Acyclic tautomers in crystalline carbohydrates: the keto forms of 1-deoxy-1-carboxymethylamino-D-2-pentuloses (pentulose-glycines)

NMR and X-ray diffraction studies on acyclic carbohydrate keto tautomers of N-(1-deoxy-d-erythro-2-pentulos-1-yl)- and N-(1-deoxy-d-threo-2-pentulos-1-yl)glycines (ribulose-glycine, 1 and xylulose-glycine, 2), are reported. In aqueous solutions, both 1 and 2 exist as a tautomeric mixture, with the beta-furanose as a major form, followed by the alpha-furanose and the acyclic keto tautomer. Both 1 and 2 crystallize as zwitterions in the acyclic keto form. Solid-state 13C NMR spectra reveal the existence of two crystallographically independent molecules in 2 but only one in crystalline 1. The structure of 2 was solved by X-ray diffraction; in the crystal, two conformational isomers, 2a and 2b in a 1:1 ratio, were identified. The conformers are packed in stacks of similar molecules, with a system of intermolecular hydrogen bonds involving all hydroxyl, ammonium, and carboxyl groups. The structural data indicate that the sugar portion of acyclic 2 adopts conformations similar to those proposed for the parent d-xylulose.