X-ray studies on crystalline complexes involving amino acids and peptides. XVII. Chirality and molecular aggregation: the crystal structures of DL-arginine DL-glutamate monohydrate and DL-arginine DL-aspartate

Abstract

DL-Arginine DL-glutamate monohydrate and DL-arginine DL-aspartate, the first DL-DL amino acid-amino acid complexes to be prepared and x-ray analyzed, crystallize in the space group P1 with a = 5.139(2), b = 10.620(1), c = 14.473(2) Å, α = 101.34(1)°, β = 94.08(2)°, γ = 91.38(2)° and a = 5.402(3), b = 9.933(3), c = 13.881(2) Å, α = 99.24(2)°, β = 99.73(3)°, γ = 97.28(3)° , respectively. The structures were solved using counter data and refined to R values of 0.050 and 0.077 for 1827 and 1739 observed reflections, respectively. The basic element of aggregation in both structures is an infinite chain made up of pairs of molecules. Each pair, consisting of a L- and a D-isomer, is stabilized by two centrosymmetrically or nearly centrosymmetrically related hydrogen bonds involving the α-amino and the α-carboxylate groups. Adjacent pairs in the chain are then connected by specific guanidyl-carboxylate interactions. The infinite chains are interconnected through hydrogen bonds to form molecular sheets. The sheets are then stacked along the shortest cell translation. The interactions between sheets involve two head-to-tail sequences in the glutamate complex and one such sequence in the aspartate complex. However, unlike in the corresponding LL and DL complexes, head-to-tail sequences are not the central feature of molecular aggregation in the DL-DL complexes. Indeed, fundamental differences exist among the aggregation patterns in the LL, the LD, and the DL-DL complexes.