A crystallographic titration of the dipeptide L-isoleucyl-L-isoleucine

The dipeptide L-Ile-L-Ile has been crystallized in four different protonation states, including as a cation, a zwitterion and an anion, as well as a dimeric cation in which two peptide molecules, linked by a strong -COO...H...OOC- hydrogen bond, share an overall positive charge of +1. This unprecedented series of crystal structures exhibits differences in hydrogen-bonding capabilities, conformational properties and molecular packing arrangements. The crystallization of L-Ile-L-Ile as an anion was achieved by slow evaporation from an NH(3)(l) solution to give the first ever ammonium salt of a peptide (or amino acid).