AN EXPERIMENTAL METHODOLOGY FOR MEASURING VOLUME CHANGES IN PROTON-TRANSFER REACTIONS IN AQUEOUS-SOLUTIONS

A fast perturbation in proton concentration can be induced in aqueous solution using a pulsed ultraviolet laser and suitable photolabile com pounds which, upon photoexcitation, irreversibly release protons. The volume change and the rate constant for the reaction of the photodetac hed protons with proton-accepting groups in solution can be monitored using time resolved photoacoustics, A typical proton concentration jum p of 1 mu M can be obtained with a 200-mu J laser pulse at 308 nm. Rea ction dynamics from 20 ns to 5 mu s can be easily followed. The method ology we establish represents a direct, time-resolved measurement of t he reaction volume in proton transfer processes and an extension to th e nanosecond-microsecond range of traditional relaxation techniques, s uch as stopped-flow. We report example applications to reactions invol ving simple molecules and polypeptides. (C) 1998 Elsevier Science B.V. All rights reserved.