Mediator-assisted photocurrent extraction from the thylakoids
Introduction
Due to the growing demand for new energy resources, the capture and conversion of bio-energy in the photosynthetic organisms have received a lot of new attention. Photosynthetic reaction centers absorb solar energy and produce ATP and NADPH, which are the biological currencies for living organisms. The photosynthesis also provides the by-product of O2 during the process of absorbing CO2 and splitting H2O. The process begins when the light energy is absorbed by the reaction centers which contain chlorophyll pigments. In eukaryotic photosynthetic cells, the reaction centers are located on the thylakoid, the internal membrane structure of chloroplasts. On the membrane of the thylakoid, the electron flows following a Z-scheme mode [1], a standard model used to describe the electron transfer pathway in the light reactions of plant photosynthesis. The solar energy is captured to energize electrons taken from water. Photosystem II (PS II) P680 complex is excited into P680*. The high energy electron from P680* is then transported along the electron transport chain to photosystem I (PS I). These electrons subsequently reduce chlorophylls in P700+ of PS I and finally reduce NADP+ to NADPH, making biological energy ATP used by the organisms.
Applying artificial electron mediators is an effective approach to study the mechanism of the electron transport chain in photosynthetic membranes. A lot of progresses have been made in understanding the effects of artificial electron mediators to the natural transport chain and the details of electron transportation [2], [3], [4]. Beginning in the 1930s, ferric salts have been used to study the oxygen evolution to the chloroplast suspensions in the action of light [5]. Benzoquinones [6], [7], [8], which have relatively high redox potential, have been studied as artificial electron acceptors reduced by PSII. Phenylenediamine and diaminodurene (2,3,5,6-tetramethyl-p-phenylene diamine (DAD) [9] have been reported as the acceptors at PSII as well. 2,6-dichlorophenol-indophenol (DCIP) [10] or phenazine methosulfate (PMS) [11], [12], [13], on the other hand, could promote cyclic photophosphorylation at the site of PSI.
Other approaches to harvest the photoelectrons have been explored by several groups recently. For example, direct extraction of excited photosynthetic electrons from a single living algal cell using a nano-probe could generate pico-ampere bioelectricity [14]. Others have designed bio-solar cell with thylakoids linked anode[15], [16], [17]. Thylakoid electrode modified with nanotubes has been shown effective to enhance the photocurrent[18], [19], [20], [21].
In this paper, PMS was used as the artificial electron mediator to study the photocurrent from isolated thylakoid membrane structure. Comparing with our previous work with BQ, a much larger photocurrent has been observed with a similar peak effect as a function of the PMS concentration ().
Section snippets
Methods
To study the effect of electron mediators, isolated thylakoids were used. The thylakoid pellets were extracted from spinach leaves using a thylakoid isolation process [22], [23]. Fresh spinach leaves were initially grinded in a pH = 7.7 grinding buffer consisting of 0.3 M NaCl, 3 mM MgCl2, 30 mM Tricine/NaOH [24] using a kitchen blender, and filtered through an 8-layer cheesecloth filter. The filtrate was then centrifuged at 2500 × g (relative centrifugal force) for 4 minutes. The dark green pellet was
Results
Two types of experiments including the frequency dependence of the absorption and the effect of DCMU on blocking the electron transport cycle were performed to verify the origin of the photocurrent.
Electron flow in natural transport chain
Two kinds of photosystems take part in the electron transport on the thylakoid membranes of chloroplasts, photosystem II (P680) and photosystem I (P700). Under light illumination, the photons are absorbed by the reaction centers in the photosystems, the reaction centers are induced to their excited states. The excited state P680* makes it possible for the electrons to transfer to the plastoquinone, cytochrome b6f complex and plastocyanin. These electrons will be then acquired by P700 in PSI.
Conclusions
Studies of photo-induced current in the presence of electron mediator PMS molecules demonstrate the feasibility of harvesting high energy electrons from the thylakoid membrane. The frequency dependence of the photocurrent as well as the vanishing power exhibited by DCMU suggest that those electrons are from the photosynthesis of the reaction centers. A large peak in the current is observed as a function of PMS concentration, suggesting an optimal condition for electron extraction. The three
Acknowledgements
We thank Prof. Larry Brand, Prof. Zhongmin Lu, Prof. Ken Voss, Prof. Vincent Moy and Dr. Shradha Prabhulkar for providing some necessary equipment for the experiments and many useful discussions.
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