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Cutinase-catalyzed polymer synthesis
New paradigms in polymer synthesis are needed to meet increasing demands for structural complexity without concurrent increased environmental burden. This requires catalysts that are selective while operating under mild conditions. Work in our laboratory has provided many examples that immobilized Candida antarctica Lipase B, in nonaqueous media, is active for a surprisingly wide range of polyester and polycarbonate synthetic reactions occurring by step-condensation and ring-opening polymerization. Surprisingly, the majority of enzyme-catalysts studied for polymerization reactions have been from the lipase family where Lipase B from Candida antarctica has been most effective.
Scheme 12: A wide range of polycondensation reactions are efficiently catalyzed by immobilized Humilica insolens cutinase.
Cutinases are extracellular fungal enzymes whose natural function is catalyzing the hydrolysis of ester bonds in cutin, a lipid-polyester found in the cuticle of higher plants (Figures 11 and 12). With molecular weights of around 20 kDa, cutinases are the smallest members of the serine a/b hydrolase superfamily. Thus far, published work on cutinase-catalyzed biotransformations has focused on polyester degradation and esterification or transesterification of small molecules.
Figure 13. Polycondensation between 1,4-CHDM and various carboxylic diacids (C4, C6, C8, and C10, respectively) over the temperature range of 50-90 oC showing obtained molecular weights (Mn).
One goal of our research group is to expand the range of enzymes that are useful for polymerization reactions. To that end, investigations with the cutinase from Humicola insolens(HiC) showed it also is highly active for polyester synthesis reactions (Scheme 12). Immobilized HiC showed optimal activity at 70 °C and catalyzed a broad range of condensation and lactone ring-opening polymerizations in bulk and using toluene as solvent, yielding high molecular weight polyesters. On the basis of an extensive literature review and experience in our laboratory, we conclude that the activity of HiC for polyester synthesis reactions is rivaled only by Lipase B from Candida antarctica. Work is in progress to better define the activity, stability, and recyclability of immobilized Humicola insolens catalysts.
References
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Hunsen M, Abul A, Xie WC, Gross RA, Humicola insolens cutinase-catalyzed lactone ring-opening polymerizations: kinetic and mechanistic studies BioMacromolecules, 9(2), 518-522 Published: FEB 2008
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Mo Hunsen, Abul Azim, Harald Mang, Sabine R. Wallner, Asa Ronkvist, Wenchun Xie, and Richard A. Gross, A Cutinase with Polyester Synthesis Activity Macromolecules , 40(2), 148-150(2007).
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