INFLUENCE OF THE NONPLANARITY OF THE AMIDE MOIETY ON COMPUTED CHEMICAL-SHIFTS IN PEPTIDE ANALOGS - IS THE AMIDE NITROGEN PYRAMIDAL

The dependence of peptide NMR chemical shifts on the nonplanarity of the amide nitrogen environment has been studied ab initio. At both the GIAO-SCF and the correlated GIAO-MP2 levels, each 10 degrees deviation from 180 degrees of the H-t-N-C degrees-O dihedral angle of formamide deshields the carbonyl carbon (C degrees) by about 2 ppm. The NMR chemical shift of the amide nitrogen, however, remains almost constant when the H-t-N-C degrees-O dihedral angle is between 140 degrees and 180 degrees. An alpha-helical (1a) and a beta-pleated sheetlike structure (1b) of N-formylglycine amide (1) were optimized at different levels of theory. The RMP2-FC/6-31G*-, RHF/6-31G*-, and AM1-optimized geometries of the alpha-helical structure have an H-t-N-C degrees-O dihedral angle of about -160 degrees, and the calculated Delta delta C degrees deviates by only 1-3 ppm from the experimental range. However, Delta delta C degrees for the RHF/3-21G geometries, which have a planar amide moiety in both an alpha-helical and a beta-pleated sheetlike structure, deviates by about 8 ppm from the experimental range, Delta delta C degrees between an alpha-helical and a beta-pleated sheetlike structure was computed for N-acetyl-N-methylglycine amide (2) at the GIAO-SCF/6-311G*//AM1 level as a function of the H-t-N-C degrees-O dihedral angle. The experimental range (2-5 ppm) for Delta delta C-13 C degrees is reached when the dihedral angle is smaller than -160 degrees, whereas Delta delta C-13 C-alpha and Delta delta N-15 N-amide do not depend significantly on H-t-N-C degrees-O. The differences in chemical shifts of C degrees for the smaller N-formyl-N-methylglycine amide (3) computed with both the GIAO-SCF and GIAO-MP2 methods for alpha-helical and beta-pleated sheetlike conformations increase by 1.5 ppm for each 10 degrees deviation from 170 degrees of the H-t-N-C degrees-O dihedral angle. Semiempirical computations on a model alpha-helix consisting of eight glycine residues give a H-t-N-C degrees-O dihedral angle of -164 degrees in good agreement with the ab initio results for our dipeptide models.