Abstract
Low-temperature Mössbauer and Electron Spin Resonance (ESR) studies on paramagnetic horse methemoglobin demonstrate the existence of a low spin state of hemoglobin which is not identical to the typical Hb+-OH low spin state. The amount of this species was found to increase when samples were gradually cooled instead of being quenched into liquid nitrogen. This state is found in the pH range from 6.0 to 8.9, although an increase of the relative population of this state is found below pH 6.9. This species is formed from Hb+-H2O and Hb+-OH. Temperature studies indicate that this species can be reversibly formed from the usual high spin and low spin species even at 210 K, where water is already completely frozen, ruling out a direct perturbation due to the freezing process. These results are interesting in terms of conformational equilibrium in the region of the heme pocket. The relative intensity of the various species is explained in terms of energy barriers between the different conformations which limit the rate at which the usual high spin and low spin states are converted to this state.
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