X-ray magnetic circular dichroism (XMCD) is a new experimental technique with tremendous potential for site-specific characterization of metals in clusters and complex proteins. Dr. Cramer proposes to conduct the first-ever XMCD measurements on carefully selected paramagnetic transition metal complexes, and to compare the experimental results with recently developed theoretical simulation programs. XMCD experiments will then be conducted on small proteins with known metal sites, and the results compared with data from other methods such as Mossbauer and EPR. With the experimental and theoretical basis of XMCD established, he will apply the technique to magnetic and structural questions about metal centers in the metalloenzymes nitrogenase, hydrogenase, and photosystem II. To conduct these experiments, he will employ circularly-polarized soft x-rays emitted as synchrotron radiation from electron storage rings such as the National Synchrotron Light Source. Samples will be placed in the strong magnetic field of a superconducting magnet. Photoabsorption will be measured directly by the transmission method on thin films, and indirectly using sample photocurrent or fluorescence detection techniques. Site-specific characterization of metals in nitrogenase, hydrogenase, and photosystem II will allow better understanding and synthetic modelling of their important catalytic activities. The development of this new technique will also provide the bioinorganic community with a new tool for magnetic and site- specific characterization, and the XMCD techniques will be generally applicable to nearly all paramagnetic transition metal complexes.
