SCANNING-TUNNELING-MICROSCOPY OF A MONOLAYER OF REACTION CENTERS

Langmuir-Blodgett monolayers of photosynthetic reaction centres from Rhodobacter sphaeroides have been studied by scanning tunnelling microscopy. Freshly deposited films were studied both in the dark and in the light. In the dark, images revealed molecular structure with 64 angstrom and 30 angstrom periodicities, which correspond to protein and sub-unit sizes known from X-ray crystallography, while no periodic structure appeared in the light due to the tip action on the excited proteins. STM voltage-current measurements showed the charge separation in single protein molecules in the film and their different behaviour in the dark and light. Together with surface potential measurements at the macroscopic level, they indicated the preservation of reaction centre activity in the monolayer. By fixing the protein layer with glutaraldehyde, it was possible to prevent the perturbing tip action and obtain a periodic molecular structure with 30 angstrom spacing even in the light. After heating at 150-degrees-C, the unfixed film reorganized itself into a long-range ordered state with a hexagonal structure of 27 angstrom spacing but with no activity.