High and low spin state mixture in methemoglobin and metmyoglobin.

The mixture of low and high iron spin states is studied by electron spin resonance in methemoglobin and in metmyoglobin between 6K and 100K. The crystals contain iron (Fe3) exclusively in the high spin state, while powdered samples show a mixture of high and low spin iron. We detected, for the first time, the low spin state in metmyoglobin at low temperatures. The ratio of high to low spin concentrations (k-1) varies exponentially with inverse of temperature in both proteins, only the absolute value is greater in myoglobin. The slope of K-1 depends on the cooling rate and on the temperature range. The results are qualitatively explained assuming a temperature dependent distribution of crystical field around the cristal value, delta c.