Resonance Raman studies of methemoglobin derivatives at room temperature and 77 K.
Raman spectra in preresonance with the Soret absorption band are reported for the following methemoglobin derivatives: cyanide, cyanate, thiocyanate, hydroxy-, azide, and fluoride methemoglobin at 285 K and 77 K. For the mixed-spin derivatives, Raman intensity is observed to shift from the high-spin marker band (approx. 1480 cm-1) to the low-spin marker band (approx. 1505 cm-1) upon cooling to 77 K. In addition, Raman spectra of cyanate methemoglobin were taken as a function of temperature, and the log of the intensity ratio I1480/I1505 was found to be a linear function of 1/T, indicating a thermally activated process. We interpret these results as observations of temperature-induced spin transitions. In the case of cyanate methemoglobin we find the enthalpy and entropy differences between the high-spin and low-spin states to be deltaH = 600 +/- 40 cal x mol-1 and deltaS = 4.7 +/- 0.7 cal x mol-1 x K-1. The high-spin to low-spin ratio for cyanate methemoglobin determined by our experiment disagrees with the value reported for magnetic susceptibility measurements.
D006801:Humans; D008706:Methemoglobin / Q000031:analogs & derivatives*; D013059:Spectrum Analysis, Raman; D013696:Temperature; D013816:Thermodynamics
D008706:Methemoglobin