Mossbauer spectroscopy is a technique of nuclear physics which is particularly well adapted to probe the chemical behavior of iron via the electric and magnetic interactions between the 57 Fe nucleus and the atomic electrons. Applied to the study of hemoproteins and synthetic analogues, it can provide in many cases information not available by other means. Continuing and extending our present methods of measurement and analysis, the technique will be used to probe the active site conformations and iron states in terms of its electronic wave functions. An understanding of the iron-free and oxygenated sites of hemoglobin and myoglobin at this basic level will facilitate the use of the Mossbauer technique to examine specific questions relating to cooperative oxygenation and allosteric interactions in general. We intend to exploit the possibilities opened up by our low temperature photodissociation work, and the quaternary structure control possible in modified and artifical hemoproteins in this work. The specificity of the Mossbauer technique for the 2% abundant isotope, 57Fe, allows the use of this technique to examine selectively labeled hemoglobin subunits in tetramers which are in all chemical respects identical to the natural materials. Thus the question of subunit inequivalence may be studied in an unambiguous way. In addition to the main effort, devoted to physiologically relevant states of hemoglobin and myoglobin, we intend to continue our line of investigation into the nature of both ferrous and ferric intermedite spin hemoproteins (peroxidases, cytochrome c') and their synthetic analogues, and to participate in the investigation of cytochrome c function via model compound studies. Both protein and model compound studies will be carried out in collaboration with established workers in these fields.