A simple light-driven transmembrane proton pump.

Abstract

Light-induced lipophilic porphyrin/aqueous acceptor charge separation across a single lipid-water interface can pump protons across the lipid bilayer when the hydrophobic weak acids, carbonylcyanide m-chlorophenylhydrazone and its p-trifluoromethoxyphenyl analogue, are present. These compounds act as proton carriers across lipid bilayers. In their symmetric presence across the bilayer, the positive currents and voltages produced by the photogeneration of porphyrin cations are replaced by larger negative currents and voltages. The maximum negative current and voltage occur at the pH of maximum dark conductance. The reversed larger current and voltage show a positive ionic charge transport in the same direction as the electron transfer. This transport can form an ion concentration gradient. The movement of protons is verified by an unusual D20 isotope effect that increases the negative ionic current by 2-to 3-fold. These effects suggest that an interfacial pK shift of the weak acid caused by the local electric field of photoformed porphyrin cations/acceptor anions functions as the driving force. The estimated pumping efficiency is 10-30%. Timeresolved results show that proton pumping across the bilayer occurs on the millisecond time scale, similar to that of biological pumps. This light-driven proteinless pump offers a simple model for a prebiological energy transducer.Light-driven proton pumps that sustain proton concentration gradients across cellular membranes by consuming light energy are important in supplying energy in the form of ATP to many cells. The mechanisms of biological ion pumps involve transmembrane electron transport (1) or light-induced isomerization of pigments (2), which drive changes in protein conformations (3). The complexities of the integral membrane proteins constructing these pumps have so far prevented a complete description of the ion pumping mechanisms. A light-driven ion pump composed of simple molecules and a lipid bilayer would serve as a useful model and as a possible analogue of the prebiological steps leading to the actual photobiological ion pumps. A defining characteristic of an ion pump is its ability to move ions across bilayers against a concentration or potential gradient. Since the transmembrane transport of protons requires carriers or channels that have one or more proton binding sites (4), a thermodynamic requirement for proton pumps is to cycle through a change in proton binding constants at the two membrane interfaces. We now describe a lipid bilayer system composed of lipophilic magnesium porphyrin (P) and aqueous electron acceptor (A) as the energy generating component and lipophilic weak acids as proton carriers, which has this desired characteristic.Previous experiments have shown that a lipid bilayer can be photocharged on the nanosecond time scale by electron transfer from an excited porphyrin in the bilayer to an acceptor in the aqueous phase (5) and that it takes >0.1 s for PI to cross the bilayer (6). We have recently demonstrated the pumping of tetr...

Membrane growth can generate a transmembrane pH gradient in fatty acid vesicles

“…The remainder of the energy loss may be due to several factors, including the fast relaxation of micelles into metastable structures and entropic increases resulting from alterations in the structure of water surrounding the micelle or vesicle. The observed energy efficiency is similar to that of other energy transduction systems based on pH gradients (52); for example, the energy efficiency of photosynthetic conversion of absorbed red light into reduced carbon is 34% (60). In comparison with these systems, however, energy transduction is achieved in oleate-arginine vesicles with only a few chemical components, namely oleate and a buffer by using an impermeant cation.…”

“…As predicted, further acidification was observed upon addition of fresh micelles (Table 2). Taken together, these results indicate that vesicle growth was not limited by intrinsic properties of the membrane, but rather that growth stops when the ''back pressure'' of the proton gradient equals the driving force for growth (52,53). In this case, the vesicle interior and exterior were initially buffered at pH 8.0 with 0.2 M arginine-bicine.…”

Self-organization of self-assembled photonic materials into functional devices: Photo-switched conductors

“…The charge-transfer reactions in similar metalloporphyrin assemblies (both H-bond-and metalmediated) in organic solvents are on the same scale (30). Thus, it is reasonable to expect that the assemblies studied here have generally similar electron-transfer rates and are consistent with other transmembrane currents (10)(11)(12)(13)(14)(15)(16). Because the time constant for the amplifier is 0.01-0.1 s for these experiments, the time constant to turn on the device is as of yet unknown.…”

“…A Grotthus-type mechanism was proposed to be the basis of conduction across the bilayer, wherein the initial charge transfer led to the spontaneous formation of an ion chain of alternating lipophilic porphyrin cations and lipophilic anions that spans the bilayer (11,12). Later devices (13,14) used other charge carriers such as C 60 and protonophores, which led to a device that pumped protons across the membrane (15,16). The substantial changes in the electrostatics of the bilayer itself were also proposed to play a significant role in the conduction mechanism (22).…”

Coupling of Proton Transport across Planar Lipid Bilayer and Electron Transport Catalyzed by Membrane-bound Enzyme <small>D</small>-Fructose Dehydrogenase

“…Then, Koland, et al indirectly observed the electron transport by a membrane-bound enzyme across a liposomal membrane containing adequate redox compounds. 10 Although Sun and Mauzerall have reported a transmembrane proton pump driven by light by use of the lipid bilayer containing magnesium octaethylporphyrin and hydrophobic weak acids, 11,12 the proton transport was qualitatively proved by the dependence of pH. The author's group also identified the electron transport across the planar BLM in the presence of D-fructose dehydrogenase (FDH) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) within the BLM by electrochemical analysis.…”