Water-membrane interface, proton diffusion dynamics

Time-Resolved Dynamics of Proton Diffusion at a Water-Membrane Interface... 

The manner in which protons diffuse is a reflection of the physical properties of the environment, the geometry of the diffusion space, and the chemical composition of the surface that defines the reaction space. The biomembrane, with heterogeneous surface composition and dielectric discontinuity normal to the surface, markedly alters the dynamics of proton transfer reactions that proceed close to its surface. Time-resolved measurements of fast, diffusion-controlled reactions of protons with chromophores and fluorophores allow us to gauge the physical, chemical, and geometric characteristics of thin water layers enclosed between phospholipid membranes. Combination of the experimental methodology and the mathematical formalism for analysis renders this procedure an accurate tool for evaluating the properties of the special environment of the water-membrane interface, where the proton-coupled energy transformation takes place. 

In this work, we have approaehed the understanding of proton transport with two tasks. In the first task, deseribed above, we have sought to identify the moleeular-level stmeture of PFSA membranes and their relevant interfaees as a funetion of water content and polymer architecture. In the second task, described in this Section, we explain our efforts to model and quantify proton transport in these membranes and interfaces and their dependence on water content and polymer architecture. As in the task I, the tool employed is molecular dynamics (MD) simulation. A non-reactive algorithm is sufficient to generate the morphology of the membrane and its interfaces. It is also capable of providing some information about transport in the system such as diffusivities of water and the vehicular component of the proton diffusivity. Moreover, analysis of the hydration of hydronium ion provides indirect information about the structural component of proton diffusion, but a direct measure of the total proton diffusivity is beyond the capabilities of a non-reactive MD simulation. Therefore, in the task II, we develop and implement a reactive molecular dynamics algorithm that will lead to direct measurement of the total proton diffusivity. As the work is an active field, we report the work to date. 

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