Esterification of phenylacetic and 2-phenylpropionic acids by mycelium-bound carboxylesterases

Lyophilized mycelia of Aspergillus oryzae and Rhizopus oryzae were used in phosphate buffer to study the hydrolysis of ethyl esters of phenylacetic and 2-phenylpropionic acids and in organic solvents to synthesize, by direct acylation, several esters of these acids, namely ethyl, propyl, butyl, pentyl, hexyl and isoamyl esters. Ethanol acylations with 2-phenylpropionic acid were also performed at different temperatures and in different solvents. In all cases, the conversion yield progressively increased with temperature and the best results were obtained at 50 °C in n-heptane. The use of A. oryzae led to the preferential formation of the (S)-enantiomer of ethyl 2-phenylpropionate with enantiospecificity increasing with temperature, whereas R. oryzae behaved oppositely. The molar conversions of these reactions also increased with the alcohol molecular weight (MW), likely because of the higher hydrophobicity and chemical affinity of the products for the solvent. A. oryzae proved to be more effective and quicker than R. oryzae in all the esterifications, probably due to a more favorable microenvironment. The kinetics of direct ethanol acylations were also studied in various solvents, specifically dimethylsulfoxide, dioxane, acetonitrile, tetrahydrofurane, pyridine, diisopropyl ether, benzene, toluene, n-heptane, isooctane, and pentadecane. The starting formation rates of ethyl esters with either biocatalyst were shown to progressively increase with the ratio of log P to the solvent molecular weight.