Biocatalysis in Polymer Science - American Chemical Society

Chapter 18

Downloaded by CORNELL UNIV on September 21, 2016 | http://pubs.acs.org Publication Date: December 10, 2002 | doi: 10.1021/bk-2003-0840.ch018

Modification of Cellulose Solids by EnzymeCatalyzed Transesterification with Vinyl Esters in Anhydrous Organic Solvents Jiangbing Xie and You-Lo Hsieh Fiber and Polymer Science, University of California at Davis, Davis, CA 95616

Enzyme-catalyzed transesterification of several cellulose solids in organic media has been investigated. Protease and lipase enzymes were made soluble in organic media through ion-paired enzyme-surfactant complexes. Of the enzymes studied, Subtilisin Carsberg was found to be most catalytically active in the transesterification of cellulose in anhydrous pyridine. Following transesterfiction with vinyl propionate and vinyl acrylate, the presence of ester carbonyl groups on acylated cellulose were confirmed by FTIR and the modified cellulose surfaces became hydrophobic. From reactions on specifically substituted cellulose, the enzyme-catalyzed transesterification was confirmed to regioselectively target the primary hydroxyl group of cellulose.

© 2003 American Chemical Society

Gross and Cheng; Biocatalysis in Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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Introduction Enzymes are natural catalysts in biological systems. They have found many industrial applications including food processing, detergent additives, and textile finishing (/). Despite the conventional belief that enzymes could only function in an aqueous environment, the pioneering work of Klibanov et. al. (2-3) demonstrated that solid enzymes dispersed in anhydrous organic solvents also exhibited high catalytic activity. A variety of enzyme-catalyzed reactions in organic solvents have since been reported. These findings offer opportunities to use enzyme to catalyze reactions, such as esterification and transesterification, that are impossible to carry out in aqueous medium due to thermodynamics and/or kinetics reasons. The advantages of enzyme catalysis in organic syntheses are many: (i) Enzymes promote reactions that are difficult or impossible to emulate using traditional synthetic sequence, sometimes resulting in shortcuts. (ii) Enzymes exhibit enantioselectivity and regioselectivity in certain reactions, preferentially reacting with specific stereoisomer and/or site, offering easy control of chemical structure. (iii) Enzyme-catalyzed reactions are usually performed under mild conditions with regard to temperature, pH, and pressure, improving energy efficiency. (iv) Enzymes are natural catalysts without detrimental properties to either human or the natural environment. (v) The products from enzymatic reactions are biodegradable, the most attractive aspect of these reactionsfroman environmental perspective. (vi) Enzymes can be recycled for repeated use, providing an advantage of resource conservation. Enzyme-catalyzed transesterifications have been described as follows (4). It begins with binding of the ester donor, or acylating agent, to the active site of the enzyme to form an enzyme-ester complex which then transforms into the acyl-enzyme intermediate with the concomitant release of the alcohol product (Scheme 1). The acyl-enzyme intermediate then combines with nucleophiles, such as compounds containing hydroxyl groups, to form secondary binary complex, which finallyfreethe enzyme and form the transesterified product. Many lipases and proteases have been explored to catalyze esterification or transeterification reactions in organic solvents. The choice of enzymes is limited by the reaction systems and their catalyzing activities vary a great deal. For instance, out of dozens of commercially available Upases, only very few exhibit


219 low activities in pyridine (5-6). No lipase was found active in DMF and DMSO yet. However, a few proteases, such as chymotrypsin and subtilisin, have shown relatively high activities in the highly polar pyridine (6) and DMF (5) in catalyzing reactions of soluble substrates, such as sugars.

+ RZOH

m


Enzyme

R30H

Nucleophile

Enzyme-substrate complex

acylenzyme

alcohol product

+ R1C00R3 Product

Scheme 1: Mechanism of enzyme-catalyzed transesterification Various organic solvents,fromnon-polar to polar, have been documented to support enzymatic transesterifications. Generally speaking, the more hydrophobic or less polar the reaction medium, the more catalytically active the enzyme (2-3, 7). This is attributed to the fact that hydrophilic solvents are more capable of striping bonded waterfromthe enzymes, deteriorating their stability and activity. In addition, water, either originally contained in the polar solvents or deprivedfromthe enzymes, causes hydrolysis of the ester product and is unfavorable to transesterification. On the other hand, the solubility of substrates may need to be considered in solvent selection. Often, highly polar solvents, such as pyridine and DMF, are necessary to dissolve the substrates. In most cases, special esters are chosen as acylating agents to facilitate the reaction thermodynamically and/or kinetically. Activated esters, such as trihaloalkyl esters, are commonly used to increase the electrophilicity of the acylating agent (5). The bulky leaving groups are also desirable in reducing the rate of reverse reaction through steric hindrance. Vinyl esters are also excellent acylating agents which not only accelerate the rate of acyl transfer but also shift the reaction equilibrium forward continuously because the enol byproducts tend to tantomerize to ketones or aldehydes.


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Enzyme-catalyzed transesterifications in organic solvents have been explored for the synthesis of both macromolecules and small molecules. Polyesters can be synthesized by either ring-opening self-condensation of hydroxyl ester (A-B type) (8-10), or condensation between diester and diol (AA and B-B type) (11-15). Although most are synthesized from aliphatic monomers, polyesters with molecular weight ranging from 400 to 1,400 daltons have also been reported from aromatic monomers (14, 16). The reaction rates and molecular weight are highly dependent of the solvent medium and the removal of byproducts (10-15, 17). Some polyesters by enzyme-catalyzed transesterification have been characterized by narrower molecular weight distribution, more uniform structure, and, for chiral monomers, optically active products (77, 18-19). Enzyme-catalyzed transesterification has also been studied for the acylation of small molecules which contain either mono-hydroxyl groups (alcohols) (2-3, 19-20) or multi-hydroxyl groups (sugars) (5-7, 21-23). The regioselectivity of reactions on sugars is particularly significant because the great advantage over conventional reactions involving tedious block-deblock processes. Enzymes are insoluble in organic solvents. Most enzyme-catalyzed transesterifications, either synthesis of polyesters or acylation of small molecules, are carried out with suspended enzymes but soluble reactants in organic solvents. However, many reactants are either insoluble in organic media, or only soluble in solvents that may denature enzymes. For insoluble substrates, conventional suspension technology cannot support reactions with insoluble enzymes because of their poor interaction. Dordick et. al. (24-26) developed a method to solublize enzymes in hydrophobic organic solvents through the formation of enzyme-surfactant ion pairs. This approach differs from other methods in which enzymes are made soluble in organic solvents by either chemical modification or encapsulation in reversed micelles. The advantages of this approach include the extremely low water content (

Biocatalysis in Polymer Science - American Chemical Society

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