Effects of Substitution Site on Acetyl Amylose Biodegradability by Amylase Enzymes

The site-selective syntheses of water soluble (6- O )- and (2- O /3- O )-acetyl amylose polymers (substituted at primary and secondary hydroxyl functionalities, respectively) were carried out. On the basis of 1 H NMR analyses regiospecificities of >95% were achieved. In addition, routine chemical methods which did not employ protection−deprotection steps provided water soluble (2- O /3- O /6- O )-acetyl amylose polymers. To maintain water solubility, the polymer degree of substitution (ds) was maintained at <0.70. The biodegradation characteristics of these products as a function of site and ds were studied by exposures to the α-amylases from Bacillus subtilis, Bacillus licheniformis, and Aspergillus oryzae . Quantitation of the biodegradation rate and percent were carried out using the dinitrosalicylic acid (DNS) reducing sugar assay. Common to all three α-amylases was that these enzymes degraded (2- O /3- O )-acetyl amylose polymers much more rapidly and to greater extents than (6- O )-acetyl amylose derivatives of similar ds's and molecular weights ( M v ). The rate of and percent degradation of (2- O /3- O /6- O )-acetyl amylose polymers was intermediate to that of (2- O /3- O )- and (6- O )-acetyl amylose polymers. Thus, the importance of site of substitution on the biodegradability of acetyl amylose polymers was demonstrated. Interestingly, when low ds (∼0.20) acetyl amylose polymers were exposed to the exoglycosidase from sweet potatoes (β-amylase), little to no polymer degradation was observed. This is believed to result from the rapid formation of substituted chain ends that are not degraded by the β-amylase, thus terminating further chain degradation events.