AN AB-INITIO QUANTUM-CHEMICAL INVESTIGATION OF C-13 NMR SHIELDING TENSORS IN GLYCINE, ALANINE, VALINE, ISOLEUCINE, SERINE, AND THREONINE - COMPARISONS BETWEEN HELICAL AND SHEET TENSORS, AND THE EFFECTS OF (X)(1) ON SHIELDING

The carbon-13 nuclear magnetic resonance shielding surfaces for the is otropic and anisotropic shielding components, sigma(11), sigma(22), an d sigma(33), for C-alpha in N-formylglycine amide, and C-alpha and C-b eta in N-formylalanine amide, N-formylvaline amide (chi(1) = 180 degre es, -60 degrees, 60 degrees), N-formylisoleucine amide (all chi(1) = - 60 degrees conformers), N-formyl serine amide (chi(1) = 74.3 degrees), and N-formylthreonine amide (chi(1) = 180 degrees, -60 degrees, 60 de grees) have been computed at the Hartree-Fock level by using large, lo cally dense basis sets. The results for C-alpha in glycine and alanine show the expected similar to 4-5 ppm increase in isotropic shielding of sheet over helical geometries, and the overall breadths of the shie lding tensors are very similar for both helical and sheet fragments (\ sigma(33) - sigma(11)\ similar to 31-37 ppm). However, for each of the C-beta substituted amino acids (valine, isoleucine, serine, and threo nine) our results for C-alpha indicate not only the expected similar t o 4-5 ppm increase in shielding of sheet fragments over helical ones b ut also a large increase in the overall tensor breadths for sheet resi dues over helical ones, and a change in tensor orientation. On average ; the sheet residues have \sigma(33) - sigma(11)\ similar to 34 ppm, w hile on average the helical value is only similar to 22 ppm. For each C-beta substituted amino acid, the results for C-alpha also show that \sigma(22) - sigma(11)\(sheet) >> \sigma(22) - sigma(11)\(helix). For C-beta, the helical and sheet tensor breadths are in general much more similar for a given amino acid, although the actual magnitudes vary w idely from one amino acid to another. Since the individual C-alpha ten sor elements, sigma(11), sigma(22), and sigma(33), are all quite sensi tive to not only the backbone torsion angles, phi, psi, but also to th e side chain torsion angle, chi(1), as well, these results suggest tha t it will in many instances be possible to deduce both backbone (phi,p si) and side chain (chi(1)) torsion angles from an experimental determ ination of the three principal elements of the C-13(alpha) shielding t ensor, results which can be confirmed in some cases with data on C-bet a (and C-gamma). Such an approach, based on quantum chemical calculati ons, should be useful in determining the structures of both crystallin e, noncrystalline, and potentially even soluble peptides and proteins, as well as in refining their structures, using shielding tensor eleme nts.