Molecular structure of <scp>L</scp>‐lysyl‐<scp>L</scp>‐alanyl‐<scp>L</scp>‐alanine: A tripeptide found in histone H1

Abstract

Abstract The crystal structure of L ‐lysyl‐ L ‐alanyl‐ L ‐alanine hydrochloride has been determined by x‐ray diffraction. The peptide is in zwitterionic form with the carboxylic group deprotonated, and with positive charges both in the amino terminal and ϵ‐amino groups of lysine. Crystals are monoclinic, space group P2 1 and Z = 4, with two peptide molecules in the asymmetric unit, which show different conformations. While one molecule has torsional angles for the Lys‐Ala peptide bond (φ 2 , φ 2 ) in the β‐pleated sheet region, the values for the other molecule are close to those for the α‐helix. This molecular flexibility is of interest for the study of H1 histone, which contains this sequence repeated several times. The two lysine residues show fully extended side chains. Two methanol molecules and two acetonitrile molecules are also present in the unit cell. An extensive network of hydrogen bonds and ionic interactions stabilize the crystal structure.