X-ray studies on crystalline complexes involving amino acids and peptides. Part XX. Crystal structures of DL-arginine acetate monohydrate and DL-lysine acetate and a comparison with the corresponding L-amino acid complexes

X-ray studies on crystalline complexes involving amino acids and peptides XXXIV. Novel mode of aggregation, interaction patterns and chiral effects in the maleic acid complexes of DL- and L- arginine

Amino acid - carboxylic acid complexes provide useful information in relation to molecular interactions in present day biological systems and to prebiotic self-organisation. The crystal structures of the complexes of maleic acid with DL- arginine (orthorhombic; Pca2(1); a=15.9829, b=5.4127, c=16.1885; R=0.0522 for 956 reflections) and L- arginine (triclinic; P1; a=5.2641, b=8.0388, c=9.7860, alpha=106.197, beta=97.275, gamma=101.64; R=0.039 for 1749 reflections) have been determined. The complexes are made up of positively charged zwitterionic arginine molecules and negatively charged semi-maleate ions which contain an intramolecular symmetric O-H-O hydrogen bond. In both the structures, the amino acid molecules aggregate into layers. In each layer, S2 head-to-tail sequences are interconnected through specific intermolecular interactions between alpha-carboxylate and guanidyl groups, an arrangement observed for the first time in crystal structures involving arginine. The carboxylate-guanidyl interactions are of different types in the two complexes and consequently aggregation patterns in them exhibit substantial differences. Interactions between the amino acid layers involve the semi-maleate ions in both the complexes. In addition, water-bridges also exist in the L complex. The full potential of the guanidyl group for specific interactions is realized in both the structures. The L complex contains an array of water-mediated salt bridges. The structures demonstrate that the effect of chirality on molecular aggregation can span a wide range.

X-ray studies on crystalline complexes involving amino acids and peptides. XXVII. Effect of chirality, specific interactions and characteristic aggregation patterns in the structures of arginine and its complexes with formic acid

The crystal structures of DL-arginine dihydrate, DL-arginine formate dihydrate and L-arginine formate have been determined and refined using X-ray crystallographic techniques. The three structures, along with other related ones, demonstrate the conformational variability of arginine. The amino acid molecules aggregate essentially in a similar manner in DL-arginine dihydrate and in the known structure of L-arginine dihydrate; the effects arising out of the reversal of the chirality of half the amino acid molecules are absorbed by small local adjustments. However, such a reversal leads to profound differences in aggregation in DL-arginine and L-arginine formates, in contrast to the situation in the corresponding acetates. Thus the effect of chirality on biomolecular aggregation cannot be easily predicted or even rationalized. Arginine-carboxylate interactions in the complexes primarily involve the guanidyl groups and contain specific interactions. Indeed the primary mode of arginine-carboxylic acid aggregation is substantially invariant in the arginine complexes of succinic, acetic and formic acids.