Variation in protein C α ‐related one‐bond J couplings

Four types of polypeptide 1 J CαX couplings are examined, involving the main‐chain carbon C α and either of four possible substituents. A total 3105 values of 1 J CαHα , 1 J CαCβ , 1 J CαC′ , and 1 J CαN′ were collected from six proteins, averaging 143.4 ± 3.3, 34.9 ± 2.5, 52.6 ± 0.9, and 10.7 ± 1.2 Hz, respectively. Analysis of variances (ANOVA) reveals a variety of factors impacting on 1 J and ranks their relative statistical significance and importance to biomolecular NMR structure refinement. Accordingly, the spread in the 1 J values is attributed, in equal proportions, to amino‐acid specific substituent patterns and to polypeptide‐chain geometry, specifically torsions ϕ, ψ, and χ 1 circumjacent to C α . The 1 J coupling constants correlate with protein secondary structure. For α‐helical ϕ, ψ combinations, 1 J CαHα is elevated by more than one standard deviation (147.8 Hz), while both 1 J CαN′ and 1 J CαCβ fall short of their grand means (9.5 and 33.7 Hz). Rare positive ϕ torsion angles in proteins exhibit concomitant small 1 J CαHα and 1 J CαN′ (138.4 and 9.6 Hz) and large 1 J CαCβ (39.9 Hz) values. The 1 J CαN′ coupling varies monotonously over the ϕ torsion range typical of β‐sheet secondary structure and is largest (13.3 Hz) for ϕ around − 160 . All four coupling types depend on ψ and thus help determine a torsion that is notoriously difficult to assess by traditional approaches using 3 J . Influences on 1 J stemming from protein secondary structure and other factors, such as amino‐acid composition, are largely independent. Copyright © 2008 John Wiley & Sons, Ltd.