Helix-Capping Histidines: Diversity of N–H···N Hydrogen Bond Strength Revealed by (2h)J(NN) Scalar Couplings

[Image: see text] In addition to its well-known roles as an electrophile and general acid, the side chain of histidine often serves as a hydrogen bond (H-bond) acceptor. These H-bonds provide a convenient pH-dependent switch for local structure and functional motifs. In hundreds of instances, a histidine caps the N-terminus of α- and 3(10)-helices by forming a backbone NH···Nδ1 H-bond. To characterize the resilience and dynamics of the histidine cap, we measured the trans H-bond scalar coupling constant, (2h)J(NN), in several forms of Group 1 truncated hemoglobins and cytochrome b(5). The set of 19 measured (2h)J(NN) values were between 4.0 and 5.4 Hz, generally smaller than in nucleic acids (∼6–10 Hz) and indicative of longer, weaker bonds in the studied proteins. A positive linear correlation between (2h)J(NN) and the difference in imidazole ring (15)N chemical shift (Δ(15)N = |δ(15)Nδ1 – δ(15)Nε2|) was found to be consistent with variable H-bond length and variable cap population related to the ionization of histidine in the capping and noncapping states. The relative ease of (2h)J(NN) detection suggests that this parameter can become part of the standard arsenal for describing histidines in helix caps and other key structural and catalytic elements involving NH···N H-bonds. The combined nucleic acid and protein data extend the utility of (2h)J(NN) as a sensitive marker of local structural, dynamic, and thermodynamic properties in biomolecules.