Crystal structures of (2R,4R)-2-(polyhydroxyalkyl)-1,3-thiazolidine-4-carboxylic acids: condensation products of l-cysteine with d-hexoses.

We report herein the first crystal structures of (4-carboxy-1,3-thiazolidin-2-yl)pentitols [2-(polyhydroxyalkyl)thiazolidine-4-carboxylic acids], condensation products of l-cysteine with d-galactose and d-mannose: 2-(d-galacto-pentahydroxypentyl)thiazolidine-4-carboxylic acid hydrate, Gal-Cys·H(2)O (1), and 2-(d-manno-pentahydroxypentyl)thiazolidine-4-carboxylic acid hydrate, Man-Cys·H(2)O (2). In 1 and 2 the compounds crystallize as zwitterions, with the carboxylic groups deprotonated and the thiazolidine N atoms protonated. The sugar moiety and carboxylate group are in a cis configuration relative to the thiazolidinium ring, which adopts different conformation: twisted (T) on C(ß)-S in 1, and S-puckered envelope (E) in 2. The carbon chain of the galactosyl/mannosyl moiety remains in an extended zig-zag conformation. The orientation of the sugar O2 atom with respect to the thiazolidinium S and N atoms is trans-gauche in 1 and gauche-gauche in 2. The molecular conformation is stabilized by the intramolecular N-H⋯O(Cys) contacts in both 1 and 2 and by the additional N-H⋯O(Man) interaction in 2. The crystal packing of orthorhombic 1 and monoclinic 2 is determined mainly by N/O/C-H⋯O hydrogen bonds forming ribbons linked to each other by direct and water-mediated O/C-H⋯O/S contacts.