X-ray studies on crystalline complexes involving amino acids and peptides. XXV. Structures of DL-proline hemisuccinic acid and glycyl-L-histidinium semisuccinate monohydrate and a comparative study of amino-acid and peptide complexes of succinic acid

Abstract

DL-Proline hemisuccinic acid, C₅H₉NO₂.½C₄H₆ O₄, M_(r) = 174.2, P2₁/c, a = 5.254 (1), b = 17.480 (1), c = 10.230 (I) Å, β = 119.60 (6)° Z = 4, D_m = 1.41 (4), D_x = 1.42 g cm^-3, R = 0.045 for 973 observed reflections. Glycyl-L-histidinium semisuccinate monohydrate, C₈H₁₃N₄O₃⁺.C₄H₅O₄^-.H₂O, M_r = 348.4, P2₁, a = 4.864 (1), b = 17.071 (2), c = 9.397 (1) Å, β = 90.58°, Z = 2, D_m = 1.45 (1), D_x = 1.48 g cm^-3, R = 0.027 for 1610 observed reflections. Normal amino-acid and dipeptide aggregation patterns are preserved in the structures in spite of the presence of succinic acid/semisuccinate ions. In both the structures, the amino-acid/dipeptide layers stack in such a way that the succinic acid molecules/semisuccinate ions are enclosed in voids created during stacking. Substantial variability in the ionization state and the stoichiometry is observed in amino-acid and peptide complexes of succinic acid. Succinic acid molecules and succinate ions appear to prefer a planar centro-symmetric conformation with the two carboxyl (carboxylate) groups trans with respect to the central C=C bond. Considerable variation is seen in the departure from and modification of normal amino-acid aggregation patterns produced by the presence of succinic acid. Some of the complexes can be described as inclusion compounds with the amino acid/dipeptide as the 'host' and succinic acid/semisuccinate/succinate as the 'guest'. The effects of change in chirality, though very substantial, are not the same in different pairs of complexes involving DL and L isomers of the same amino acid.