Solvent deuterium isotope effects on phosphodiester cleavage catalyzed by an extraordinarily active Zn(II) complex

The effect of increasing pL on the extraordinary catalytic activity of a dinuclear Zn²⁺ complex toward cleavage of uridine 3′-4-nitrophenyl phosphate (UpPNP) in H₂O and D₂O was determined. This change from H₂O to D₂O causes an increase from 7.8 to 8.4 in the apparent pK_a of a catalytic functional group, but has little effect on the activity of the active form of the catalyst toward cleavage of UpPNP, so that there is no primary kinetic SDIE on the cleavage reaction from movement of a proton at the rate-determining transition state. It is concluded that essentially all of the rate acceleration for this catalyst is due to electrostatic stabilization of the transition state by interactions between opposing cationic and anionic charges.