Electron paramagnetic resonance studies of hole mobility and localisation in haemoglobin

Abstract

In our electron paramagnetic resonance studies of the effects of ionizing radiation on a range of proteins, we have drawn the important general conclusion that ejected electrons can migrate over large distances, and are trapped selectively at electron-affinic sites, whereas the electron-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 froms of myoglobin and haemoglobin. In these studies, radiation converted Fe^III centres to Fe^II centres, and FeO₂ centres into FeO^–₂ centres in high yields, showing the efficiency of electron migration and specific capture. However, working with somewhat different systems, one of us clearly observed the production of Fe^III centres on irradiation and hence interpreted the results in terms of long-range hole migration.

Our present aim was to work together to see if we could discover some fault in one or other system that would either disprove or support the theory of rapid hole mobility.

The results confirm that, for pure isolated haemoglobin, only electrons are free to migrate, and the holes are rapidly trapped by proton loss. However the conflicting results are also correct in the proper context: irradiation of red blood cells does generate Fe^III centres efficiently. The results are discussed in terms of the possible presence of the Fe^IV state in these systems, or of some [Fe^III—Fe^III] dimer species which is EPR silent, but which forms [Fe^II—Fe^III] units on electron capture, thereby giving detectable Fe^III signals.