The influence of a free α-ammonium group in the substrate upon trypsin-catalyzed transesterification

1. 1. By comparison of rates, measured by using titration, of trypsin-catalyzed hydrolysis of either l-arginine methyl ester (AME) or l-lysine methyl ester (LME) with those determined by paper chromatography analysis, it was possible to ascertain that pK_a of the α-ammonium group of both esters differs by about -1.8 units from that of the respective free amino acids. 2. 2. In aqueous alcohol solutions, the logarithm of the ratio of concentrations of products of alcoholysis to hydrolysis at a given time shows a linear relationship with the logarithm of the ratio of ionized to un-ionized species of substrate α-ammonium group. 3. 3. The ratio l-lysine ethyl ester (LEE):lysine also decreases with temperature, an effect which at least partially might be accounted for in terms of a decreased ionization of substrate α-ammonium group. 4. 4. The conclusion can be reached that k₄ > k₃ (the deacylation rate constant in alcoholysis is larger than the corresponding one in hydrolysis). Hence, the addition of alcohol exerts a driving effect on the reaction. Besides this effect, alcohols also accelerate the reaction by a dielectric constant effect similar to that produced by acetone on the trypsin-catalyzed hydrolysis of LME or to those observed with various solvents in the trypsin-catalyzed hydrolyses of α-N-substituted substrates.