n-->pi* interactions in proteins

Hydrogen bonds between backbone amides are common in folded proteins. Here, we show that an intimate interaction between backbone amides also arises from the delocalization of a lone pair of electrons (n) from an oxygen atom to the antibonding orbital (pi*) of the subsequent carbonyl group. Natural bond orbital analysis predicted significant n-->pi* 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-->pi* interactions are abundant and especially prevalent in common secondary structures such as alpha-, 3(10)- and polyproline II helices and twisted beta-sheets. In addition to their evident effects on protein structure and stability, n-->pi* interactions could have important roles in protein folding and function, and merit inclusion in computational force fields.