The relative contributions to changes in visible and near UV circular dichroism spectra of hemoglobin of heme ligation and tertiary and quaternary conformational transitions were separated by exploiting the slowing down of structural relaxations for proteins encapsulated in wet, nanoporous silica gels. Spectral signatures, previously assumed to be characteristic of T and R quaternary states, were demonstrated to be specific to different tertiary conformations. The results support the view that ligation and allosteric effectors can modulate the structural and functional properties of hemoglobin by regulating the equilibrium between the same tertiary species within both quaternary states.