Newly identified C–H⋯O hydrogen bond in histidine

Abstract

New Cδ–H⋯O histidine hydrogen bonding interactions in various proteins are identified by neutron diffraction and computationally characterized. Neutron diffraction data shows several H-bond motifs with the Cδ–H moiety in histidine side chains, including interactions in β-sheets and with coordinated waters, mostly with histidinium and τ-tautomers. In yellow protein, an active site histidine H-bonds via Cδ–H to a main chain carbonyl while the Cε–H bond coordinates a water molecule. Although the H-bonding ability of Cε–H bonds in histidine have been previously identified, analysis of neutron diffraction structures reveals Cε–H H-bonds in notable active site interactions: for the proximal histidine in myoglobin; a zinc-bound histidine in human carbonic anhydrase II; within the Ser–Asp–His catalytic triad of the trypsin active site; and a histidine in the proton shuttle mechanism of RNase A, in addition to more general roles of coordinating water and forming H-bonds with carbonyl groups in β-sheets within a number of proteins. Properties of these H-bonds were computationally investigated using 5-methylimidazole and 5-methylimidazolium as models for histidine and histidinium. The π- and τ-tautomeric states of 5-methylimidazole were investigated, as both histidine tautomers are observed in the crystal structures. The newly characterized Cε–H⋯O and Cδ–H⋯O model complexes with water and acetone meet the overwhelming majority of IUPAC H-bonding criteria. 5-Methylimidazolium forms complexes that are nearly twice as strong as the respective neutral τ-5-methylimidazole and π-5-methylimidazole complexes. While the τ- and π-tautomers form Cε–H⋯O complexes of similar strength, the τ-Cδ–H⋯O interaction is approximately twice as strong as the π-Cδ–H⋯O interaction. Calculated charges on C–H (and N–H) hydrogens not participating in the H-bond are only slightly perturbed upon complex formation, implying that formation of one H-bond does not diminish the molecule's capacity for further H-bond formation at other sites in the imidazole ring. Overall, findings indicate that the Cδ–H⋯O interaction may be important for β-sheet stability, conformation, interactions with solvent, and mechanisms in the active site. Recognition of C–H bond polarity and hydrogen bonding ability in histidine may improve molecular modeling and provide further insight into the diverse roles of histidine in protein structure-function-dynamics.