Thylakoids and photosystem I (PSI) reaction centers were imaged by scanning tunneling microscopy. The thylakoids were isolated from spinach chloroplasts, and PSI reaction centers were extracted from thylakoid membranes. Because thylakoids are relatively thick nonconductors, they were sputter-coated with Pd/Au before imaging. PSI photosynthetic centers and chemically platinized PSI were investigated without sputter-coating. They were mounted on flat gold substrates that had been treated with mercaptoacetic acid to help bind the proteins. With tunneling spectroscopy, the PSI centers displayed a semiconductor-like response with a band gap of 1.8 eV. Lightly platinized (platinized for 1 hr) centers displayed diode-like conduction that resulted in dramatic contrast changes between images taken with opposite bias voltages. The electronic properties of this system were stable under long-term storage.The photosynthetic apparatus in green plants and algae is contained in a unique cellular organelle, the chloroplast. The chloroplast is enclosed by a double membrane and contains thylakoids, which consist of stacked membrane disks (grana) and unstacked membrane disks (stromal region). Although scanning tunneling microscopy (STM) images of disrupted chloroplasts have been reported (1, 2), functional characterization of single isolated reaction centers has not yet been achieved. Thylakoids play an important role in electron transport during photosynthesis. With STM and scanning tunneling spectroscopy (STS), it is possible to investigate their operational electrooptical properties.Photosystem I (PSI) reaction centers are embedded in the thylakoid membrane. They drive the light-dependent transfer of electrons from plastocyanin (a copper-containing soluble protein located in the luminal space of chloroplast thylakoids) to ferredoxin (a [2Fe-2S]-containing soluble protein located in the chloroplast stroma). The PSI complex contains two high molecular mass subunits, the products of the psaA and psaB genes (the gene products for the chlorophyll a protein of PSI), and many low molecular mass subunits (3, 4). The PSI reaction center measured by electron microscopy is about 10 nm x 15 nm (5-7), or 7 nm x 12 nm after correction for attached detergent. Alekperov et al. (8) used STM to image photosynthetic reaction centers of purple bacteria and used the tip to transfer molecules or clusters of molecules from one area to another within the scanning range.The attachment of platinum on the reducing side of PSI has a significant effect on the electrical properties of PSI. This can be observed by transformed photobiocatalytic properties and a sustained steady-state vectorial flow of current (9, 10). In this paper, we describe the use of tunneling spectroscopy to characterize the electrical nature of bare and platinized PSI. For semiconductors, tunneling spectroscopy has been used to characterize surface states and to measure surface-state band gaps (11,12). We show that a band gap can be clearly seen in bare PSI, and platinized PSIs...

Although both processes of vision and photosynthesis are initiated by absorption of visible light, they are chemically and energetically different. In vision, light triggers a thermodynamically downhill reaction that is preloaded by dark metabolism. Photon absorption by rhodopsin activates a G-protein cascade leading to cyclic guanosine monophosphate (cGMP) hydrolysis, which in turn closes cation-specific channels to generate a nerve signal. In photosynthesis, on the other hand, absorption of photons by the special reaction center chlorophylls in Photosystems I and II (PSI, PSII) triggers charge separations across the photosynthetic membrane. This charge separation generates a voltage that is the source of Gibbs energy for the thermodynamically uphill reactions of green plant photosynthesis: oxidation of water to molecular oxygen and reduction of atmospheric carbon dioxide to sugars. We have presented a hybrid system, a new reaction that demonstrates photoactivation of mammalian cells with a plant photosynthetic light sensory system [1]. In this scheme, mammalian cells that possess no photoactivity are changed into photosensitive cells after the treatment with the PSI reaction centers, which were delivered by PSI proteoliposomes.By using the scanning surface probe microscopy (SSPM) as a diagnostic tool, we report here the measurements of the surface potential of hydrogenated soy phosphatidylcholine/cholestrol proteoliposomes with reconstituted, functional photosystem I reaction centers. The PSIproteoliposomes were imaged with the combined techniques of tapping-mode atomic force microscopy (AFM) and SSPM, illustrated in Fig. 1 and Fig.2. The apparent range of liposome diameters was 70-100 nm. The AFM-SSPM technique uses a slender cantilever probe with a slightly blunt apex. It provides accurate voltage measurements but exaggerates lateral dimensions. The theory for this technique has been developed by Jacobs et al. [2]. provide additional information on the techniques used for working with single PSI reaction centers. The oneto-one correspondence between the AFM and SSPM liposome images is evident in Fig. 1 anf Fig.2. The images were obtained under illumination with a diode laser at 670 nm, near the absorption maximum of chlorophyll (671 nm) in PSI-proteoliposomes. The AFM image of Fig. 1 illustrates the gross geometric structure of the PSI-proteoliposome, whereas the electrostatic SSPM image [ Fig. 2] reveals a finer grained pebble-like structure in the surface potential map, suggesting a close packing of the PSI reaction centers in the liposome membrane. 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.