n→π* Interactions in Proteins

Hydrogen bonds between backbone amides are common in folded proteins. Here, we show that an intimate interaction between backbone amides likewise arises from the delocalization of a lone pair of electrons (n) from an oxygen atom to the antibonding orbital (π*) of the subsequent carbonyl group. Natural bond orbital analysis predicted significant n→π* interactions in certain regions of the Ramachandran plot. These predictions were validated by a statistical analysis of a large, non-redundant subset of protein structures determined to high resolution. The correlation between these two independent studies is striking. Moreover, the n→π* interactions are abundant, and especially prevalent in common secondary structures such as α-, 3(10)-, and polyproline II helices, and twisted β-sheets. In addition to their evident effects on protein structure and stability, n→π* interactions could play important roles in protein folding and function, and merit inclusion in computational force fields.