Multifrequency electron
paramagnetic
resonance (EPR) spectra of the Cu(II) site in nitrous oxide reductase (N2OR)
from Pseudomonas stutzeri confirm the assignment of the low
field
g
value at 2.18 consistent with the seven line pattern observed at 9.31
GHz,
10 K. S-band spectra at 20 K are better resolved than the X-band
spectra
recorded at 10 K. The features observed at 2.4, 3.4, 9.31 and 35 GHz
are
explained by a mixed-valence [Cu(1.5)..Cu(1.5)] S = 1/2 species
with the unpaired electron delocalized between two equivalent Cu
nuclei.
The resemblance of the N2OR S-band spectra to
the
spectra for the EPR-detectable Cu of cytochrome c oxidase
suggests
that the S-band spectrum for cytochrome c oxidase measured
below
30 K may also contain hyperfine splittings from two approximately
equivalent
Cu nuclei.