X-RAY STUDIES ON CRYSTALLINE COMPLEXES INVOLVING AMINO-ACIDS AND PEPTIDES - XXXIII - CRYSTAL-STRUCTURES OF L-ARGININE AND DL-ARGININE COMPLEXED WITH OXALIC-ACID AND A COMPARATIVE-STUDY OF AMINO-ACID OXALIC-ACID COMPLEXES

The DL- and L-arginine complexes of oxalic acid are made up of zwitter ionic positively charged amino acid molecules and semi-oxalate ions. T he dissimilar molecules aggregate into separate alternating layers in the former. The basic unit in the arginine layer is a centrosymmetric dimer, while the semi-oxalate ions form hydrogen-bonded strings in the ir layer. In the L-arginine complex each semi-oxalate ion is surrounde d by arginine molecules and the complex can be described as an inclusi on compound. The oxalic acid complexes of basic amino acids exhibit a variety of ionization states and stoichiometry. They illustrate the ef fect of aggregation and chirality on ionization state and stoichiometr y, and that of molecular properties on aggregation. The semi-oxalate/o xalate ions tend to be planar, but large departures from planarity are possible. The amino acid aggregation in the different oxalic acid com plexes do not resemble one another significantly, but the aggregation of a particular amino acid in its oxalic acid complex tends to have si milarities with its aggregation in other structures. Also, semi-oxalat e ions aggregate into similar strings in four of the six oxalic acid c omplexes. Thus, the intrinsic aggregation propensities of individual m olecules tend to be retained in the complexes.