Temperature dependent quaternary state relaxation in sol-gel encapsulated hemoglobin

Samples of human adult hemoglobin (HbA) encapsulated in a wet porous sol-gel are prepared under aerobic and anaerobic conditions. Resonance Raman spectroscopy is used to compare equilibrium deoxyHbA to the nonequilibrium deoxy species generated by deoxygenating an encapsulated oxyHbA sample. The spectra of the deoxygenated samples as a function of delay subsequent to deoxygenation reveal a marked slow down by the gel of the two phases of relaxation: the tertiary relaxation associated with the transition from the liganded R to deoxy R conformations and the quaternary relaxation associated with the deoxy R to deoxy T transition. The temper-ature dependence (4-80°C) of the relaxation indicates that the internal viscosity of the gel is greatly enhanced at the lower temperatures. At 80°C the tertiary and quaternary relaxations occur over minutes to hours, respectively, whereas at 4°C both relaxations are essentially frozen. These results demonstrate the impressive potential of using sol-gel encapsulation as a means of studying substrate binding induced conformational changes in proteins.