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Theoretical calculations of 17 O and 14 N nuclear quadrupole coupling (NQC) constants (χ) and asymmetry parameters (η) for small R-helix and β-sheet protein fragments have been carried out using the density functional theory. This computational study is intended to shed light on the differences between the two major structural elements found in the secondary structure of proteins. Specific NQDR spectra are computationally simulated for the 17 O and 14 N nuclei inherent in protein backbones. The separate signals resulting from R-helices and β-sheet models are predicted to be experimentally distinguishable for 17 O but not for 14 N. In particular, we predict that the differences in χ (in MHz) between R-helix and β-sheet proteins in solution are ∆χ( 17 O) ) 0.53(15) and ∆χ( 14 N) ) 0.14( 16), with the standard deviations in parentheses. It is found that 17 O NQC parameters of proteins are dependent on the particular conformation of the backbone, specifically on the hydrogen bond angle θ ) ∠H-N‚‚‚O and the backbone dihedral angle ψ ) ∠NC-C(O)N. Due to this, 17 O NQC parameters are observably different in R-helices and β-sheets. Conversely, 17 O NQC parameters are not dependent on the length of the hydrogen bond R O‚‚‚N , as had been previously thought, nor are they dependent on either the hydrogen bond dihedral angle ξ ) ∠N-CdO‚‚‚H or the backbone dihedral angle φ ) ∠C-(O)C-NC(O). We also conclude that, unlike 17 O NQC parameters, 14 N NQC parameters of proteins are within the uncertainties identical for both R-helices and β-sheets. Finally, differing residues on protein side chains do not significantly affect the NQC parameters of the backbone CdO and NH groups, and can be modeled computationally by using glycine.

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