Ground state and transition state effects in the acylation of a-chymotrypsin in organic solvent-water mixtures

The kinetics of the -chymotrypsin catalysed hydrolysis of p-nitrophenyl acetate and N-acetyl-L-tryptophan methyl ester have been investigated at pH 7·85 and ionic strength 0·2M in solutions containing from 1 to 20% v/v dioxan or propan-2-ol. The solubilities of the two esters in these media were also measured. The ratio of the activity coefficients of the two transition states shows a much smaller variation with the organic solvent content of the medium than does the ratio of the activity coefficients of the substrates in their ground states, from which it is concluded that the substrate undergoes significant desolvation in the activation process. After allowing for the increased stability of the ester brought about by the addition of organic solvent, the inhibitory effect of dioxan can be represented by the simple formation of an inactive 1 : 1 complex of dioxan with the enzyme