Although the peptide CαH group has historically not been thought to form hydrogen bonds within proteins, ab initio quantum calculations show it to be a potent proton donor. Its binding energy to a water molecule lies in the range between 1.9 and 2.5 kcal/mol for nonpolar and polar amino acids; the hydrogen bond (H-bond) involving the charged lysine residue is even stronger than a conventional OH··O interaction. The preferred H-bond lengths are quite uniform, about 3.32 Å. Formation of each interaction results in a downfield shift of the bridging hydrogen's chemical shift and a blue shift in the CαH stretching frequency, potential diagnostics of the presence of such an H-bond within a protein.