Twenty-one different organic solvents were assayed as possible reaction media for the synthesis of butyryl esters from trimethylammonium alcohols in dry conditions catalyzed by immobilized Candida antarctica lipase B. The reactions were carried out following a transesterification kinetic approach, using choline and L-carnitine as primary and secondary trimethylammonium alcohols, respectively, and vinyl butyrate as acyl donor. The synthetic activity of the enzyme was strictly dependent on the water content, the position of the hydroxyl group in the trimethylammonium molecule, and the Log P parameter of the assayed solvent. Anhydrous conditions and a high excess of vinyl butyrate over L-carnitine were necessary to synthesize butyryl-L-carnitine. The synthetic reaction rates of butyryl choline were practically 100-fold those of butyryl-L-carnitine with all the assayed solvents. In both cases, the synthetic activity of the enzyme was dependent on the hydrophobicity of the solvent, with the optimal reaction media showing a Log P parameter of between -0.5 and 0.5. In all cases, 2-methyl-2-propanol and 2-methyl-2-butanol were shown to be the best solvents for both their high synthetic activity and negligible loss of enzyme activity after 6 days. (C) 2003 Wiley Periodicals, Inc.