Catalytic activity, enzymes enzymatic polymerization reaction

Apart from CALB other enzymes were shown to be able to successfully polymerize lactones. For instance Lipase PS-30, immobilized on Celite, was used as catalyst to study PDL-ROP under bulk reaction conditions. Poly(PDL) with M = 62000 and PDI 1.9 was reported [76]. Gross and coworkers could show that Humicola insolens cutinase (HiC) showed a high catalytic activity for enzymatic ROP of e-CL and PDL [87]. Poly(e-CL) with M n = 16000 (M /lVf = 3.1), in >99% yields was produced in bulk (70 °C, 24h) with 0.1% w/w immobilized HiC. Furthermore, using immobilized HiC in toluene (70 °C, 24h), PDL was converted to poly(PDL) (99% yield) with M = 44600 and Mw/M = 1.7. 

Lipases have also been used as initiators for the polymerization of lactones such as /3-bu tyro lac tone, <5-valerolactone, e-caprolactone, and macrolides.341,352-357 In this case, the key step is the reaction of lactone with die serine residue at the catalytically active site to form an acyl-enzyme hydroxy-terminated activated intermediate. This intermediate then reacts with the terminal hydroxyl group of a n-mer chain to produce an (n + i)-mer.325,355,358,359 Enzymatic lactone polymerization follows a conventional Michaelis-Menten enzymatic kinetics353 and presents a controlled character, without termination and chain transfer,355 although more or less controlled factors, such as water content of the enzyme, may affect polymerization rate and the nature of endgroups.360... 

In vitro enzymatic polymerizations have the potential for processes that are more regio-selective and stereoselective, proceed under more moderate conditions, and are more benign toward the environment than the traditional chemical processes. However, little of this potential has been realized. A major problem is that the reaction rates are slow compared to non-enzymatic processes. Enzymatic polymerizations are limited to moderate temperatures (often no higher than 50-75°C) because enzymes are denaturated and deactivated at higher temperatures. Also, the effective concentrations of enzymes in many systems are low because the enzymes are not soluble. Research efforts to address these factors include enzyme immobilization to increase enzyme stability and activity, solubilization of enzymes by association with a surfactant or covalent bonding with an appropriate compound, and genetic engineering of enzymes to tailor their catalytic activity to specific applications. 

Reports on the use of monomeric MPc complexes for the electrocatalysis of phenols are rare, hence the use of some polymeric MPc complexes will be included in this section. Biosensors based on enzymes have been developed for many electrochemical analyses. The use of MPc complexes as part of the enzyme improves the sensitivities of the biosensors. Ozsoz et al. used CoPc monomer dispersed in mushroom tissue electrode as a catalyst for the analysis of phenolic compounds The enzymatic reaction between mushroom and the phenolic compounds was coupled with the catalytic activity of CoPc. The CoPc dispersed electrodes gave shorter response times and lower potentials compared to conventional tissue biosensors . 

Lipase-catalyzed ring-opening polymerization of lactones was carried out over a wide temperature range of between 40 and 120 °C. The reaction temperature of higher than 80 °C was greater than that used in the conventional enzymatic reactions. It has been reported that imder relatively dry conditions, enzymes, such as lipases, proteases, and esterases, were catalytically active at temperatures around 90-120 °C [3-5]. Turner et al. demonstrated that lipase actually catalyzed the transesterification of octadecanol with palmityl stearate at 130 °C using lipase CA [6]. 

---