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

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 pKa of the free phenol group upon enzymatic hydrolysis has been synthesised. Aryl β-glycosides with a pKa 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 kcat versus pKa is biphasic with an upward curvature at low pKa values suggesting a change in transition-state structure depending on the aglycon.