Vol 49, No 6 (2018)

Comparison of the relative differences between the experimental and calculated values of proton–proton vicinal scalar constants obtained from nuclear magnetic resonance spectra of two structurally similar organic molecules makes it possible to increase the efficiency of using Karplus relationship for experimental detection and quantitative description of the conformational distortions in molecules under investigation. Advantages and disadvantages of the proposed approach are considered on the examples of the study of two pairs of rigid steroid molecules in solution where they are in a single conformation. These steroids have potential biological activity which is determined by their structural and conformational features. There is only one structural difference in each pair of compared steroids. The influence of small modifications such as substituent variation on the molecules spatial structure was investigated by the joint use of molecular optimization methods (semi-empirical and molecular mechanic) to determine the dihedral angles and Karplus-type equation of C.A.G. Haasnoot, F.A.A.M. de Leeuw and C.A. Altona to calculate vicinal constants in ethane fragments. It was shown that the usage of relative differences of experimental and calculated vicinal constants for conformational analysis of rigid molecules eliminates systematic errors of vicinal constant calculations. Such approach allows us to detect small distortions between conformations of comparable molecules with high accuracy, which are concluded in the differences of the corresponding dihedral angles within not more than 10°–15°. The proposed approach is a more sensitive way of studying small specific features of the spatial structure of molecules in comparison with the known methods on the basis of Karplus-type equation in which the absolute values of experimental and calculated vicinal proton–proton constants are compared.

Aggregation of decyltrimethylammonium bromide and cetyltrimethylammonium bromide (CTAB) in D₂O has been studied. Spin–lattice relaxation time and self-diffusion coefficient of surfactant molecules were measured at concentrations below and above surfactant critical micelle concentration. The aggregation properties of conventional surfactant, CTAB, examined by nuclear magnetic resonance (NMR) and molecular dynamic (MD) simulation, were compared with the properties of double-tail analog, N,N,N′,N′-tetramethyl-N,N′dihexadecyl-1,4-butan di-ammonium di-bromide (BCTA). Both NMR and computer simulation methods suggest that micellization is a stepwise process and the pre-micellar aggregates take place in a solution at concentration below critical micelle concentration. According to MD simulation Gemini surfactant, BCTA, forms worm-like micelles, whereas CTAB, which may be considered as its “monomer”, forms only elongated micelles.

Magic-angle sample spinning (MAS) nuclear magnetic resonance (NMR) measurements are treated for heterogeneous systems with nanometer dimensions. An appreciable line narrowing in the MAS NMR spectra of the embedded molecules may be achieved also in the cases when the molecules still possess an appreciable local mobility. It appears that the MAS frequencies are of comparable order of magnitude as the frequencies which characterize the random molecular motional processes and which compete with MAS. It will be shown that this behavior may occur if inhomogeneous local magnetic fields due to susceptibility effects have a dominating influence on the widths and shapes of the resonance NMR lines. Properties of these local fields are described. Spectra simulations are carried for molecules embedded in these heterogeneous systems when the coherent averaging by MAS is superimposed by random local motions. This situation may occur for molecules contained in nanoporous solids and also for heterogeneous systems like membranes and biological tissues with flexible components like water, lipids, and small peptides. Several examples are treated which reveal advantages and limitations of these experiments and their theoretical interpretation.