Lateral proton conduction at a lipid/water interface. Effect of lipid nature and ionic content of the aqueous phase

Abstract

Fast lateral proton conduction was observed along the lipidiwater interface using a fluorescence technique. This conduction can be detected for a large number of lipids, both phospholipids and glycolipids. The efficiency of the proton transfer is dependent on the molecular packing of the host lipid at a given surface pressure. The proton conduction which is present in the liquid expanded state is abolished by the transition to the liquid condensed state. The proton transfer is affected slightly by the ionic content of the aqueous subphase except in the case of calcium which can inhibit the conduction along phosphatidylglyceroethanolamine.We suggest that the transfer of the protons occurs along a bidimensional hydrogen-bond network formed from the polar head groups, their water molecules of hydration and the water molecules which are intercalated between the lipid molecules.One of the basic requirements for the occurrence of a 'localized' mechanism of energy transduction [l -41 is the existence of a proton pathway along the membrane. Using a fluorescence method with lipid monolayers spread at the air/ water interface, we were able to detect such a proton pathway along the polar heads of phosphatidylethanolamine [5, 61. This technique provided experimental evidence that protons moved 20 times faster along the polar heads ('localized pathways') than in the bulk aqueous phase ('delocalized pathways'). This process occurs as long as the monolayer is structured, i.e. that its surface potential is different from zero. The time required by protons to move from one position to another was not affected by the packing of the film, although the number of protons-apparently transferred was strongly controlled by this, structural parameter. A systematic investigation of the physical parameters modulating this process showed that it depended on a bidimensional diffusion of protons along or within the polar heads.Using a laser-pulse-induced pH jump, Gutman and coworkers were able to demonstrate that surface groups on a high-molecular-mass body, such as neutral red bound to a detergent micelle, exchange protons at a very fast rate [7, 81. This observation was tentatively explained by the close proximity of the reactants and by the two-dimensional nature of the system 191. More recently, this approach was extended Correspondence to J.

Probing of Membrane's Surface by Dynamic Measurements of Proton Diffusion

“…On the basis of the many experiments we have carried out, we reject the notion of Prats et al (11) that the ordering of water on the surface of membrane (12) has any accelerating effect on the diffusion of proton (11).…”

Single channel H+ currents through reconstituted chloroplast ATP synthase CF0-CF1.

“…Likewise we determined the H+ buffer capacity of our proteoliposomes between pH 5 and pH 8 (for experimental details see Junge et al, 1979) to be of the order of 1 mM. This higher surface concentration of protons (we observed one channel in an area of -1 /um2) in combination with the possibly enhanced H+ diffusion at the membrane surface (Prats et al, 1987) would indeed be sufficient to allow up to -1 s steady-state channel currents with a rate of 106 H+/s. The observed independence (between pH 5.5 and pH 8) of the channel unit conductance from the bulk pH is consistent with the above mechanism.…”

Hydrogen Bonding Structure and Dynamics of Water at the Dimyristoylphosphatidylcholine Lipid Bilayer Surface from a Molecular Dynamics Simulation

“…7 An alternative view postulates that proton diffusion occurs via a hop and turn mechanism, possibly across the bulk. 2 Experimental studies of water diffusion at hydrophobic interfaces suggest diffusion coefficients for water around 10 -10 to 10 -11 cm 2 /s [8][9][10][11] compared to the bulk water coefficient of 10 -5 cm 2 /s.…”