X-ray studies on crystalline complexes involving amino acids and peptides.XXXVI. Crystal structures of hydrated glycyl-L-histidine and L-histidyl-L-alanine complexes with oxalic acid

The crystal structures of the complexes of oxalic acid with glycyl-L-histid ine and L-histidyl-L-alanine were determined. The three crystallographicall y independent peptide molecules in the complexes have closed conformations. The terminal carboxyl group of the dipeptide and the oxalate ion in the Gl y-His complex exhibit unusual ionization states and are connected by a symm etric O- - -O hydrogen bond. The peptide aggregation in the complex is almo st identical to that. in the corresponding semisuccinate complex and is sim ilar to one of the predicted aggregation patterns for Ala-Ala, demonstratin g that dipeptide aggregation is controlled primarily by main-chain interact ions and is substantially unaffected by disturbing influences such as those arising from polar side chains ions and water molecules. The peptide molec ules in the highly pseudosymmetric crystals of the His-Ata complex, however , exhibit a hitherto unobserved aggregation pattern. Thus, in spite of the repeated occurrence of a few patterns, the possibility of the existence of new patterns needs to be taken into account.