Enzymatic synthesis in biphasic aqueous-organic systems. II. Shift of ionic equilibria

Abstract

Ionic equilibria in ‘water-water-immiscible organic solvent’ systems have been studied. It has been shown that in such systems shift of the apparent p K value of acids and bases takes place (compared to aqueous solutions), the value of the shift being rather high, up to 5 and more pH units (with 2,4-dinitrophenyltryptophan as an acid and neutral red as a base). The p K shift of ionogenic reagents observed in biphasic systems can be used in preparative organic synthesis for increasing the yield of end products in enzyme-catalyzed reactions. In connection with this, the physico-chemical reasons for the equilibrium shift in a chemical reaction that involves one or two ionogenic reagents are theoretically analyzed. The above approach has been tested with two α-chymotrypsin-catalyzed reactions, i.e., synthesis of N- benzoyl- l-phenylalanine ethyl ester (from NBz- lPhe-OH and ethanol) and synthesis of N- acetyl- l-tryptophanyl - l- leucine amide (from NAc- lTrp-OH and lLeu-NH 2). In water the equilibria in these reactions are shifted almost entirely towards the starting reagents with the yield of end product being negligibly low. In biphasic systems consisting of chloroform + 5% (v/v) water or ethyl acetate + 2% (v/v) water, the yield of both the ester and the dipeptide reaches 100%.