Large Photovoltages Generated by Plant Photosystem I Crystals

Abstract

Light energy conversion to electrical energy in photosynthesis and in photovoltaic devices is initiated by separation of charges that generates a photovoltage across an energy barrier in the system. [1][2][3] In conventional devices, mostly based on semiconductors, [ 4 ] the photovoltage is limited by the bandgap energy. Anomalous photovoltaic effects, in which the generated photovoltage is larger than the bandgap, have been reported in polar materials such as ferroelectric crystals, [ 5 , 6 ] but also in various amorphous and polycrystalline semiconductors. [7][8][9][10] Can a photoactive biological protein also generate an anomalous photovoltage? Natural candidates are photosynthetic reaction centers such as plant photosystem I (PSI), which are highly effi cient light energy converters found in thylakoid membranes inside the chloroplasts of green plants. [ 11 , 12 ] Here we show that these dried micrometer-thick crystalline biological protein complexes made of plant PSI [ 13 ] can generate unprecedented high photovoltages (well above 10 V) when placed on a conducting solid surface and measured by Kelvin probe force microscopy (KPFM). The measured photovoltages give rise to internal electric fi elds as large as 100 kV cm − 1 , which are among the highest values ever reported in an inorganic material system. We suggest that these high values result from depletion of the P700 electron donor centers and trapping centers in the PSI complex. We also present an equation depicting the dependence of the large photovoltage phenomenon in plant PSI crystals on light intensity. The unprecedented photovoltage, the microscale dimensions, and the high quantum yield make the PSI crystals intriguing units for potential use in optoelectronic devices.Photosynthetic reaction centers such as PSI evolved over a billion years to become highly effi cient light energy converters.The nanosized membrane complex containing chlorophyll molecules can generate a photopotential of approximately 1 V with a quantum effi ciency of ≈ 100%. [ 11 ] In the crystals of plant PSI, the complexes are serially oriented in stacks of over several hundred layers ( Figure 1 ) that absorb most of the visible light. [ 12 -14 ] We show that such dried PSI crystals generate very large photovoltages compared to any semiconductor, ferroelectric, and/or organic crystals.Each PSI complex consists of 18 polypeptides including four light-harvesting protein chlorophyll complexes binding a total of over 170 light-harvesting chlorophyll and over 30 carotenoid molecules. [ 14 ] The electron transport chain contains a chlorophyll a special pair (P700), four monomeric chlorophyll a molecules, two phylloquinones, and three [4Fe-4S] iron-sulphur clusters (Figure 1 a). P700, which undergoes light-induced charge separation in 1 ps, transfers an electron across the protein through the sequential carriers within 250 ns to the fi nal acceptor. [ 11 ] The redox potential of the primary donor P700 is + 0.43 V and that of the fi nal acceptor (4Fe-4S) F B is -0.53 V, producing a red...

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