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The results of electron-nuclear double resonance and electron paramagnetic resonance (EPR) studies on the hydrogen peroxide compound of yeast cytochrome c peroxidase are inconsistent with previous proposals for the source of the EPR signal in this compound, in particular with its identification with an aromatic amino acid radical such as would arise by oxidation of a tryptophanyl side chain. The present observations lead us to propose that the EPR signal is associated with a cluster containing at least one methionine and in which proximate side chains share the charge created by loss of one electron.

The reversible dioxygen adduct of manganese porphyrin (Mn(Por)) is five coordinate and of intermediate spin state [S = 3/2) unlike the adducts of cobalt, iron, and chromium porphyrins. We have prepared a series of Mn(Por)(02) complexes, where Por = para-substituted tetraphenylporphyrin ([T(p-X)PP]; X = F, H, ti-BuO) or octaethylporphyrin (OEP), and have examined their magnetic properties. The EPR parameters of the four complexes are qualitatively similar, but do differ within narrow limits: -D ~2-3.2 cm-1; E/D ~V3; -/t(55Mn) ~52-54 X 10-4 cm-1; -5(55Mn) ~86-88 X 10-4 cm-1. We have also remeasured the effects of 170 substitution, and at low temperature (~10 K) find that it is possible to obtain the value of the small l70 hfs constant: (|70) ~2.3 X 10_4cm_1. Solely on the basis of the magnetic resonance parameters obtained, it is possible to rule out most possible combinations of an odd-electron configuration and Mn-02 binding geometry. These results and qualitative bonding considerations support the original description in terms of a Mnlv-022~valency formalism with a symmetric, edge-on dioxygen.

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