Near infrared transient absorption spectra of photolyzed myoglobin at low temperatures

We have measured the transient absorption spectrum of photolyzed sperm whale carboxymyoglobin as a function of temperature and time after photolysis. Our measurements extend from 60K to 220K and from three microseconds to thirty seconds. We see an absorption band near 758 nm which is known as band VIII. Three microseconds after photolysis at 60K, the center of band VIII is 766 nm, red-shifted from the peak seen in deoxy myoglobin at this temperature. Band VIII shifts with time towards the deoxy value with a nonexponential time course. We conclude that this shift is caused by unrelaxed degrees of freedom in the protein at these temperatures. We present a model in which each protein has several "blocking residues" which relax following photolysis by surmounting enthalpy barriers which range from ten to fifty kJ/mol. These motions do not affect the rate of ligand rebinding.U of I OnlyThesi