Configurational stability and molecular dynamics of acetal-linked pyruvate substituents in polysaccharides

Pyruvate groups occur naturally in many microbial polysaccharides as nonsaccharidic components and significantly affect their physicochemical and biological properties. The configuration of the acetal carbon of pyruvate groups is mainly influenced by the favoured equatorial orientation of the methyl group. Evaluation of conformational energies has been carried out to assess the relative stabilities of the R and S isomers as a function of configuration and torsional angles for several residue models, including methyl 4,6-O-(1-carboxyethylidene)-alpha-D-galactopyranoside (1). Different levels of theoretical approach are used ranging from ab initio, semiempirical (AM1), and molecular mechanics (MM) methods up to molecular dynamics (MD). The higher stability of the isomer R of 1 was demonstrated by all of the methods used, thus giving full agreement with the NMR data on the natural compounds.