Evidence for ion chain mechanism of the nonlinear charge transport of hydrophobic ions across lipid bilayers

Sun, Kai; Mauzerall, D.

doi:10.1016/s0006-3495(96)79225-1

Cited by 9 publications

(20 citation statements)

References 28 publications

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“…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.…”

Section: Resultssupporting

confidence: 58%

“…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).…”

Section: Design Features and Principlesmentioning

confidence: 99%

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

Drain

2002

Proc. Natl. Acad. Sci. U.S.A.

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Linear porphyrin arrays self-assembled by either hydrogen bonding or metal ion coordination self-organize into lipid bilayer membranes. The length of the transmembrane assemblies is determined both by the thermodynamics of the intermolecular interactions in the supermolecule and by the dimension and physical chemical properties of the bilayer. Thus, the size of the porphyrin assembly can self-adjust to the thickness of the bilayer. An aqueous electron acceptor is placed on one side of the membrane and an electron donor is placed on the opposite side. When illuminated with white light, substantial photocurrents are observed. Only the assembled structures give rise to the photocurrent, as no current is observed from any of the component molecules. The fabrication of this photogated molecular electronic conductor from simple molecular components exploits several levels of self-assembly and selforganization.

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Section: Resultssupporting

confidence: 58%

Section: Design Features and Principlesmentioning

confidence: 99%

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

Drain

2002

Proc. Natl. Acad. Sci. U.S.A.

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“…4). These timeresolved results show that the driving force of the proton pumping must be the light-generated interfacial P+/A-charge separation, which lives for seconds (10), but in the present system is limited by the 60-ms charge leakage time of the proton carrier doped bilayer.…”

mentioning

confidence: 62%

A simple light-driven transmembrane proton pump.

Sun

Mauzerall²

1996

Proc. Natl. Acad. Sci. U.S.A.

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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...

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“…A detailed description of the materials and apparatus can be found elsewhere (Sun and Mauzerall, 1996;Mauzerall and Drain, 1992). Typically, the lipid solution consists of 18 mM diphytanoylphosphatidylcholine and 3.6 mM magnesium octaethylporphyrin in n-decane.…”

Section: Methodsmentioning

confidence: 99%

“…The symmetric photoformation of P+ cations in the lipid bilayer greatly increases the transmembrane ionic current carried by lipophilic anions such as tetraphenylborate (TPhB-). This is termed the photogating effect (Drain et al, 1989;Drain and Mauzerall, 1992;Mauzerall and Drain, 1992;Sun and Mauzerall, 1996). The translocation of ions across the lipid membrane is thus markedly affected by the intramembrane ion interactions.…”

Section: Introductionmentioning

confidence: 99%

Charge transfer across a single lipid-water interface causes ion pumping across the bilayer

Sun¹,

Mauzerall²

1996

Biophysical Journal

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The photoformation of magnesium-porphyrin cations (P+) at a single lipid bilayer-water interface can pump lipophilic borate anions completely across the lipid bilayer and causes an actual reversal of the photovoltage. The system consists of a lipid bilayer containing magnesium octaethylporphyrin, an aqueous or interfacial electron acceptor on one side, and chloro- or fluoro-substituted tetraphenylborate in both aqueous electrolyte solutions. With 1-micros pulsed illumination, an immediate positive photovoltage is observed, which decreases on the microsecond and millisecond time scales. On the time scale of seconds, as the P+ cation concentration decays in reverse electron transfer, the voltage swings negative to a value almost equal to its initial value and finally decays with a half-time (approximately 20 s) longer than the time constant of the system (approximately 5 s). Thus, an ion gradient across the membrane is formed, trapped by the nonlinear relation between ion mobility and ion concentration. Continuous light illumination confirms that negative charge moves in the direction opposite that of the initial photoinduced electron transfer. Steady-state measurements indicate an ion pumping efficiency of approximately 30%. This simple mechanism may be a progenitor of photobiological ion pumps.

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Evidence for ion chain mechanism of the nonlinear charge transport of hydrophobic ions across lipid bilayers

Cited by 9 publications

References 28 publications

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

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

A simple light-driven transmembrane proton pump.

Charge transfer across a single lipid-water interface causes ion pumping across the bilayer

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