Correlation of 2J couplings with protein secondary structure

Geminal two-bond couplings (2J) in proteins were analyzed in terms of correlation with protein secondary structure. NMR coupling constants measured and evaluated for a total six proteins comprise 3999 values of 2JCaN', 2JC'HN, 2JHNCa, 2JC'Ca, 2JHaC', 2JHaCa, 2JCbC', 2JN'Ha, 2JN'Cb, and 2JN'C', encompassing an aggregate 969 amino-acid residues. A seamless chain of pattern comparisons across the spectrum datasets recorded allowed the absolute signs of all 2J coupling constants studied to be retrieved. Grouped by their mediating nucleus, C’, N’ or Ca, 2J couplings related to C’ and N’ depend significantly on f,y torsion-angle combinations. beta turn types I, I’, II and II’, especially, can be distinguished on the basis of relative-value patterns of 2JCaN', 2JHNCa, 2JC'HN, and 2JHaC'. These coupling types also depend on planar or tetrahedral bond angles, whereas such dependences seem insignificant for other types. 2JHaCb appears to depend on amino-acid type only, showing negligible correlation with torsion-angle geometry. Owing to its unusual properties, 2JCaN' can be considered a ’one-bond’ rather than two-bond interaction, the allylic analog of 1JN'Ca, as it were. Of all protein J coupling types, 2JCaN' exhibits the strongest dependence on molecular conformation, and among the 2J types, 2JHNCa comes second in terms of significance yet was hitherto barely attended to in protein structure work.