| Abstract
| - Theoretical calculations of 17O and 14N nuclear quadrupole coupling (NQC) constants (χ) and asymmetryparameters (η) for small α-helix and β-sheet protein fragments have been carried out using the density functionaltheory. This computational study is intended to shed light on the differences between the two major structuralelements found in the secondary structure of proteins. Specific NQDR spectra are computationally simulatedfor the 17O and 14N nuclei inherent in protein backbones. The separate signals resulting from α-helices andβ-sheet models are predicted to be experimentally distinguishable for 17O but not for 14N. In particular, wepredict that the differences in χ (in MHz) between α-helix and β-sheet proteins in solution are Δχ(17O) =0.53(15) and Δχ(14N) = 0.14(16), with the standard deviations in parentheses. It is found that 17O NQCparameters of proteins are dependent on the particular conformation of the backbone, specifically on thehydrogen bond angle θ = ∠H−N···O and the backbone dihedral angle ψ = ∠NC−C(O)N. Due to this, 17ONQC parameters are observably different in α-helices and β-sheets. Conversely, 17O NQC parameters are notdependent on the length of the hydrogen bond RO···N, as had been previously thought, nor are they dependenton either the hydrogen bond dihedral angle ξ = ∠N−CO···H or the backbone dihedral angle φ = ∠C(O)C−NC(O). We also conclude that, unlike 17O NQC parameters, 14N NQC parameters of proteins arewithin the uncertainties identical for both α-helices and β-sheets. Finally, differing residues on protein sidechains do not significantly affect the NQC parameters of the backbone CO and NH groups, and can bemodeled computationally by using glycine.
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