PIXE ANALYSIS OF PROTEINS FROM A PHOTOCHEMICAL CENTER

In oxygen evolving photosynthetic organisms, light is absorbed and its energy used for the conversion of chemical products in two photosyste ms: PSI and PSII. Each photosystem is composed of a protein core which binds a pigment antenna and a Reaction Center (RC), RC of PSI is cons idered an ''Iron-Sulfur'' type, There are six components that particip ate in the charge separation after light absorption occurs in PSI: the center chlorophyll P700, two accepters Ao and A(1) and three FeS cent ers F-X, F-A and F-B. However, the exact number of polypeptides, their exact molecular weight, their relative abundances and the active comp onents associated to those polypeptides remain still to be completely characterized. In particular the Fe-S centers have been difficult to d etect in a direct way in a gel band, because the amount of centers inv olved is under the detection limits of the conventional techniques. Th is study has been undertaken to explore the capability of particle ind uced X-ray emission (PIXE) to detect in a qualitative way the presence of Fe in some of the protein bands obtained by Sodium Dodecyl Sulfate Polyacrylamide Gel Electrophoresis (SDS-PAGE) from the PSI complex. T he complex was isolated from membranes of thermophilic cyanobacteria: Synechochoccus sp. The polyacrylamide gel electrophoresis of the compl ex shows eight subunits of 66, 60-65, 14, 13, 9, 8 and 7 KDa. In-air P IXE was performed at 2 MeV and proved to be an adequate tool for direc t identification of the iron present in the gel bands. (C) 1998 Elsevi er Science B.V.