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Poster Presentation 2-34
Measurements of Light-induced Electric Potential from Photosystem I and Photosystem I Reconstituted Liposomes
Ida Lee, Tanya Kuritz, Charlene Sanders, James W. Lee and Elias Greenbaum
Chemical Sciences Division, Oak Ridge National Laboratory, P. O. Box 2008, Oak Ridge, TN 37831-6194
Telephone: (865) 241-6695; Fax: (865) 574-1275; E-mail: leei@ornl.gov
The Photosystem I (PSI) reaction center is one of the pigment-protein complexes responsible for the photosynthetic conversion of light energy to chemical energy. Using the technique of Kelvin probe force microscopy (KFM), we have measured the photovoltages generated from single PSI reaction centers in a heterostructure comprised of PSI, organosulfur molecules, and atomically flat gold. Illumination of the reaction centers was accomplished with a diode laser at wavelength 670 nm. The average values of the light minus dark voltages relative to the substrate for the four PSIs were –1.13 + 0.14V and –1.20 + 0.19V at diametrical peripheries and –0.97 + 0.04 V at the center. Under illumination, the potentials of the central region of the PSIs were typically more positive than the periphery by 6-9 kT, where kT is the Boltzmann energy at room temperature. These energies suggest a possible mechanism whereby negatively charged ferredoxin, the soluble electron carrier from PSI to the Calvin-Benson Cycle, is anchored and positioned at the reducing end of PSI for electron transfer.
We have also measured the surface potential of hydrogenated soy phosphatidylcholine/cholestrol proteoliposomes with reconstituted, functional PSI reaction centers. The surface potential of the proteoliposomes was found to be in the range of 10 – 70 mV with particle-like structures on each liposome’s surface, which is ~1 mV. This demonstrates that the KFM may have the ability to resolve individual PSI reaction centers in liposome membranes.
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