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

Ionic equilibria in "water-water-immiscible organic solvent" systems have been studied. It has been shown that in such systems shift of the apparent pK 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 pK 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 alpha-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-NH2). 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%.