Dynamic studies of proton diffusion in mesoscopic heterogeneous matrix: I. Concentrated solutions of sucrose

Biochemical systems lose their homogeneity at a mesoscopic scale; physical parameters vary sharply over a scale of a few nanometers. In this manuscript, we demonstrate how proton diffusion studies can report the microscopic properties of inhomogeneous systems. The method used for this purpose was the laser induced proton pulse and the reaction followed was the recombination of a proton with pyranine anion (8 hydroxy pyrene 1,3,6 trisulfonate) either in the excited state (subnanosecond dynamics) or in the ground state (microsecond time-scale measurements). The observed signals were analyzed by numeric integration of differential rate equations pertinent to the diffusion controlled reaction between proton and pyranine anion. The accuracy of the methodology was verified by measuring the dielectric constant of sucrose solutions. The results we obtained are identical with those published in the International Critical Tables (1933. Vol. VI, 82–101). The diffusion coefficient of proton was found to be independent of the sucrose concentration, up to 2M solution where the sucrose makes up 45% of the volume. This observation is interpreted in terms of the microscopic heterogeneity of the solution: the proton diffuses in the aqueous space between the sucrose molecules, while the continuity of the aqueous phase is maintained by the Brownian motion of the sucrose molecule, which allows the proton to pass between them at an unhindered rate.