EFFECT OF MOLECULAR VIBRATIONAL MOTION ON NMR PARAMETERS IN ZWITERANION LAlanine-LAlanine

V. Sychrovský, P. Bouř, V. Špirko, J. Šebestík, M. Buděšínský

Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, 166 10 Praha

Calculation of NMR shifts as well as spin-spin coupling constants [1] was performed for the zwiter-anionic form of dipeptide LAla-LAla including the effect of water solvent. Experiment was done for LAla-LAla molecule labelled by ¹³C, ¹⁵N isotopes, what enables the complete NMR spectra assignment. The NMR parameters were calculated for different molecular geometries of a di-peptide in order to systematically sample the conformational space corresponding to the mutual rotation of both amino acids along the peptide bond. The analysis of surfaces (i.e. NMR properties calculated on the grid of smoothly varying geometry parameters) of calculated NMR parameters revealed their distinct partitioning according to their character. The single-dimensional character of NMR parameter is attributed to the autonomous behaviour within single amino acid unit, while for the two-dimensional NMR parameters the molecular motion of individual amino acids are coupled. The vibration correction for each NMR parameter was calculated as an average value of the parameter over the anharmonic vibrational molecular motion. Prediction of molecular geometry from the calculated values of NMR parameters is discussed.

[1] Sychrovsky V., Grafenstein J., Cremer D.; J.Chem. Phys. 113(9), 3530-3547, 2000.