Almost all biochemical studies of protein structure and function utilize spectroscopic probes of one sort or another. For metalloproteins the active site can often be monitored directly by optical spectroscopy or by magnetic resonance techniques. This research aims at achieving a full utilization of such probes through theoretical calculations of the electronic structures of the active sites of heme proteins and iron-sulfur proteins. The principal computational tool will be molecular orbital calculations using the Xa scattered wave model, an approach that is especially appropriate for transition metal complexes. Studies on the heme proteins will concentrate on deoxy- and met-hemoglobin, and other paramagnetic species, thus complementing earlier work on diagmagnetic hemes. We will attempt to assign observed electronic transitions and to provide a specific model for the spin distribution seen in Mossbauer, nmr and esr spectra. Calculations on the iron sulfur proteins will extend previous work on 1Fe and 2Fe clusters to the 4Fe4S* moieties found in a variety of proteins. This will provide for the first time a unified theoretical picture of these active sites. Attention will be paid to the effects of distortions from idealized symmetry as a mechanism by which the irons may be rendered spectroscopically inequivalent, and structural and electrostatic perturbations caused by the protein environment will be considered, with special attention paid to the effect of NH-S hydrogen bonds. Finally, the possible participation of these clusters in hydrogenase activity will be investigated.