Structural evidence for the aromatic-(i+1) amine hydrogen bond in peptides: L-tyr-L-tyr-L-Leu monohydrate.

In crystalline Tyr-Tyr-Leu monohydrate, an aromatic-(i+1) amine hydrogen bond is observed, that is a weak hydrogen-bond-type interaction between an aromatic side chain and N-H of the next peptide group in the main chain. Unlike the better investigated aromatic-(i+2) amine hydrogen bonds, which can adopt almost ideal geometries, the geometry of the discussed interaction is very distorted because of steric constraints. Presumably, this kind of weak hydrogen bond is only formed as a last resort if N-H finds it impossible to engage in the much stronger conventional hydrogen bonding with O-atom acceptors.