Nanoscale Photosynthesis, the Photophysics of Neural Cells, and Artificial Sight

Elias Greenbaum Oak Ridge National Laboratory and The University of Tennessee, P.O. Box 2008, Bethel Valley Road, Oak Ridge, TN 37831-6194, U.S.A. e-mail: exg@ornl.gov URL: http://www.ornl.gov/awards/cf/cfcitations/cfbios/greenbau.htm

This presentation provides an overview of recent progress in molecular electronics and green plant photosynthesis. We have demonstrated that the isolated Photosystem I (PSI) reaction center is a nanoscale molecular diode and photovoltaic device. One goal of our research is construction of molecular electronic devices from these nanoscale structures. Progress has been demonstrated by direct electrical contact of emergent electrons from the PSI reaction center by nanoparticle platinum precipitation. The stable diode properties of isolated reaction centers combined with the ability to orient PSI by self-assembly on a planar surface makes this structure a potential building block for 2-D and possibly 3-D devices. A second goal of our research is restoration of photoreceptor activity to people who are blind from retinal degenerations such as age-related macular degeneration or retinitis pigmentosa. We extracted and purified integral membrane PSI reaction centers from spinach leaves and measured their open and closed circuit photovoltages. The open circuit value is at least 1 V whereas the closed circuit value is at least 0.6 V. A quantitative analysis of the known physical properties of PSI reaction centers and voltage-gated ion channels indicates that PSI may be able to optically trigger opening of the channels. The cell membrane can be depolarized or hyperpolarized depending on the orientation of the PSI reaction center in the membrane. PSI-proteoliposomes will be used as the delivery vehicle. We have successfully inserted PSI reaction centers into liposome membranes and, using P700 absorption spectroscopy, demonstrated that the reaction centers retain their functional activity in the liposomes. We have also obtained microscopic evidence that the liposomes are capable of fusing with the membranes of retinoblastoma cells.