The common valence states and electron configurations of iron are ferrous, Fe(II)3d6, and ferric, Fe(III)3d5. Hemoglobin and myoglobin carry out their physiological roles with the metal remaining ferrous. Iron in the cytochromes cycles between ferrous and ferric. In the “resting” state, the iron in peroxidases and catalase is ferric. These enzymes gain one or two oxidizing equivalents after reacting with peroxides, and higher oxidation states for the iron in the catalytic intermediates have been discussed, Fe(IV)3d4 in the form of the ferryl ion Fe(IV)o2+2, and Fe(V)3d3. Compatible with the ferrous configuration are three spin states, S = 2,1,0 (diamagnetic) of which the first and last are found for deoxygenated and oxygenated hemoglobin (and myoglobin) respectively (Chap. 6-3). Similarly, three spin states are possible for the ferric configuration, S = 5/2 (high-spin), 3/2 (mid-spin), and 1/2 (low-spin). Loew (Harris, 1968a,b; Har-ris-Loew, 1970) has reviewed and extended the theory of spin state stability for the ferric ion in tetragonal complexes; the high- and low-spin states of ferric heme are greatly favored over the state of intermediate spin and over spin-mixed states. In ferrous hemoglobin and myoglobin, coordination numbers of five and six are associated with the high-spin (as in deoxy-hemoglobin) and low-spin (as in oxy-hemoglobin) states respectively. The ferric ion in hemeproteins is six-coordinated; changes in spin-state accompany changes in the sixth position, the ligand at that position producing a ligand field strength in accordance with its location in the spectrochemical series (Cotton and Wilkinson, 1972; Phillips and Williams, 1966). In small heme compounds (Hoard, 1968) and in hemeproteins (Chap. 2-2c), displacement of the iron from the heme plane is associated with the spin state changing from low to high.
KeywordsSpin State ENDOR Spectrum Mossbauer Spectrum Polarization Ratio Heme Plane
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