(71c) Chemical and Enzymatic Synthesis of Carotenoid-Based Polymers From Renewable Material

Carotenoids are a class of naturally occurring antioxidants responsible for the coloration of many species and are important as nutritional supplements and in medical treatments. Many carotenoids contain fully conjugated carbon backbones, making them potentially useful as charge carriers. Using metabolic engineering techniques, we have designed a system to produce large quantities of carotenoids from carbohydrates. These carotenoids contain synthetically useful functional groups such as carboxylic acids. We have designed both chemical- and enzyme-catalyzed polymer syntheses based on the structure of these molecules.

We have synthesized tri-block copolymers consisting of a carotenoid unit flanked by blocks of a poly(hydroxyalkanoate) using lipase B from Candida antarctica as the catalyst. Lipases can catalyze the formation of ester bonds in organic reaction media with high selectivity. Further, they function under mild conditions, preventing degradation of the carotenoids. As a model system, we used crocetin, a C20 diacid from the gardenia plant, as the carotenoid subunit. Because the carotenoid is very bulky, lipases cannot normally accommodate it as an initiator for ring-opening polymerization of lactones. The addition of a linker molecule such as 1,3-propanediol, to each end of the carotenoid allows the modified carotenoid to participate in the polymerization. We can use crocetin 1,3-propanediol diesters as initiators for ring-opening polymerization to make polymers of molecular weight between 2,000 and 30,000 g mol-1 with polydispersity ~1.5 before fractionation. We have produced these polymers on the gram scale and can control the molecular weight and rate of polymerization by varying reactant concentrations. We observed slower reactions with diol-containing carotenoids such as astaxanthin and bixin, and can ultimately produce polymers of Mn ~ 20,000 using these carotenoids as initiators for ring-opening polymerization. We have also synthesized condensation polymers of several thousand molecular weight using dicarboxylic acid esters and hydroxyl-functionalized carotenoids.