Reaction selectivities of lipases for lipid ester modification reactions in microaqueous media

An evaluation of reaction selectivity was conducted for various lipases toward fatty acids (FA) in microaqueous media, as influenced by alcohol co-substrate, water activity (a_w) and immobilization. Lipase preparations, from Burkholderia cepacia, Rhizomucor miehei and Candida antarctica B were studied in multicompetitive reaction mixtures with n-chain FA (C4–C18:X series, where X = 0–3 double bonds) in ester modification reactions in tert-butyl methyl ether with polyols (glycerol, 1,2-propanediol (1,2-PD), or 1,3-PD) or their acetates as alcohol co-substrates. FA selectivity patterns (with optima for C8, C10, C12, C16, and/or C18:2) and the fold-differences in FA selectivity (ranging from 0.8 to 16.7) were characteristic for each lipases and were further modulated by the choice of alcohol co-substrate used.; Each lipase exhibited a characteristic response of FA selectivity to changes in a_w (0.19, 0.69 and 0.90) and immobilization for esterification reactions with 1,3-PD. In general, the immobilization of lipases onto Celite had some, but limited impact on FA selectivity relative to the free enzyme, whereas profound differences in FA selectivity in response to changes in a _w were observed for resin-bound lipases. Candida rugosa lipase selectivity toward C4–C6 was particularly affected by changes in a_w.; Synthesis of specifically structured lipids (SL) enriched in palmitic (C16:0) and oleic (C18:1) acids at the sn-2 and sn-1(3) positions, respectively, of triacylglycerols (TAG), was evaluated for R. miehei and B. cepacia lipase-catalyzed transesterification and interesterification reactions with lard and olive oil as co-substrates in the presence or absence of isooctane. In general, transesterification reactions in isooctane were most effective for preparing SL, with good TAG yields (>76 mol%) with the desired FA regio-distribution (with >71 mol% 16:0 at the sn-2 position and >56 mol% 18:1 at the sn-1(3) position of TAG) over 24 hr of reaction.; These findings illustrate that in addition to the intrinsic FA selectivities of lipases, some reaction parameters (choice of alcohol co-substrate, a _w and immobilization) can be selected to further modulate reaction selectivity. Multiple dimensions of control of reaction selectivity, if identified and understood, are conducive for the design of processes leading to a precise rearrangement of FA along the glycerol backbone to yield value-added SL.