X-ray studies on crystalline complexes involving amino acids and peptides. XXXV. Invariance and variability in amino acid aggregation in the complexes of maleic acid with L-histidine and L-lysine

Abstract

The crystal structures of complexes of maleic acid with l-histidine and l-lysine have been determined. The two crystallographically independent amino acid molecules in the l-histidine complex have different closed conformations, while the lysine molecule in its complex has the most favourable conformation sterically with an all-trans sidechain trans to the -carboxylate group. The maleic acid molecules exist as semimaleate ions of similar conformation and contain a symmetric O H O hydrogen bond. Amino acid cations and semimaleate anions aggregate into alternate layers in both the structures. The arrangement of molecules in the histidine layer in l histidine semi- maleate is closer to that in the crystals of the free amino acid than in other l-histidine complexes. On the other hand, the arrangement of lysine molecules in its semi-maleate complex is different from any observed so far. However, the well established characteristic interaction patterns involving amino and carboxylate groups still play a major role in holding the molecules together in the crystal of the complex