Hydrolysis of glycosyl fluorides

Other inverting glucosidases which conform to the pattern of direct hydrolysis of glycosyl fluorides having the correct anomeric configuration, and transglycosylation with inversion if the anomeric configuration is opposite to that of the natural substrates are trehalase from rabbit renal cortex and from the yeast Candida tropicalis, and ) -D-xylosidase from Bacillus pu-milis. ... 

Hehre and coworkers showed that beta amylase from sweet potatoes, an inverting, a-specific exo-(l 4)-glucanase, catalyzes the hydrolysis of jS-maltosyl fluoride with complex kinetics which indicated the participation of two substrate molecules in the release of fluoride ion. Furthermore, the reaction was strongly accelerated by the addition of methyl ) -maltoside. Hydrolysis of a-maltosyl fluoride, on the other hand, obeyed Michaelis-Menten kinetics. The main product with both a- and yj-maltosyl fluoride was )S-maltose. The results with )3-maltosyl fluoride were interpreted by the assumption of a glycosylation reaction preceding hydrolysis by which a malto-tetraoside is formed by the replacement of fluoride ion by a second substrate molecule or added methyl -maltoside (see Scheme 5). 

Evidence for a glycosyl-enzyme intermediate of finite lifetime with inverting a-D-glycosidases, and details of its reaction, came from studies with 2,6-anhydro-l-deoxyhept-l-enitols and glycosyl fluorides. - Analysis of hydration and hydrolysis products on the one hand, and of glycosyla-tion products on the other, indicated an intermediate that could be approached by water from the yff-face only of the ring, and by other glycosyl acceptors only from the a-face (see Schemes 4 and 5 This can be considered a proof of the principle of microscopic reversibility of chemical reactions. 

In this case the predominance of glycosylation of C4-carbinols over substrate hydrolysis is in a mixture where the molar concentration of water was more than 3000 times that of the a-maltosyl fluoride. In other experiments the substrate concentration could be further reduced at least 5-fold without altering the fact of a substantially greater extent of maltosidic linkage synthesis than hydrolysis. We now are referring to a very dilute 4 mM, solution of substrate with which, if the hydrolase concept were valid, hydrolysis ought to have been predominant. 

In the first case, typical reactions are the formation of the methyl fi-D-glucoside from a-D-glucopyranosyl fluoride and of its 2-amino-2-deoxy derivative from 2-amino-2-deoxy-a-D-glucopyranosyl fluoride, on treatment with sodium methoxide in methanol. The products from the reaction of aqueous bases with the glycosyl fluorides depend on the concentration of alkali. At low concentrations, the normal hydrolysis products are formed. At higher concentrations of base, if the proper (trans) steric relation exists between C-6 and the fluorine atom at C-1, anhydro compounds are formed, as in (21) from (20). 

---