Correlation of (2)J couplings with protein secondary structure

Geminal two-bond couplings ((2)J) 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 (2)J(CalphaN'), (2)J(C'HN), (2)J(HNCalpha), (2)J(C'Calpha), (2)J(HalphaC'), (2)J(HalphaCalpha), (2)J(CbetaC'), (2)J(N'Halpha), (2)J(N'Cbeta), and (2)J(N'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 (2)J coupling constants studied to be retrieved. Grouped by their mediating nucleus, C', N' or C(alpha), (2)J couplings related to C' and N' depend significantly on phi,psi torsion-angle combinations. beta turn types I, I', II and II', especially, can be distinguished on the basis of relative-value patterns of (2)J(CalphaN'), (2)J(HNCalpha), (2)J(C'HN), and (2)J(HalphaC'). These coupling types also depend on planar or tetrahedral bond angles, whereas such dependences seem insignificant for other types. (2)J(HalphaCbeta) appears to depend on amino-acid type only, showing negligible correlation with torsion-angle geometry. Owing to its unusual properties, (2)J(CalphaN') can be considered a "one-bond" rather than two-bond interaction, the allylic analog of (1)J(N'Calpha), as it were. Of all protein J coupling types, (2)J(CalphaN') exhibits the strongest dependence on molecular conformation, and among the (2)J types, (2)J(HNCalpha) comes second in terms of significance, yet was hitherto barely attended to in protein structure work.