynamics simulations are becoming an invaluable complement to experimental results. Copyright © 2004 John Wiley & Sons, Ltd.

The importance of vicinal and long-range interresidue effects in determining the stability of the collagen triple helix has been investigated by quantum mechanical (QM) and molecular mechanical (MM) computations on suitable model polypeptides, taking into account solvent effects by the polarizable continuum model (PCM). At the QM level, the PII conformation corresponds to an energy minimum for pentapeptide analogues incorporating the sequence Gly-Pro-Pro-Gly, irrespective of the down or up puckering of the pyrrolidine ring. However, our computations indicate that the alternation of down and up prolines characterizing collagen and collagen-like peptides is not due to an intrinsic preference of the Pro-Pro-Gly sequence. This result is confirmed by MM computations of longer polypeptides. Next, MM computations on model triple helices show that a better packing is obtained for specific values of backbone dihedrals, which, in turn, favor the alternation of down and up prolines along each chain.

High-level ab initio g and A tensor components have been calculated for PD-tempone and tempo-palmitate (TP) radical spin probes dissolved in n-pentyl and n-hexyl cyanobiphenyl liquid crystals. Solvent effects have been included in the proposed approach by means of the polarizable continuum model, allowing for solvent anisotropy. An in-depth analysis of the electronic structure of probes was performed to choose a suitable model for TP and make the calculations more accessible. Computed magnetic tensor components have been compared with corresponding values measured in the rigid limit. The quality of the results suggests the use of quantum-mechanical data to determine the order parameter of the nematic from experimental electron-spin resonance measurements.