Charge separation across membranes

Natural photosynthetic reaction centers, which are responsible for the conversion of light energy into chemical energy in photosynthetic organisms, employ a multistep electron transfer strategy to achieve charge separation across membranes with a total quantum yield near unity. Thus, at each intermediate step ... 

Figure 1. Schematic representation of the artificial photosynthetic reaction center by a monolayer assembly by A-S-D triad and antenna molecules for light harvesting (H), lateral energy migration and energy transfer, and charge separation across the membrane via multistep electron transfer (a) Side view of mono-layer assembly, (b) top view of a triad surrounded by H molecules, and (c) energy diagram for photo-electric conversion in a monolayer assembly.

Potentiometric responses to cationic guests by neutral hosts can be interpreted on the basis of charge separation across the membrane interface with cationic host-guest complexes at the membrane side and hydrophilic counteranions at the aqueous side (Figure 3a). The amount of the cationic complexes at the membrane side of the interface is determined by the lipophilicity of the guest, the stability... 

For the series of the compounds with n = 1-4 the lifetime of D + -P-Q was found to decrease exponentially with the distance between Q and D+ with ae = 3.3 A [166]. This value considerably exceeds the value of ae 1.25 A found for P -L-Q [151] and may arise from the amide group insertion into the linking bridge in contrast to hydrocarbon spacers in the latter molecules. In Ref. [167], triad molecule with n = 1 has shown the capability of facilitating light-induced charge separation across a bilayer lipid membrane. 

An important aspect of the function of photosynthetic complexes is their asymmetric arrangement in respect to the membrane and to the external and internal phases of the cellular compartments. This arrangement allows the catalysis of vectorial electron transfer and the performance of electrical work by promoting charge separation across the membrane dielectric barrier. It allows also in some cases the net translocation of protons across the membrane. These two processes are at the basis of the mechanism of energy conservation in photosynthesis coupled to the formation of ATP, which is added, in oxygenic photosynthesis, to the conservation of redox energy in the form of reduced pyridine nucleotide coenzymes. 

Natiual photosynthesis, as we have already noted, seems to observe these tenets. The A value of reaction centre protein is <0.5 eV, which is in the ballpark of the -AG° values for the primary light-induced charge separation across a membrane, but much smaller than -AG° values for the highly exergonic unwanted back ET step. Also the reaction centre cofactors are so positioned as to provide molectrlar ET stepping stones for the... 

A FIGURE 8-33 Photoelectron transport, the primary event in photosynthesis. After absorption of a photon of light, one of the excited special pair of chlorophyll a molecules in the reaction center left) donates an electron to a loosely bound acceptor molecule, the quinone Q, on the stromal surface of the thylakoid membrane, creating an essentially irreversible charge separation across the membrane right). The electron cannot easily return through the reaction center to neutralize the positively charged chlorophyll a. 

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