The soft x-ray region between 500 and 1000 eV has been untouched for biochemical spectroscopy. This is due in part to experimental difficulties caused by the short pathlengths for x-rays in this region. However, there have been dramatic improvements in x-ray optical designs, window materials, and low energy detectors in recent years. Progress has also been made in the theoretical interpretation of L-edge spectra, such that ligand field splittings, oxidation state and spin-state can be extracted. Thus, high resolution bulk spectroscopy of 3d transition metal-containing proteins now appears both feasible and scientifically rewarding. For initial studies, we will assemble a data base of spectra for low molecular weight complexes of known structure. Experiments will then be conducted on the small proteins myoglobin, cytochrome c, and stellacyanin. Finally, we propose to examine 1) the manganese and calcium in the oxygen- evolving complex of photosystem II of chloroplasts, 2) the iron and copper in the dioxygen-reducing enzyme cytochrome oxidase, and 3) the nickel and iron sites of the membrane bound hydrogenase from Azotobacter vinelandii. The latter three enzymes can be prepared in oriented dehydrated membrane multilayers, and in the first two cases anisotropy has already been observed in EPR and K edge XANES and EXAFS spectra. These multilayer samples have relatively high metal concentration, low water contents, and vacuum stability, making them ideal candidates for spectroscopy in the vacuum x-ray region. From the interpretation of these spectra, we plan to obtain information about the ligand field, local geometry, and orientation of the metal centers.
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