Carica papaya lipase: An effective biocatalyst for esterification resolution of (RS)-2-(chlorophenoxy)propionic acid

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 (ΔΔH) in the present reaction conditions. A linear enthalpy-entropy compensation effect between ΔΔH and the difference of activation entropy (ΔΔS) 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.