Enzymatic regioselective acylation ofcarbohydrates for synthesis of biodegradable polymers

Abstract

Lipases and proteases from different commercial sources were screened for their ability to catalyze the transesterification of glucose and activated N-blocked phenylalanine, tBoc-L-Phe-COOTFE. A commercial protease from Bacillus licheniformis, Optimase M-440,was found to be most effective for this purpose. The enzyme showed a broad substrate specificity toward various monosaccharides. Various monosaccharides and their derivatives(hexosamines, glycosides and sugar alcohols) were effectively acylated with tBoc-L-Phe-COOTFE by Optimase M-440 in pyridine. On a basis of $^13C$-NMR analysis, glucose was acylated only at the C-6 position. No di- and triesters were generated because of steric hindrance. D-amino acids also can be incorporated effectively into carbohydrates with Optimase M-440. Various solvents were tested as a reaction medium for carbohydrate acylation study. Hydrophobicity of organic solvents was shown to be independent of the transesterification activity of Optimase M-440. Disaccharides were also shown to be acylated as effectively as monosaccharides with amino acid esters. Acylation of disaccharides with activated amino acid ester, tBoc-L-Phe-COOTFE resulted in the formation of diesters, which are appropriate monomers for polymerization. Various disaccharides could be acylated with amino acid ester in pyridine. Solubility of disaccharides in pyridine could be judged from the solubility data in tert-butanol and melting points. Bulky carbohydrates such as raffinose could be also nucleophiles in the Optimase M-440 catalyzed acylation. $^13C$ NMR data enabled us to assign structures to the mono- and diester products with the sucrose monoester identified 1``-O-, 6-O-, and 6``-O-monoesters and diesters identified as 1``,6-O- and 1``,6``-O-diesters. In case of trehalose, the monoesters were identified as 6-O- and 3``-O-monoesters and diester products identified as 6,6``-O- and 6,3``-O-diesters Analysis of acylation site indicated sucrose was acylated at...