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An insight into photosynthesis in artificial membranes

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Page 1: An insight into photosynthesis in artificial membranes

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An insight into photosynthesis in artificial membranes An understanding of the mechanisms of photosynthesis is useful for the potential development of artificial biomembranes which could behave photosynthetically. It is believed that the initial chemical reaction in photosynthesis is the successive emission of electrons from a reaction centre - photosystem 2, PS2 - followed by the oxidation of water.

surfactant concentration was observed. Among those surfactants tested, Na-cholate exhibited the least apparent acceleration, while the C-16 CTAB had the maximum effect among cationics, and a C-14 hydrocarbon zwitterion, SB-14, accelerated the DPIP reduction by a factor of 32, the highest of all those surfactants tested. In every case the surfactant exhibited maximum acceleration of the reduction effect at a concentration approximate to its critical micelle concentration, CMC.

In the case of soya bean asolectin, a mixture of various

For the determination of light- dependent electron transport from the PS2 reaction centre, diphenylcarbazide, DPC, has often been used in place of water as an electron donor, and 2,6- dichlorophenol indophenol, DPIP, has often been used as an electron acceptor. Study of PS2 extracted from thylakoid membranes with surfactants has shown that, in the dark, DPIP is reduced faster by DPC in the presence of a surfactant than without it. The properties of surfactant micelles as catalysts has been extensively studied, and micelles and phospholipid vescicles are considered as precursors of biomembranes.

Despite numerous investigations on the mechanism of micellar catalysis, no theoretical system has yet been established. Researchers at the National Chemical Laboratory for Industry, Tokyo, Japan, have been conducting research into the reduction of DPIP by micelles and vescicles to elucidate the mechanism for the reduction/ oxidation process in DPIP/DPC systems. In particular, a recent research programme focussed on the mechanism of acceleration of DPIP reduction caused by surfactants.

A number of surfactants were used and in all cases a dependence of DPIP reduction on


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(A) Surfactants: SDS X LDAO O Na-cholate - A

(B) Surfactants: trimethyl ammonium bromides

n-decyl A n-dodecyl • n-tetradecyl- O cetyl X

(C) Surfactants: SB-10 A SB-12 • SB-14 0 SB-16 X

Membrane Technology 9

Page 2: An insight into photosynthesis in artificial membranes

phospholipids such as phosphatidyl choline and phosphatidyl ethanolamine, it was observed as remarkable that the acceleration rate was not affected by whether the asolectin was sonicated or not. It was considered that a single-wall small l iposome could be formed with sonication, and a multilamellar large l iposome might be formed without sonication. Thus the reaction might be accelerated by the lipid, irrespective of the structure of the liposome.

In the range 15-50°C, the temperature dependence of the DPIP reduction reaction is decreased by the addition of surfactant.

It is postulated that the redox reaction between DPC and DPIP proceeds as follows:

- DPC donates two electrons to DPIP

- this converts DPC to diphenylcarbazone diphenylcarbazone converts progressively to 5-hydroxy-2,3- diphenyl-tetraoleum-betain, donating two electrons to DPIP

- DPIP is thus reduced with two electrons

When diphenylcarbazone was used as electron donor, DPIP reduction occurred without surfactant. The results indicated that transfer of electrons to DPIP from DPC is a rate-limiting step in the reaction sequence.

The motive force of DPIP reduction is the difference between the reduction and oxidation potentials of DPIP and DPC, and the redox potentials are closely related to solution pH. However, the significance of the surfactant micelles in the redox potentials of the reactants could not be elucidated. From the results, it was considered that DPIP might locate in the region susceptible to both hydrophilic and hydrophobic effects of the micelles.

Considering micellar structure as a packed state, so that even hydrophobic compounds might be unable to penetrate easily into

Variation with temperature of DPIP reduction accelerated by surfactants



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the micellar interior, it is probable that reactants are concentrated near the micellar surface, where reaction might take place.

Applying a model of this type, it is hypothesised that the mechanism for acceleration of DPIP reduction might occur at the interface between micelle and the outer aqueous phase. Reduced DPIP would then relocate to a less hydrophobic region (e.g. the outer aqueous phase), while diphenylcarbazone would

transfer to a more hydrophobic one (e.g the micellar surface phase).

Further investigation is planned to analyse the contribution of hydrophilic groups of each surfactant. []

Adapted from a paper "Acceleration of reduction of DPIP by surfactants with DPC as reductant," S Miyairi et al, Membrane, Vol 16, No1 48- 51(1991)

10 Membrane Technology