Kinetic analysis of proton transfer between reactants adsorbed to the same micelle. The effect of proximity on the rate constants

The dense packing of protogenic enzymes on the coupling membrane can furnish a route for a rapid proton flux which may avoid the adjacent bulk phase. In order to evaluate the role of proximity between reactants on the rate constant of proton transfer we generated a model system consisting of 2-naphthol and pH indicator (bromocresol green) both adsorbed on the same micelle of unchanged detergent. Excitation of the 2-naphthol by a short intensive laser pulse lowers its pK with subsequent synchronized proton ejection. The discharged protons are detected by their reaction with the indicator using a fast transient absorption technique. Evidence is produced that under certain conditions all of the observed proton flux represents proton transfer between 2-naphthol and indicator molecules sharing the same micelle. In this model system the entire proton flux proceeds through an aqueous phase fully accessible to phosphate ions. The high proximity of the reactants (the separation can not exceed approximately 6 nm) has a marked effect on the rate constant of the reaction k = 2.0 +/- 0.5 X 10(11) M-1 s-1. In spite of this extremely fast rate of reaction we observe unhindered competition, for the surface discharged proton, between the surface-bound reactants and phosphate ions in the bulk. Thus even in proton transfer between closely packed reactants on an interface, the diffusion of the proton is not limited to the interface. This finding implies that on bioenergetic surface the electrochemical potential of the proton on the surface will equal that of the bulk.