Topological Evidence of Previously Overlooked Ni+1-H···Ni H-Bonds and Their Contribution to Protein Structure and Stability.

Hydrogen-bonds (H-bonds) between backbone N-H donors and CO acceptors are central to our understanding of protein structure and stability. However, while interactions between backbone N atoms and the N-H of the following residue are also common, they have been ignored as potential H-bonds due to their bent geometry and the assumption that the amide N is a poor H-bond acceptor. Recently, we reported indirect experimental evidence that these interactions constitute functional H-bonds. We now report a combined atoms in molecules and noncovalent interaction theoretical analysis of electron density that unambiguously supports the characterization of these interactions as H-bonds. The calculations further suggest that the Ni+1-H···Ni H-bonds are largely electrostatic in nature and, importantly, that they make a significant contribution to stability. Thus, given their apparently frequent occurrence, Ni+1-H···Ni H-bonds likely make critical, but previously unrecognized, contributions to protein structure and function.