The multicopper proteins,
nitrous-oxide
reductase (N2OR) and cytochrome c
oxidase (COX), were investigated by EPR spectroscopy at microwave
frequencies
2.4 - 35 GHz. Our results support a Cu-Cu interaction in COX and N2OR.
At least 10 lines in the 2.7-GHz, 12 lines in the 4.6-GHz and 14 lines
in the 9.2 GHz spectra were resolved for N2OR.
Eight copper lines at 2.7 GHz, about nine lines at 4.6 GHz and about
six
lines at 9.2 GHz were resolved for COX. Simulations of the EPR spectra
were consistent with most of the resonances of the multiline spectra,
including
regions in the center of the spectra where overlap of the three
seven-line
patterns is proposed. These simulations indicated that Cu-Cu
interaction,
in a nuxed-valence [Cu(1.5)...Cu(1.5)],
S = 1/2 site is consistent
with, if not proof of, the unusual spectral features observed for N2OR
and COX.