Photosensitized electron transport across lipid vesicle walls: Enhancement of quantum yield by ionophores and transmembrane potentials

Abstract

The photosensitized reduction of heptylviologen in the bulk aqueous phase of phosphatidylcholine vesicles containing EDTA inside and a membrane-bound tris(2,2'-bipyridine)ruthenium(2+) derivative is enhanced by a factor of6.5 by the addition ofvalinomycin in the presence ofK+. A 3-fold stimulation by gramicidin and carbonyl cyanide m-chlorophenylhydrazone is observed. The results suggest that, under these conditions, the rate of photoinduced electron transfer across vesicle walls in the absence of ion carriers is limited by cotransport of cations. The rate of electron transfer across vesicle walls could be influenced further by generating transmembrane potentials with K+ gradients in the presence of valinomycin. When vesicles are made with transmembrane potentials, interior more negative, the quantum yield of heptylviologen reduction is doubled, and, conversely, when vesicles are made with transmembrane potentials, interior more positive, the quantum yield is decreased and approaches the value found in the absence of valinomycin.In recent years, light-induced electron-transfer processes have been extensively investigated with the aims of understanding the mechanism of natural photosynthesis and of designing artificial systems that will decompose water by sunlight to produce chemical energy in the form of H2 (1-3). A basic concept in the design of such systems is the use of dyes to photosensitize electron-transfer reactions that produce chemical species capable of oxidizing and reducing water. A major problem accompanying the dissociation of water by sunlight involves the back reactions of the intermediary redox species, whereby the potential energy of the photochemical process is degraded. One way to control the forward and backward reactions is to separate the photooxidized and photoreduced species by a phospholipid vesicle wall (4-9).As a model for studying photosensitized electron transfer across vesicle walls, we have used the system described earlier (9, 10). An amphiphilic Ru2+ complex incorporated in the membrane mediates vectorial electron transfer from EDTA trapped in the inner compartment of the vesicle suspension to heptylviologen added to the bulk aqueous phase. Recent evidence suggests that electron transfer through the vesicle wall can be accomplished by an electron-exchange mechanism between

Photoinduced electron transfer across membranes

“…A similar effect is exerted by valinomicin -a carrier of potassium ions, or gramicidin which creates ion channels in membranes [69]. Moreover, the creation of the negative potential (50-100mV) inside the vesicle prior to the illumination leads at room temperature to rising ~ of PET to outside the vesicle up to 0.44%, while • = 0.04% is observed in the absence of the potential [69]. If none of the indicated ionophores is present, the electric charge appearing upon electron transfer is compensated apparently due to either the electric breakdown of the membrane or the diffusion across the membrane of the components of the buffer which is always present in vesicle suspensions.…”

“…via providing the transfer of the ions through the membrane simultaneously with electron transfer. Indeed, in Systems 8 and 19 a significant (by a factor of 3 to 10) increase of has been found after efficient carriers of protons were added into the system [49,50,[67][68][69]. A similar effect is exerted by valinomicin -a carrier of potassium ions, or gramicidin which creates ion channels in membranes [69].…”

“…Indeed, in Systems 8 and 19 a significant (by a factor of 3 to 10) increase of has been found after efficient carriers of protons were added into the system [49,50,[67][68][69]. A similar effect is exerted by valinomicin -a carrier of potassium ions, or gramicidin which creates ion channels in membranes [69]. Moreover, the creation of the negative potential (50-100mV) inside the vesicle prior to the illumination leads at room temperature to rising ~ of PET to outside the vesicle up to 0.44%, while • = 0.04% is observed in the absence of the potential [69].…”

“…3 and 4. Calvin and his co-workers [67][68][69] have used as the photosensitizer (bpy)2 " • Ru(2 C16bpy) 2 +, a surfactant analog of Ru(bpy)~ + complex (System 19, Table 1), rather than the natural pigment Chl. Just as Chl, (bpy)2Ru(2C16bpy) 2÷ is embedded into the membrane owing to long hydrocarbon substituents, while EDTA and MV z÷ serve as electron donor and acceptor respectively.…”

Quantum capture and redox storage

“…We have shown by kinetic arguments that this electron transport is the rate-limiting step in the overall reaction. If that is so, then it should be possible to accelerate electron transfer by putting the proper potential across the membrane by selectively adding ions (such as potassium and sodium) on the inside and outside of the membrane with an ionophore in the membrane, a molecule which allows only passage of potassium ions (9). By varying the concentration of ions in the membrane system the potassium ion can be made to flow through the membrane down a concentration gradient while the sodium will be unable to come back.…”