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

• Published in: Biochemistry (Easton) 2015-11, Vol.54 (46), p.6896-6908
• Main Authors: Preimesberger, Matthew R, Majumdar, Ananya, Rice, Selena L, Que, Lauren, Lecomte, Juliette T. J
• Format: Article
• Language: English
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/acs.biochem.5b01002

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 310-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 15N chemical shift (Δ15N = |δ15Nδ1 – δ15Nε2|) 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.