The kyotorphin precursor, N-carbobenzoxyl-L-tyrosine-L-arginine amide (N-CBZ-L-Tyr-L-ArgNH(2)), was synthesized from N-CBZ-L-tyrosine ethyl ester (N-CBZ-L-TyrOEt) and L-arginine amide (l-ArgNH(2)) by using alpha -chymotrypsin. Eutectic mixtures were formed by mixing the substrates in the presence of water and/or organic solvents as adjuvants. The eutectic temperature was obtained with a 0.45 l-ArgNH(2) mole fraction. It was lowered to 23 degreesC by addition of 10% (v/w) water and 5% (v/w) dimethyl formamide, thus maintaining homogeneous liquid states at the reaction temperature of 30 degreesC. After 9 h the solutions became solidified and no further reaction took place. Approximately 90% (mol/mol) conversion was achieved from the substrate mixtures with substrate contents responsible for more than 80% (w/w) of the total mixture.