Relaxation times have been
obtained
with time-domain EPR for the dinuclear mixed valence [CuA(1.5) ...
CuA(1.5)]
S = 1/2 center in nitrous oxide reductase, N2OR,
from
Pseudomonas stutzeri, in the Tn5 mutant defective in copper
chromophore
biosynthesis, in a synthetic mixed valence complex, and in type 1 and 2
copper complexes. Data confirmed that the intrinsic electron
spin-lattice
relaxation time, T1, for N2OR
in the temperature range of 6-25 K is unusually short for copper
centers.
At best, a twofold increase of T1 from g^
to g|| was measured.
Optimized fits
of the saturation-recovery data were obtained using both
double-exponential
and stretched-exponential functions. The temperature dependence
of
the spin-lattice relaxation rate of mutant N2OR
is
about T 5.0 with the
stretched-exponential
model or T 3.3 and T 3.9
for the model using the sum of two exponentials. These T1s
are intrinsic to the mixed valence [CuA(1.5) ...
CuA(1.5)]
center, and no interaction of the second copper center in wild-type N2OR
with the [CuA(1.5) ... CuA(1.5)]
center has been observed. The T1 of the
mixed
valence center of N2OR is not only shorter than
for
monomeric square planar Cu(II) complexes, but also shorter than for a
synthetic
mixed valence complex, Cu2{N[CH2CH2NHCH2CH2NHCH2CH2]3N}.
The short T1 is attributed to the
vibrational
modes of type 1 copper and/or the metal-metal interaction in [CuA(1.5)
... CuA(1.5)].