Tertiary-Structure Relaxation in Hemoglobin: A Transient Raman Study

Tertiary relaxation in photodissociated hemoglobins has been monitored by using time-resolved resonance Raman scattering. It is observed that the submicrosecond tertiary-structure relaxation which precedes the switch in quaternary structure is sensitive to factors influencing R state stability. At pH 7 and lower, substantial relaxation involving the iron-proximal histidine linkage occurs over the time scale of geminate recombination. Because the structural parameter associated with the variation in this linkage has been implicated in the protein modulation of ligand binding, this result supports the idea of a dynamic barrier which increases on the time scale of geminate recombination. On the basis of these findings a model is presented which accounts for solution-dependent variations in ligand reactivity within a given quaternary structure.