The enzymatic acylation of polar dipeptides was investigated. First, the Novozym435 (R)-catalyzed acylation of Lys-Ser, HCl exhibiting three potential acylable sites was carried out in organic media (2-methyl-2-butanol, oleic acid) and in an ionic liquid ([Bmim](+)[PF(6)](-)). in these reactions, the chemoselectivity of the acylation was exclusively in favour of the NE-lysine acylation and the efficiency (Substrate conversion) was demonstrated to be under control of the peptide solubility. The use of [Bmim](+)[PF(6)](-) permitted to significantly improve the dipeptide solubility, and to enhance both substrates conversion and initial rates of acylation reaction. In the three reaction media used, the O-acylated derivative of the dipeptide was never detected suggesting a weak reactivity of the serine hydroxyl group due to its molecular environment and particularly to the presence of a free carboxylic group known for its electroattractor property. Last, the acylation of a natural dipeptide (carnosine), exhibiting a very low solubility in organic solvents, was also performed. Carnosine was Successfully N-acylated in 2-methyl-2-butanol, and a yield of 39% was obtained when improving the substrate solubility: a better dispersibility was obtained by application of a high pressure on the reaction medium just before starting the reaction. (C) 2008 Elsevier Ltd. All rights reserved.