Chemoenzymatic synthesis of 6ω-S-α-d-glucopyranosyl-6ω-thiomaltooligosaccharides: their binding to Aspergillus niger glucoamylase G1 and its starch-binding domain

A coupling reaction of cyclodextrin glucosyltransferase (CGTase) with glucose and 6-deoxy-6-iodo-cyclomaltoheptaose (1), in the presence of glucoamylase, followed by acetylation, led to a convenient synthesis of acetylated 6III-deoxy-6III-iodo-maltotriose (2) and 6IV-deoxy-6IV-iodo-maltotetraose (3). Nucleophilic displacement of the iodine atom of these protected maltotriose and maltotetraose analogs by the activated form of 2,3,4,6-tetra-O-acetyl-1-S-acetyl-1-thio-α-d-glucose (4) afforded peracetylated 6III-S-α-d-glucopyranosyl-6III-thiomaltotriose (5) and 6IV-S-α-d-glucopyranosyl-6IV-thiomaltotetraose (6) in high yield. The interaction of OH-free tetra- and penta-saccharides (7 and 8) with both glucoamylase G1 from Aspergillus niger as well as its isolated starch-binding domain fragment were studied by UV difference spectroscopy. It was found that the starch-binding domain has higher affinity for 7 and 8 than for maltotetraose and maltopentaose.