Side-chain 2H and backbone 15N relaxation data have been collected at multiple temperatures in the samples of the SH3 domain from alpha-spectrin. Combined analyses of the data allowed for determination of the temperature-dependent correlation times tauf characterizing fast methyl motion. Molecular dynamics simulations confirmed that tauf are dominated by methyl rotation; the corresponding activation energies approximate methyl rotation barriers. For 33 methyl groups in the alpha-spectrin SH3 domain the average barrier height was thus determined to be 2.8 +/- 0.9 kcal/mol. This value is deemed representative of the "fluid" hydrophobic protein core where some barriers are increased and others are lowered because of the contacts with surrounding atoms, but there is no local order that could produce systematically higher (lower) barriers. For comparison, the MD simulation predicts the average barrier of 3.1 kcal/mol (calculated via the potential of mean force) or 3.4-3.5 kcal/mol (rigid barriers after appropriate averaging over multiple MD snapshots). The latter result prompted us to investigate rigid methyl rotation barriers in a series of NMR structures from the Protein Databank. In most cases the barriers proved to be higher than expected, 4-6 kcal/mol. To a certain degree, this is caused by tight packing of the side chains in the NMR structures and stems from the structure calculation procedure where the coordinates are first annealed toward the temperature of 0 K and then subjected to energy minimization. In several cases the barriers >10 kcal/mol are indicative of van der Waals violations. The notable exceptions are (i) the structures solved using the GROMOS force field where tight methyl packing is avoided (3.0-3.6 kcal/mol) and (ii) the structure solved by means of the dynamic ensemble refinement method (Lindorff-Larsen, K.; Best, R. B.; DePristo, M. A.; Dobson, C. M.; Vendruscolo, M. Nature 2005, 433, 128) (3.5 kcal/mol). These results demonstrate that methyl rotation barriers, derived from the experiments that are traditionally associated with studies of protein dynamics, can be also used in the context of structural work. This is particularly interesting in view of the recent efforts to incorporate dynamics data in the process of protein structure elucidation.

Rapid advances in solid-state MAS NMR made it possible to probe protein dynamics on a per-residue basis, similar to solution experiments. In this work we compare methyl 2H relaxation rates measured in the solid and liquid samples of alpha-spectrin SH3 domain. The solution data are treated using a model-free approach to separate the contributions from the overall molecular tumbling and fast internal motion. The latter part forms the basis for comparison with the solid-state data. Although the accuracy of solid-state measurements is limited by deuterium spin diffusion, the results suggest a significant similarity between methyl dynamics in the two samples. This is a potentially important observation, preparing the ground for combined analysis of the dynamics data by solid- and solution-state NMR.

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Peculiarities of quadrupolar relaxation in electrolyte solutions were established via comparison of the data obtained from proton and deuteron resonances. It has been shown that quadrupole coupling constants (QCC) of deuterons depend not only on internal electron structure of molecule or ion, but on solution structure as well. To interpret the experimental results quantumchemical calculations of QCC of deuterons in different molecular complexes simulating different solution substructures were carried out. Density functional theory (DFT) method with hybrid B3LYP functional was used for all calculations.

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Characteristic features of the structure of Ca 2+ hydration shells were considered. The results of quantum chemical calculations were compared with experimental data obtained from the study of nuclear magnetic relaxation of deuterons in aqueous solutions of calcium salts. The influence of the basis set and computational procedure on the calculated 2 D quadrupole cou pling constants (QCC) in isolated water molecule was investigated. The 2 D QCC in molecular clusters (D 2 O) 5 and Ca 2+ (D 2 O) n (n =6, 8, 10, 18) were calculated using the B3LYP/6 31++G** density functional method.Key words: ab initio quantum chemical calculations, quantum chemical calculations, densi ty functional theory, B3LYP functional, molecular clusters, 2 D quadrupole coupling constants, NMR relaxation.Aqueous solutions of salts have been studied for long; however, their microstructure is not known with certainty. Nuclear magnetic relaxation measurements provide infor mation on the features of ion microstructure and ion hy dration in such molecular systems. 1 Among NMR relax ation processes, an important place is occupied by qua drupole coupling of nuclei with the spin I > 1/2. Its inten sity depends on the product of the nuclear electric quadru pole moment by the electric field gradient (EFG) at the nuclear site 1,2 called the quadrupole coupling constant (QCC). An independent determination of QCC permits more correct interpretation of experimental NMR relax ation data. Using quantum chemical methods, one can calculate the EFG tensors at the nuclear sites and, hence, the QCC of nuclei in various molecular complexes. How ever, the results of calculations depend on the computa tional method, on the basis set used to solve the electronic Schrödinger equation, and on the structure of the molecu lar clusters used to simulate the ion hydration shells. 3,4 Therefore, in the preliminary step one should thoroughly assess the influence of the above factors on the final out come by comparing the results of calculations with reli able experimental data.In this work, the local environment of an ion in aque ous electrolyte solutions is simulated by clusters contain ing different numbers of water molecules. The aim of this work is to elucidate the applicability of the cluster ap proach for the description of the structure of ion hydration shells and to calculate the 2 D QCC in these clusters using the density functional theory. Nuclear magnetic relaxation in aqueous electrolyte solutionsNMR relaxation is the process by which the equilibri um distribution of nuclei between the Zeeman energy states in an external magnetic field is established as a result of interaction between atomic nuclei with the environment. The type of interaction resulting in the equilibrium distri bution is called the relaxation channel (or mechanism). If there are several independent relaxation channels, the over all spin lattice relaxation rate can be expressed by the relation 1,2 ,(where T 1 k is the relaxation time for the kth channel. Most often, the condition for fast exchange ...

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