MOLECULAR ELECTRONICS OF A SINGLE PHOTOSYSTEM-I REACTION-CENTER - STUDIES WITH SCANNING-TUNNELING-MICROSCOPY AND SPECTROSCOPY

Thylakoids and photosystem I (PSI) reaction centers were imaged by sca nning tunneling microscopy. The thylakoids were isolated from spinach chloroplasts, and PSI reaction centers were extracted from thylakoid m embranes. Because thylakoids are relatively thick nonconductors, they were sputter-coated with Pd/Au before imaging. PSI photosynthetic cent ers and chemically platinized PSI were investigated without sputter-co ating. They were mounted on flat gold substrates that had been treated with mercaptoacetic acid to help bind the proteins. With tunneling sp ectroscopy, the PSI centers displayed a semiconductor-like response wi th a band gap of 1.8 eV. Lightly platinized (platinized for 1 hr) cent ers displayed diode-like conduction that resulted in dramatic contrast changes between images taken with opposite bias voltages. The electro nic properties of this system were stable under long-term storage.