A Cutinase with Polyester Synthesis Activity

New paradigms in polymer synthesis are needed to meet increasing demands for structural complexity without a concurrent increased environmental burden. This requires catalysts that are selective while operating under mild conditions. Some enzymes, in nonaqueous media, have proven to be surprisingly active for a wide range of polyester and polycarbonate synthetic reactions. Reactions leading to polymers include step-condensation, transesterification, and ring-opening polymerization.1 Surprisingly, the majority of enzymes studied for polymerization reactions have been from the lipase family with Lipase B from Candida antarctica as the dominant enzyme. This paper reports for the first time that a cutinase from Humicola insolens (HiC) has been found to have this unusual characteristic of catalyzing polyester synthesis. 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.2 With molecular weights of around 20 kDa, cutinases are the smallest members of the serine R/â hydrolase superfamily.3 Thus far, the majority of published work on cutinase-catalyzed biotransformations have focused on degradation of polyesters4 and on the esterification or transesterification of small molecules.5 Herein, we report that the cutinase from Humicola insolens (HiC), obtained from Novozymes, has promising activity for lactone ring-opening and condensation polymerization reactions. Previous work has shown that, for polymerization reactions, it is preferable to immobilize enzymes on high surface area supports.6 This increases the enzymes accessibility to high molecular weight substrates that must diffuse to and from the catalyst active site. Furthermore, it is well-known that immobilization of enzymes on solid supports often increases its thermal stability.7 Many literature reports on lipase-catalyzed polymerizations use Lipase B from Candida antarctica immobilized on Lewatit beads.8 Hence, as a starting point for studies of HiC activity for polyester synthesis, this enzyme was similarly immobilized by physical adsorption onto Lewatit beads. A study was performed to determine HiC activity for polyester synthesis via condensation reactions. A series of diols and diacids were selected that differ in chain length (see Scheme 1). Diacids were used in the free acid form without activation by esterification with groups such as vinyl or halogenated alkyl esters. Indeed, many literature reports on lipase-catalyzed polyester synthesis have relied on activation of diacids with such leaving groups.9 First, polymerizations were performed between adipic acid and diols of differing chain length (see Table 1, entries 1-3). With an increase in diol chain length from C4 to C6 and C8, Mn increased from 2700 to 7000 and 12 000, respectively. The polydispersity (Mw/Mn) values were between 1.4 and 1.6, well below that for statistically random condensation polymerization reactions. In another series of reactions, 1,4-cyclohexanedimethanol (1,4-CHDM) was used as the diol, and the diacid chain length was varied from C4 to C10 (Table 1, entries 4-7, and Figure 1). Inspection of Table 1 shows that Mn values were 900, 4000, 5000, and 19 000 for the C4, C6, C8, and C10 diacids, respectively. Hence, these experiments show that HiC has good activity for polycondensation polymerizations and that HiC activity is dependent on diol and diacid chain length. Increase in the diol and diacid chain length to C8 and C10, respectively, resulted in polyesters of increased molecular weight. Preference of HiC for longer chain length building blocks is reminiscent of substrate specificity observed for Candida antarctica Lipase B-catalyzed polycondensations.10 Furthermore, cutin, the natural substrate hydrolyzed by cutinases, consists of long chain (e.g., C16, C18) hydroxyacids. 1,4-Cyclohexanedimethanol was found to be a good substrate for HiC-catalyzed polycondensation reactions. This is useful since the cyclohexyl ring can impart greater rigidity to the resultant polyesters. Low polydispersities for some polymerizations in Table 1 indicate HiC is “chain selective”.11 In other words, chain growth occurs where HiC has higher activity for formation and/or esterifications between oligomers of certain chain lengths. Interestingly, chain selectivity during polymerization reactions was similarly observed for Candida antarctica Lipase B-catalyzed polycondensation reactions.11 Initial investigations of immobilized HiC thermal stability were conducted by performing polycondensation reactions at different temperatures (see Figure 1). Polymerizations between 1,4-CHDM and diacids with chain lengths C4 to C10 were performed at 50, 60, 70, 80, and 90 °C. Good activity was found over a large temperature range with an optimum at 70 °C. Large decreases in HiC activity occurred for reactions performed at 80 and 90 °C. Since polymerization reactions are often run in viscous reaction media, the ability to perform enzyme-catalyzed reactions at relatively higher temperatures, approaching 100 °C, is desirable. Inspired by the finding that HiC is active for polycondensation polymerizations, studies were performed to assess HiC activity for lactone ring-opening polymerizations (Scheme 2). HiC-catalyzed ROP was carried out in either bulk or toluene. To determine the relationship between reaction temperature and enzyme activity for lactone ring-opening polymerizations, -caprolactone (1) in bulk was taken as the model system. * Corresponding author. E-mail: rgross@poly.edu. Table 1. HiC-Catalyzed Polycondensation Reactions between Various Alcohols and Diacidsa