Kinetic analysis of the protonation of a surface group of a macromolecule

The dynamics of proton transfer between a surface-attached acidic moiety and the bulk of the solution was measured using the laser-induced proton pulse technique. [Gutman, M., Huppert, D. and Pines, E. (1981) J. Am. Chem. Soc. 103, 3709-3713]. The model system for this study consists of pH indicators (either neutral red or bromcresol green) adsorbed on Brij 58 micelles, as defined targets for protonation and a non-adsorbed proton emitter (2-naphthol-3,6-disulfonate) for generation of protons in bulk. The reaction was measured with 50-ns time resolution over a time period of about 200 microseconds. The results were analyzed by a numerical solution of the coupled nonlinear differential equation corresponding with the reaction system. [Gutman, M., et al. (1983) J. Am. Chem. Soc. 105, 2210-2216]. Quantitative analysis reveals two independent reactions which govern the observed dynamics: (a) a diffusion-controlled reaction between the proton and the surface targets; (b) translocation of the protonated target between the hydration layer of the interface and a more hydrophobic one. The contribution of the translocation reaction to the dynamics of surface protonation is more pronounced for compounds like carboxylates or phenolates which increase their hydrophobicity upon protonation. Amines and azoaromatic structures are more hydrophilic in their protonated states, the dynamics of their protonation is less affected by post-protonation distribution within the microenvironments of the interface. The interrelation between the partial rate constants and the macroscopic time constants and equilibrium parameters is analyzed.