X-ray studies on crystalline complexes involving amino acids and peptides. XXIV. Different ionization states and novel aggregation patterns in the complexes of succinic acid with DL-and L-histidine

Abstract

Diffusion of acetonitrile into an aqueous solution of DL-histidine and succinic acid in 1:3 molar proportions results in the crystals of DL-histidine hemisuccinate dihydrate [triclinic, P1, a = 7.654(1), b = 8.723(1), c = 9.260(1) Å, α = 77.23(1), β = 72.37(1) and γ = 82.32 (1)°]. The replacement of DL-histidine by L-histidine in the crystallization experiment under identical conditions leads to crystals of L-histidine semisuccinate trihydrate [orthorhombic, P2₁2₁2₁, a = 7.030 (1), b = 8.773 (1), and c = 24.332 (3) Å]. The structures were solved using counter data and refined to R values of 0.056 and 0.054 for 2356 and 1778 observed reflections, respectively. Histidine molecules in both the complexes exist in open conformation I. Succinate and semisuccinate ions in them are planar, and exactly or nearly centrosymmetric. In the DL-histidine complex, the amino acid molecules form double ribbons and the succinate ions occupy voids left behind when the double ribbons aggregate, as in inclusion compounds. In the L-histidine complex, the amino acid molecules form columns; so do the semisuccinate ions and water molecules. The two columns interdigitate to form the complex crystal. There are similarities between the molecular aggregation in the complexes and that in the crystals of L- and DL-histidine. However, the presence of succinic acid has the effect of disrupting, partially or totally, head-to-tail sequences involving amino acid molecules.