Mediator-assisted photocurrent extraction from the thylakoids

Abstract

Photocurrent extracted from the thylakoids has been studied as a function of electron mediator concentration. Phenazine methosulfate is used to facilitate the charge transfer from the thylakoid's charge transport chain to the outside medium. The photocurrent has been shown to originate from the photosynthesis on the thylakoid membranes. Comparing with a previous study using para-Benzoquinone as the mediator, a similar peak effect in the photocurrent as a function of concentration is observed, but the magnitude of the current is nearly a thousand times greater. A semi-quantitative analysis is presented to explain the data found in those systems.

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 ($C_{PMS}$).