Peroxidase catalyzed grafting of gallate esters onto the polysaccharide chitosan

Peroxidases are believed to play a role in various natural polymerization processes and it may be possible to exploit peroxidases for environmentally-friendly industrial polymer processing. We examined the potential for using horseradish peroxidase to graft the phenolic substrate dodecyl gallate (DDG) onto the polysaccharide chitosan. Several analytical approaches were used to provide evidence that DDG was grafted onto chitosan. Compared to unmodified chitosan, DDG-modified chitosan had significantly increased absorbance in the UV-visible region, and in the C-H and carbonyl-stretching regions of the IR spectra. Also, the 1H NMR spectrum of a soluble fraction of DDG-chitosan had broad peaks near 1.2 ppm consistent with the grafting of DDG onto the polymer. Additional evidence for DDG grafting was obtained in two studies in which the DDG-modified chitosan was subjected to nitrous acid hydrolysis. First, a highly modified and insoluble DDG-chitosan was suspended in 10% acetic acid. After partial hydrolysis, peaks associated with the sugar and dodecyl gallate moieties were observed to appear in the solution phase 1H NMR spectrum. Finally, the DDG-modified chitosan was hydrolyzed and fractions were separated by HPLC. One fraction showing both UV absorbance (characteristic of the phenolic) and carbohydrate reactivity to anthrone was purified by two chromatographic steps. This fraction was analyzed by both FAB-MS and electrospray-MS, and observed to have a molecular weight of 371 Da. These results provide evidence that peroxidases can be used to graft phenolic moieties onto the polysaccharide chitosan.