FREE-ENERGY PROFILES FOR H-BONDED CHAINS OF WATER-MOLECULES( CONDUCTION ALONG HYDROGEN)

The molecular mechanism for proton conduction along hydrogen-bonded ch ains, or ''proton wires,'' is studied with free energy simulations. Th e complete transport of a charge along a proton wire requires two comp lementary processes: 1) translocation of an excess proton (propagation of an ionic defect), and 2) reorientation of the hydrogen-bonded chai n (propagation of a bonding defect). The potential of mean force profi le for these two steps is computed in model systems comprising a singl e-file chain of nine dissociable and polarizable water molecules repre sented by the PM6 model of Stillinger and cc-workers. Results of molec ular dynamics simulations with umbrella sampling indicate that the unp rotonated chain is preferably polarized, and that the inversion of its total dipole moment involves an activation free energy of 8 kcal/mol. In contrast, the rapid translocation of an excess H+ across a chain e xtending between two spherical solvent droplets is an activationless p rocess. These results suggest that the propagation of a bonding defect constitutes a limiting step for the passage of several protons along single-file chains of water molecules, whereas the ionic translocation may be fast enough to occur within the lifetime of transient hydrogen -bonded water chains in biological membranes.