Abstract
A partially purified Carica papaya (pCPL) is explored as an effective biocatalyst for the esterification resolution of ( RS)-2-(chlorophenoxy)propionic acid (CPA) with trimethylsilylmethanol in anhydrous isooctane. In comparison with a crude Candida rugosa lipase (CRL), pCPL has higher enantioselectivity ( E > 100) for 2- and 4-chloro but not 3-chloro substituted CPA, in which more than an order-of-magnitude lower reactivity for the hindered ( RS)-2-(2-chlorophenoxy)propionic acid is found for both lipases. The thermodynamic analysis indicates that the enantiomeric discrimination is mainly driven by the difference of activation enthalpy (ΔΔ H) in the present reaction conditions. A linear enthalpy-entropy compensation effect between ΔΔ H and the difference of activation entropy (ΔΔ S) is demonstrated and elucidated from the regio-effect of 2-, 3- and 4-chloro substituent. The kinetic analysis indicates that pCPL is strongly inhibited by the acid substrate, but to a less extent by the alcohol substrate. With the disadvantage of decreasing the enzyme activity, the E value can be greatly enhanced when trimethylsilylmethanol concentration higher than 40 mM in isooctane is employed.
Published Version
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