The overall thesis of this Program is that hemoglobin (Hb) provides a well-defined, tractable "molecular workbench" which can be used to develop the rules by which oligomeric protein complexes undergo allosteric regulation. A central goal is to test and further develop a predictive stereochemical model that was formulated during the last funding period. The model describes how specific residues propagate ligation-induced changes in heme structure to the subunit-subunit interfaces and thereby promote the transitions from a low affinity T structure to higher affinity T-like structures, and finally to a range of R quaternary structures. The specific aims of this Project are to capitalize on advances in electron-nuclear double resonance (ENDOR) spectroscopy to address two basic issues. i) How are the critical interfaces coupled to ligand binding events at the hemes: What structural perturbations are induced at the liganded and liganded hemes of Hb ligation intermediates, and how do they function to modulate ligand affinity. ii) By measuring the actual distances between the heme-iron and critical heme-pocket residues, we will be able for the first time to accurately probe the tertiary-structure switch points of the Hb ligation pathway, and in so doing will suggest ways to test and refine the stereochemical allosteric model.