The effect of alcohols on the basic cleavage of m-nitrophenyl hexanoate by β-cyclodextrin: allosteric reaction mode switching

Abstract

Evidence is presented of a reacting guest–host system where binding of an ‘allostere' to the host inhibits its reaction with the guest by one particular mode and promotes its reaction by another. Simple aliphatic alcohols do not slow down the basic cleavage of m-nitrophenyl hexanoate by β-cyclodextrin (β-CD) to the extent required for competitive inhibition and so an additional, alcohol-mediated reaction must be taking place. Rate constants for this process correlate well with the ability of the alcohol to bind to β-CD, as do those for the analogous reaction of p-nitrophenyl hexanoate, suggesting that the alcohol is in the cavity of β-CD during the reaction. Transition state binding parameters for the alcohol-mediated reaction of the two nitrophenyl esters are very similar, and they show the same dependence on the binding ability of the alcohols. Overall, the results are consistent with a switch in the mode of reaction from cleavage of m-nitrophenyl hexanoate by aryl group inclusion (1) to its cleavage by acyl group inclusion (2), brought about by binding of a simple alcohol, acting as an ‘allostere'.