Enzymatic Methods of Polymerization

In vitro enzyme-catalyzed ROP in a nonaqueous medium has been extensively investigated during the past 15 years (see also Chapter 15). The benefits of using enzymes as catalysts instead of nonenzymatic methods include  

The Upase-catalyzed ROP of small-, medium- and large-ring lactones has often been reported. By definition. Upases hydrolyze triglycerides in an aqueous medium at one or more of the three ester bonds, and have been characterized by their drastically increased activity at the Hpid-water interface of a micellar or emulsified substrate. In a nonpolar medium, these enzymes are capable of initiating the ROP of cychc lactones, and this properly has been exploited for the synthesis of polyesters, especially from macrolides. 

Most such carboxylic ester hydrolases (Upases) contain a Ser-His-Aspartate/ Glutamate catalytic triad (Serl05-His224—Aspl87) in the active site, and share (at least in part) the common structural framework of the a,P-hydrolase fold. This fold is composed mainly of parallel P-sheet, flanked on both sides by a-helices [51]. A unique structural feature common to all lipases is a lid or flap composed of an amphiphilic a-helix peptide sequence which, in its closed conformation, prevents access of the substrate to the catalytic site [52]. When the lid is opened. 

Water is critical for enzymes because it influences enzyme structure via nonco-valent bonding. A too-low water content generally reduces enzymic activity, whereas a too-high content can also reduce the reaction rates by causing the aggregation of enzyme particles and hence diffusional limitations. The optimum water content is often within a narrow range. 

Those lipases that have been reported for the polyester synthesis are of mammalian (porcine pancreatic lipase, PPL) or fungal (Candida antartica, CA Candida rugosa, CR Candida cylindracea, CC Aspergillus niger, AN PeniciUium roureforti, PR Rhizopus delemar, RD Rhizomucor miehei, RM) or bacterial origin (Pseudomonas cepacia, PC Pseudomonas Jlourescens, PF Pseudomonas species lipase, PS) and so on. 

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