Stereoselective thioglycoside syntheses. Part 6. Aryl 4-thiomalto-oligosaccharides as chromogenic substrates for kinetic studies with α-amylase

Abstract

Nucleophitic bimolecular substitution, of either o- or p-nitrophenyl 2,3,6-tri-O-benzoyl-4-O-trifluoro-methylsulphonyl-α-D-galactopyranoside (1) or (2) with the sodium salt of 1-thio-α-D-glucopyranose in hexamethylphosphoramide afforded, after the usual deprotection sequences, o- and p-nitrophenyl 4-thio-α-maltosides (7) and (8). A similar synthetic scheme with (1) and the 1-α-thiolate of 4-thiomaltose (12) led to o-nitrophenyl 4,4′-dithio-α-maltotrioside (15). These 4-thio-oligosaccharides and their corresponding oxygen analogues were used, in comparative assays, as chromogenic substrates with porcine and human pancreatic α-amylases. In both series, enzymic velocity was higher for the maltotrioside derivatives than for the maltodisaccharides. o-Nitrophenyl glycosides behave as better substrates than the corresponding para isomers. Replacement of intersaccharide oxygen atoms by sulphur increased slightly the Michaelis constant, but had a negative effect on the hydrolysis rate. As a consequence, 4-thiomaltosylpligosaccharides were less sensitive substrates for pig pancreatic α-amylase as compared with their O-glycosyl counterparts. However, as the former class of compounds is split exclusively at the chromogenic site, they appear to be substrates of interest for direct kinetic studies with α-amylases.