(527e) Navigating Free Energy Landscape: Beyond the Minimum Energy Path

Understanding of dynamics of complex molecular systems is often facilitated by analysis of underlying free energy landscapes. In particular, activated transitions between (meta-) stable states are frequently investigated assuming that the system follows a minimum energy path (MEP) connecting these states. However, a MEP is not always the most likely path (MLP) taken by the system and the analysis based on the MEP assumption can significantly overestimate the transition rates. In this talk, we demonstrate that such discrepancies between MLP and MEP are caused by a large separation between time-scales of the degrees of freedom involved in the transition. This situation is illustrated by a case study of molecular transport across thin flexible membranes, such as surfactant mono- and bilayers. The deviation of these systems from their MEP is caused by strong coupling between fast solute movements and slow membrane undulations. The latter are too slow to enable a timely adjustment of the membrane shape to its most favorable configuration in response to the solute movement. We present a general approach based on the Wiener path integration formalism, which enables one to correctly identify the MLP and predict the transition rate. This approach leads to a definition of an effective energy, which, in addition to the equilibrium free energy, accounts for kinetic properties of the system, such as diffusion coefficients of various degrees of freedom. In essence, the free energy landscape is replaced by an effective energy landscape and the most likely path corresponds to the minimum effective energy path on this landscape.