The ionization of acidic metmyoglobin

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Abstract
The equilibrium constant for the ionization of (horse) acid metmyoglobin which may be represented as Fe+ (H2O)= FeOH+H+ was measured over a range of pH, ionic strength and temp. The effect of salts on this equilibrium is marked. Thus, addition of NaCl favors ionization up to an ionic strength of about 0.01, but beyond this the effect is reversed. Extrapolation to 0 ionic strength gives pK[image]o 9.038 [plus or minus] 0.030 at 20[degree] and 8.803 [plus or minus] 0.025 at 37[degree]. From the equilibrium constant at 2 temps. the ionization is found to be endothermic to the extent of 5.75 [plus or minus] 0.67 kg. cal./mole. [DELTA] S[degree]20 = -21.7 [plus or minus] 2.4 cal./mole deg. It is only possible to measure this ionization constant within the pH range 7.8-9.2. In more alkaline or acidic solns. significant interaction takes place between this equilibrium and, presumably, other ionizations in the molecule, and the value of the ionization constant falls steadily with increasing pH. This effect is suppressed at high ionic strengths, which suggests specific interaction between electrolyte ions in soln. and the charged groups on the protein.
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