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

The crystal structures of complexes of maleic acid with L-histidine and L-l ysine have been determined. The two crystallographically independent amino acid molecules in the L-histidine complex have different closed conformatio ns, while the lysine molecule in its complex has the most favourable confor mation sterically with an all-trans sidechain trans to the alpha-carboxylat e group. The maleic acid molecules exist as semi-maleate ions of similar co nformation and contain a symmetric O ... H ... O hydrogen bond. Amino acid cations and semi-maleate anions aggregate into alternate layers in both the structures. The arrangement of molecules in the histidine layer in L-histi dine 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 t he molecules together in the crystal of the complex.