Conformations of Ammonium 3-Deoxy-D-manno-2-octulosonate (KDO) and Methyl α- and β-Ketopyranosides of KDO: X-Ray Structure and1H NMR Analyses

Abstract

Ammonium 3-deoxy-D-manno-2-octulosonate monohydrate (KDO) crystallizes in the orthorhombic space group P2₁2₁2₁, and the cell dimensions are a - 6.9700(4)Å, b = 7.7230(4) Å, c = 23.4067 (12) Å. X-ray intensity data were measured on a diffractometer, and the structure was determined by direct methods. Least-squares refinement, which included all hydrogen atoms, converged at R = 0.034 for 1526 observed reflections. The pyranose ring exists in an almost perfect ⁵C₂ (D) chair conformation. The COO^-, 4-OH and 6-CHOHCH₂OH groups are In equatorial orientation, while the 2-OH and 5-OH groups are axial. The solution conformations of the ammonium salts of methyl α- and β-ketopyranosides of KDO were determined by high-resolution ¹H NMR spectroscopy. The conformation of the ethylene glycol side chain in the α-methyl glycopyranoside of KDO was found to be indistinguishable from that in the solid state. However, the solution conformation of the side chain is different in the β-anomer, possibly indicative of an intramolecular hydrogen bond between the 8-OH and carboxylate groups.