Proton transport

Abstract: Agricultural productivity is severely affected by soil salinity. One possible mechanism by which plants could survive salt stress is to compartmentalize sodium ions away from the cytosol. Overexpression of a vacuolar Na+/H+antiport from Arabidopsis thaliana in Arabidopsisplants promotes sustained growth and development in soil watered with up to 200 millimolar sodium chloride. This salinity tolerance was correlated with higher-than-normal levels of AtNHX1transcripts, protein, and vacuolar Na+/H+(sodium/proton) antiport activity. These results demonstrate the feasibility of engineering salt tolerance in plants.

Abstract: We apply ‘‘molecular dynamics with quantum transitions’’ (MDQT), a surface‐hopping method previously used only for electronic transitions, to proton transfer in solution, where the quantum particle is an atom. We use full classical mechanical molecular dynamics for the heavy atom degrees of freedom, including the solvent molecules, and treat the hydrogen motion quantum mechanically. We identify new obstacles that arise in this application of MDQT and present methods for overcoming them. We implement these new methods to demonstrate that application of MDQT to proton transfer in solution is computationally feasible and appears capable of accurately incorporating quantum mechanical phenomena such as tunneling and isotope effects. As an initial application of the method, we employ a model used previously by Azzouz and Borgis to represent the proton transfer reaction AH–B■A−–H+B in liquid methyl chloride, where the AH–B complex corresponds to a typical phenol–amine complex. We have chosen this model, in part, because it exhibits both adiabatic and diabatic behavior, thereby offering a stringent test of the theory. MDQT proves capable of treating both limits, as well as the intermediate regime. Up to four quantum states were included in this simulation, and the method can easily be extended to include additional excited states, so it can be applied to a wide range of processes, such as photoassisted tunneling. In addition, this method is not perturbative, so trajectories can be continued after the barrier is crossed to follow the subsequent dynamics.

Abstract: Phototaxis and photophobic responses of green algae are mediated by rhodopsins with microbial-type chromophores. We report a complementary DNA sequence in the green alga Chlamydomonas reinhardtii that encodes a microbial opsin-related protein, which we term Channelopsin-1. The hydrophobic core region of the protein shows homology to the light-activated proton pump bacteriorhodopsin. Expression of Channelopsin-1, or only the hydrophobic core, in Xenopus laevis oocytes in the presence of all-trans retinal produces a light-gated conductance that shows characteristics of a channel selectively permeable for protons. We suggest that Channelrhodopsins are involved in phototaxis of green algae.

Abstract: The integral membrane protein M2 of influenza virus forms pH-gated proton channels in the viral lipid envelope. The low pH of an endosome activates the M2 channel before haemagglutinin-mediated fusion. Conductance of protons acidifies the viral interior and thereby facilitates dissociation of the matrix protein from the viral nucleoproteins--a required process for unpacking of the viral genome. In addition to its role in release of viral nucleoproteins, M2 in the trans-Golgi network (TGN) membrane prevents premature conformational rearrangement of newly synthesized haemagglutinin during transport to the cell surface by equilibrating the pH of the TGN with that of the host cell cytoplasm. Inhibiting the proton conductance of M2 using the anti-viral drug amantadine or rimantadine inhibits viral replication. Here we present the structure of the tetrameric M2 channel in complex with rimantadine, determined by NMR. In the closed state, four tightly packed transmembrane helices define a narrow channel, in which a 'tryptophan gate' is locked by intermolecular interactions with aspartic acid. A carboxy-terminal, amphipathic helix oriented nearly perpendicular to the transmembrane helix forms an inward-facing base. Lowering the pH destabilizes the transmembrane helical packing and unlocks the gate, admitting water to conduct protons, whereas the C-terminal base remains intact, preventing dissociation of the tetramer. Rimantadine binds at four equivalent sites near the gate on the lipid-facing side of the channel and stabilizes the closed conformation of the pore. Drug-resistance mutations are predicted to counter the effect of drug binding by either increasing the hydrophilicity of the pore or weakening helix-helix packing, thus facilitating channel opening.

Abstract: Fuel cells are attractive alternatives to combustion engines for electrical power generation because of their very high efficiencies and low pollution levels. Polymer electrolyte membrane fuel cells are generally considered to be the most viable approach for mobile applications. However, these membranes require humid operating conditions, which limit the temperature of operation to less than 100 degrees C; they are also permeable to methanol and hydrogen, which lowers fuel efficiency. Solid, inorganic, acid compounds (or simply, solid acids) such as CsHSO4 and Rb3H(SeO4)2 have been widely studied because of their high proton conductivities and phase-transition behaviour. For fuel-cell applications they offer the advantages of anhydrous proton transport and high-temperature stability (up to 250 degrees C). Until now, however, solid acids have not been considered viable fuel-cell electrolyte alternatives owing to their solubility in water and extreme ductility at raised temperatures (above approximately 125 degrees C). Here we show that a cell made of a CsHSO4 electrolyte membrane (about 1.5 mm thick) operating at 150-160 degrees C in a H2/O2 configuration exhibits promising electrochemical performances: open circuit voltages of 1.11 V and current densities of 44 mA cm-2 at short circuit. Moreover, the solid-acid properties were not affected by exposure to humid atmospheres. Although these initial results show promise for applications, the use of solid acids in fuel cells will require the development of fabrication techniques to reduce electrolyte thickness, and an assessment of possible sulphur reduction following prolonged exposure to hydrogen.