Pressure effects on the hydrolysis of p-nitrophenyl and 2-naphthyl acetates catalyzed by cyclodextrins

Article abstract of DOI:10.1021/j150623a023

The rates of the hydrolysis of p-nitrophenyl acetate (PNPA) and 2-naphthyl acetate (2-NaA) catalyzed by α-, β-, and γ-cyclodextrins (CDs) were measured at pressures up to 2 kbar at 25°C and pH 8.3 in 0.02 M Tris buffer solution. At 1 bar, the rate of PNPA followed the enzymatic kinetics for α- and β-CDs and that of 2-NaA followed those for β- and γ-CDs. The substrate binding constant, K, for the 2-NaA-β-CD system decreased with increasing pressure up to 1 kbar, but the K values for other systems were constant regardless of the magnitude of pressure. The rate constants of the acylation process, k₂, for all systems up to 2 kbar increased monotonously up to 2 kbar. From the pressure dependence of K, the volume changes, ΔV, accompanying the formation of the inclusion complexes at 1 bar were 0 ± 2 cm³/mol, except for 10 ± 2 cm³/mol for 2-NaA-β-CD. The activation volumes, ΔV*, for the cleavages of ester molecules in the acylation process were in the range from -17.5 ± 2 cm³/mol for 2-NaA-β-CD to -13 ± 2 cm³/mol for PNPA-β-CD. The volume changes accompanying the formation of the inclusion complexes were explained either by the hydrophobic interaction between substrates and CD or by the conformational change of CD molecules in the inclusion process. It is suggested from the activation volumes in the acylation that the reaction process is classified into two types, one involving loose inclusion complexes for PNPA-β-CD and 2-NaA-γ-CD and the other involving tight complexes for PNPA-α-CD and 2-NaA-β-CD. The tight complexes seemed to be more solvated than the loose ones.