Electron Spin Resonance Studies of Native and Denatured Methemoglobin

pH EFFECTS

From the ¹ From the Graduate Department of Biochemistry, Brandeis University, Waltham, Massachusetts 02154

The electron spin resonance spectra of methemoglobin and denatured methemoglobin are studied as a function of pH. Satisfactory agreement is found between the literature and these studiesregarding denaturative events occurring between pH 3 and 4, between pH 4 and 5, and between pH 10 and 11; the pK for the equilibrium between methemoglobin and methemoglobin hydroxide; and the transition temperature for the denaturation of methemoglobin as a function of pH.

Only one class of denatured species is observable by electron spin resonance at 77° K over the pH range 3 to 11. Another class believed to exist gives no electron spin resonance spectra. The former class is favored over the latter when denaturation occurs at high pH, but the distribution results from seemingly irreversible effects, and is not due to a titration characteristic of denatured methemoglobin.

We conclude that the electron spin resonance method is applicable in the study of methemoglobins over the pH range of 3 to about 12.

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