Elektron transfer effects in metalloproteins: An ESR study.

The metal chromophore in metalloproteins is identified as a high affinity site for dry electrons in conformity with previous studies. Relative electron affinities for such centres in a physiologically probable valence hybrid haemoglobin have been determined. A ratio of ca. 2.5 for the electron affinity of oxyhaemog1obin over methaemog1obin is explained in terms of structural differences in the constituent forms. The phenomenon of intersubunit electron transfer in similar systems is considered in terms of haem edge-to-edge minimum separation. The hydrogen peroxide complex of iron haemoproteins is characterised as an oxo compound of iron in the +4 oxidation state (ferryl), by electronic spectroscopy and ESR spectroscopy using low temperature ?-irradiation. Variations in ESR parameters during annealing are explained in terms of structural changes at the haem site. The electron-loss centre in many proteins, consequent upon ?-irradiation, is identified as the polypeptide amide nitrogen. The nitrogen-centred radical first formed may undergo hydrogen transfer reactions to give a terminal carbon-centred radical. ESR features for the nitrogen-centred radical are interpreted in relation to predominant secondary structures in the proteins examined. The catalytic mechanism of xanthine oxidase has been studied by the use of dry electrons as the reducing substrate. The series of intermediates obtained through annealing have ESR parameters identical with those for the enzyme-substrate intermediates obtained by other workers using chemical reducing substrates. A scheme of intramolecular electron transfer is proposed.