(354f) Transport in Stochastic Systems with Non-Adiabatic Coupling Between Different Degrees of Freedom

Transport in complex molecular systems is frequently modeled by a reduced stochastic model, such as the Langevin equation. A common method of analysis of tramsport in stochastic systems with multiple degrees of freedom is based on the assumption that the system follows a minimum energy path (MEP), i.e. a path such that the free energy is minimized in all directions transversal to the path direction. This assumption effectively reduces the multi-dimensional stochastic process to a quasi-one-dimensional process confined to the MEP. However, the MEP assumption is not always valid. In particular, our earlier work has demonstrated that substantial deviations from the MEP may occur even in a relatively simple process, such as a molecular transport across a liquid-liquid interface or a lipid membrane. These deviations from the MEPs occur due to non-adiabatic coupling between translational motion of the solute molecule and thermal fluctuations of the interface.

In this talk, we present a theoretical analysis of such non-adiabatic processes. This analysis is based on applying the path integration formalism to solution of the Langevin equation. This formalism yields a probability distribution of paths and enables us to identify the most likely path (MLP) to be taken by the system. We demonstrate that the MLP may significantly deviate from the MEP, which effectively creates an additional energy barrier to transport. We investigate dependence of this dynamic barrier on several system parameters characterizing the strength of coupling between different degrees of freedom.