Toward direct determination of conformations of protein building units from multidimensional NMR experiments part II: A theoretical case study of Formyl-L-Valine amide

Chemical shielding anisotropy tensors have been determined for all twenty-s even characteristic conformers of For-L-Val-NH2 using the GIAO-RHF formalis m with the 6-31 + G* and TZ2P basis sets. The individual chemical shifts an d their conformational averages have been compared to their experimental co unterparts taken from the BioMagnetic Resonance Bank (BMRB). At the highest level of theory applied, for all nuclei but the amide proton, deviations b etween statistically averaged theoretical and experimental chemical shifts are as low as 1-3%. Correlated chemical shift plots of selected nuclei, as function of the respective phi, psi, chi (1), and chi (2) torsional angles, have been generated. On two-dimensional chemical shift-chemical shift plot s, for example,H-1(NH)-N-15(NH) and N-15(NH)-C-13(alpha), regions correspon ding to major conformational clusters have been identified, providing a bas is for the quantitative identification of conformers from NMR shift data. E xperimental NMR resonances of nuclei of valine residues have been deduced f rom 18 selected proteins, resulting in 93 H-1(alpha)-C-13(alpha) chemical s hift pairs. These experimental results have been compared to relevant ab in itio values revealing remarkable correlation between the two sets of data. Correlations of H-1(alpha) and C-13(alpha) values with backbone conformatio nal parameters (phi and psi) have also been found for all pairs (e.g. H-1(a lpha)/phi and C-13(alpha)/phi) but H-1(alpha)/psi. Overall, the appealing i dea of establishing backbone folding of proteins by employing chemical shif t information alone, obtained from selected multiple-pulse NMR experiments (e.g. 2D-HSQC, 2D-HMQC, and 3D-HNCA), has received further support.