Newly identified C-H⋯O hydrogen bond in histidine

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.