ELECTRON-PARAMAGNETIC-RESONANCE STUDIES OF HOLE MOBILITY AND LOCALIZATION IN HEMOGLOBIN

In our electron paramagnetic resonance studies of the effects of ioniz ing radiation on a range of proteins, we have drawn the important gene ral conclusion that ejected electrons can migrate over large distances , and are trapped selectively at electron-affinic sites, whereas the e lectron-loss centres, or hole centres, are rapidly trapped within the polypeptide backbone of the protein by proton loss from amide nitrogen . This view seemed to be supported in particular by work on various fo rms of myoglobin and haemoglobin. In these studies, radiation converte d Fe(III) centres to Fe(II) centres, and FeO2 centres into FeO2- centr es in high yields, showing the efficiency of electron migration and sp ecific capture. However, working with somewhat different systems, one of us clearly observed the production of Fe(III) centres on irradiatio n and hence interpreted the results in terms of long-range hole migrat ion. Our present alm was to work together to see if we could discover some fault in one or other system that would either disprove or suppor t the theory of rapid hole mobility. The results confirm that, for pur e isolated haemoglobin, only electrons are free to migrate, and the ho les are rapidly trapped by proton loss. However the conflicting result s are also correct in the proper context: irradiation of red blood cel ls does generate Fe(III) centres efficiently. The results are discusse d in terms of the possible presence of the Fe(IV) state in these syste ms, or of some [Fe(III)-Fe(III)) dimer species which is EPR silent, bu t which forms [Fe(II)-Fe(III)] units on electron capture, thereby givi ng detectable Fe(III) signals.