Effect of Salt Additives on Enzymatic Enantioselective Hydrolysis of Racemic p-Hydroxyphenylglycine Esters
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Abstract:
The effects of salt additives, enzymes and substrate structure on on enzymatic hydrolysis of racemic p-hydroxyphenylglycine ester were examined systematically, when the salt was used as additive. Among seventeen sorts of salts tested, chloride salts and oxalates have slight effect on Novozym 435-catalyzed hydrolysis of racemic p-hydroxyphenylglycine methyl ester, while potassium phosphate only increased the reaction rate but did not influence enzymatic enantioselectivity. Bicarbonate salts, ammonium salts, sodium phosphate and sodium acetate all could not only markedly accelerate the reaction but also improve its enantioselectivity in comparison with the reaction without adding salts, of which ammonium bicarbonate was showed to be the best additive for the reaction (E value of 78 was achieved). When Alcalase was used as catalyst, the initial reaction rate and E value of p-hydroxyphenylglycine methyl ester hydrolysis in tert-butanol were boosted by the addition of NH4HCO3 with 30% and nearly 3-fold, respectively, as compared to the result without the salt addition into the reaction system. However, interestingly, NH4HCO3 inhibited papain-catalyzed hydrolysis of racemic p-hydroxyphenylglycine methyl ester to some extent, decreasing the initial reaction rate with 35% and E value with 60%. For the Novozym 435-catalyzed hydrolysis of racemic p-hydroxyphenylglycine ethyl ester and butyl ester, the E value of both substrates were markedly improved by adding NH4HCO3 in the same reaction system from 5 to 25 and from 23 to 28, respectively.
