Enzymatic gelation of the natural polymer chitosan

The biopolymer chitosan was modified using the enzyme tyrosinase to convert a low molecular weight phenolic substrate (p-cresol) into a reactive o-quinone that undergoes subsequent reaction with chitosan. Spectroscopic data support the conclusion that the phenolic groups are covalently grafted at the amine site of chitosan although specific linkages could not be established due to the complexity of the chemistry. When semi-dilute chitosan solutions (<1%) were incubated with p-cresol and tyrosinase, the steady shear viscosity was observed to increase dramatically while oscillatory shear measurements indicated that the chitosan solutions had been converted into a gel, Specifically, dynamic oscillatory shear measurements showed that enzymatic reaction resulted in large increases in the complex viscosity (17 eta(8)), and storage and loss moduli (G' and G "). Further, eta* for the cresol-modified chitosan was observed to decrease with increasing frequency (omega), while G' and G " became independent of w and the loss tangent (tan delta) became less than 0.1. Incubation of these gels with the hydrolytic enzyme chitosanase showed that the cresol-modified chitosan remained biodegradable. (C) 1999 Elsevier Science Ltd. All rights reserved.