An Octanol Hinge Opens the Door to Water Transport

Despite prevalent use as a surrogate for partitioning of pharmacologically active and natural products across lipid membranes, the mechanism of solute transport across water/octanol phase boundaries remains unexplored. Using classical molecular dynamics with uniquely benchmarked forcefields, graph theoretical and cluster analysis, and Langevin dynamics, we reveal an elegant mechanism for the transport of the simplest solute, water. At equilibrium, small groups of octanol at the instantaneous interface bind water and swing like hinge of a door to bring water into a semi-organized second interfacial layer (octanol ``bilayer islands"), where water can then diffuse into bulk octanol or be returned to the aqueous phase. The fundamental forces, collective, and reversible behavior, is well-described by a double well potential energy function, satisfying the basic principles of a simple molecular machine for solute transport. Unlike other transporting machines, this example leverages the interfacial surface fluctuations rather than circumventing them, imparting new design principles for hierarchically organized structures that transport solutes across liquid/liquid phase boundaries.