Biosynthetic stereocopolymer of 3-methylmalic acid as hydrolyzable and biocompatible polyester for temporary therapeutic applications

Abstract

A mixture of (2 S,3 S) and (2 S,3 R)-3-methylaspartic acid, obtained by bioconversion of mesaconic acid in the presence of 3-methylaspartase as enzymatic catalyst, was transformed into the corresponding benzyl 3-methylmalolactonate stereoisomers using a multiple-step synthesis. A mixture of (3 R,4 R) and (3 S,4 R) (80/20 (mol/mol) ratio) benzyl-3-methylmalolactonate was transformed by anionic ring-opening polymerization to an optically active and stereoregular stereocopolymer constituted by 80 mol% of benzyl (3 R,4 S) 3-methylmalate repeating units and 20 mol% of benzyl (3 S,4 S) 3-methylmalate units, as determined by 1H NMR. The corresponding optically active poly( β-3-methylmalic acid) was obtained by catalytic hydrogenolysis of the protecting benzyl ester groups, in N-methylpyrrolidone as solvent. This functionalized hydrosoluble polyester was degraded by simple hydrolysis in a phosphate buffer at pH 7, as shown by SEC measurements. It is worth noting that the kinetic profile was equivalent to the one of poly( β-malic acid). Moreover, at 37°C, the hydrolysis was complete within six weeks, yielding to the corresponding optically active 3-methylmalic acid. In order to use this polymeric material for temporary therapeutic applications, polymers containing both diastereoisomers as repeating units as well as their ultimate products of degradation were evaluated based on harmlessness towards their environment. No toxicity was detected against a human cell line, HepG2.