Synthesis of aryl 3- O- β-cellobiosyl- β- d-glucopyranosides for reactivity studies of 1,3-1,4- β-glucanases

Abstract

A series of substituted aryl β-glycosides derived from 3- O- β-cellobiosyl- d-glucopyranose with different phenol-leaving group abilities as measured by the pK a of the free phenol group upon enzymatic hydrolysis has been synthesised. Aryl β-glycosides with a pK a of the free phenol leaving group>5 were prepared by phase-transfer glycosidation of the per- O-acetylated α-glycosyl bromide with the corresponding phenol, whereas the 2,4-dinitrophenyl β-glycoside was obtained by condensation of 1-fluoro-2,4-dinitrobenzene with the partially acetylated trisaccharide followed by acid de- O-acetylation. The aryl β-glycosides have been used for reactivity studies of the wild-type Bacillus licheniformis 1,3-1,4- β- d-glucan 4-glucanohydrolase. The Hammett plot log k cat versus pK a is biphasic with an upward curvature at low pK a values suggesting a change in transition-state structure depending on the aglycon.