Quantum chemical study on oxygen-17 and nitrogen-14 nuclear quadrupole coupling parameters of peptide bonds in alpha-helix and beta-sheet proteins

Theoretical calculations of O-17 and N-14 nuclear quadrupole coupling (NQC) constants (chi) and asymmetry parameters (eta) for small alpha -helix and beta -sheet protein fragments have been carried out using the density funct ional theory. This computational study is intended to shed light on the dif ferences between the two major structural elements found in the secondary s tructure of proteins. Specific NQDR spectra are computationally simulated f or the O-17 and N-14 nuclei inherent in protein backbones. The separate sig nals resulting from alpha -helices and beta -sheet models are predicted to be experimentally distinguishable for O-17 but not for N-14. In particular, we predict that the differences in X tin MHz) between alpha -helix and bet a -sheet proteins in solution are Delta chi(O-17) = 0.53(15) and Delta chi( N-14) = 0.14(16), with the standard deviations in parentheses. It is found that O-17 NQC parameters of proteins are dependent on the particular confor mation of the backbone, specifically on the hydrogen bond angle theta = ang leH-N . . .O and the backbone dihedral angle psi = angle NC-C(O)N. Due to t his, O-17 NQC parameters are observably different in alpha -helices and bet a -sheets. Conversely, O-17 NQC parameters are not dependent on the length of the hydrogen bond R-O . . .N, as had been previously thought, nor are th ey dependent on either the hydrogen bond dihedral angle xi = LN-C=O . . .H or the backbone dihedral angle phi = LC (O)C-NC(O). We also conclude that, unlike O-17 NQC parameters, N-14 NQC parameters of proteins are within the uncertainties identical for both alpha -helices and beta -sheets. Finally, differing residues on protein side chains do not significantly affect the N QC parameters of the backbone C=O and NH groups, and can be modeled computa tionally by using glycine.