Immobilization of Candida rugosa lipase on MSU-H type mesoporous silica for selective esterification of conjugated linoleic acid isomers with ethanol

Abstract

Candida rugosa lipases (CRLs) immobilized on MSU-H type mesoporous silicas with different pore sizes (6.0, 7.2 and 13.3nm) through physical adsorption and a cross-linking method were examined as biocatalysts for esterification of conjugated linoleic acid (CLA) isomers with ethanol in isooctane. N2 adsorption–desorption isotherms indicated that the MSU-H silicas and the immobilized CRL derivatives had mesoporous structure. Introduction of CRL molecules onto MSU-H was confirmed by Fourier transform infrared (FT-IR) spectra. The optimum conditions of the immobilization process (pH of CRL solution, 7.0; initial concentration of CRL solution, 1.6mg/ml; contact time, 8–10h) and the esterification reaction (temperature, 45°C; molar ratio of CLA to ethanol, 1:1; biocatalyst, 6% by weight of CLA) were determined. The MSU-H material with pore size of 13.3nm proved to be more suitable to immobilize CRL onto it than others. The CRL immobilized on MSU-H (pore size, 13.3nm) by cross-linking gave a loading amount 48.2mg CRL/g-dry support and a maximal hydrolytic activity of 3418.2U/g-catalyst. It also exhibited high synthetic activity and operational stability and remained 43.2–67.8% of total esterification in 48h consecutive six runs under the optimized reaction conditions. Moreover, the immobilized CRLs catalyzed 2.0–3.1 times of esterification of cis-(c)9, trans-(t)11-CLA faster than that t10, c12-CLA. The formation of ethyl esters of CLA isomers was identified by GC–MS analysis.