We use the low-temperature recombination kinetics of carbon monoxide with carp hemoglobin to determine that the R and T states of hemoglobin exhibit different low-temperature geminate recombination kinetics. The peak of the fitted Gaussian activation energy spectrum is at 1.5 kcal/mol for R state and 1.8 kcal/mol for T state. The distribution in activation energies is fit well by the Agmon-Hopfield linear strain model. The T state is fit with a stronger elastic constant than R, and has a larger displacement of the protein conformation coordinate than does the R state, indicating that the T state does have a significantly greater rigidity and also stores more strain energy in its conformational states than does R hemoglobin. The pre-exponential in the activation energy spectrum is only a factor of two greater in the R than the T state and the low-temperature activation energy spectrum does not correctly predict the difference in the on rates for R and T states at 300 degrees K, indicating that processes removed from the binding site are important in cooperativity.
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