In vitro hydrolysis of poly( l-lactide) crystalline residues as extended-chain crystallites : III. Effects of pH and enzyme

Abstract

The effects of pH and an enzyme on the hydrolysis of poly( l-lactide) [i.e. poly( l-lactic acid) (PLLA)] crystalline residues or extended-chain crystallites containing a negligibly small amount of chains in an amorphous state were investigated in aqueous solutions at pH range from −0.9 to 12.8 and in Tris–HCl buffered solution with or without proteinase K by the use of gel permeation chromatography (GPC) and differential scanning calorimetry (DSC). GPC results showed that the hydrolysis of the crystalline residues proceeded from their surface composed of very short chains with a free end along the chain direction, irrespective of pH, but that from their lateral surface could not be traced. GPC and DSC results indicated that the hydrolysis of the crystalline residues was accelerated with the pH deviation from 7, reflecting that the hydronium and hydroxide ions have catalytic effects on the hydrolysis of the crystalline residues. The dependence of hydrolysis rates estimated by GPC on the concentration of hydronium or hydroxide ions revealed that the catalytic effect of hydroxide ions on the hydrolysis is much higher than that of hydronium ions. The significant T m decrease at high and low pH means that there is a decrease in thickness of the crystalline residues by the hydrolysis. On the other hand, GPC and DSC results also showed that proteinase K has no catalytic effect on the hydrolysis of the crystalline residues, confirming the reported finding that the hydrolysis of very short chains with a free end is not accelerated by the presence of proteinase K.