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Abstract

Low temperature electron paramagnetic resonance (EPR)
spectroscopy with frequencies between 95 and 345 GHz and
magnetic fields up to 12 T have been used to study radicals and
metal sites in proteins and small inorganic model complexes. We
have studied radicals, Fe, Cu and Mn containing proteins. For
S= 1/2 systems, the high frequency method can resolve the g-
value anisotropy. It was used in mouse ribonucleotide reductase
(RNR) to show the presence of a hydrogen bond to the tyrosyl
radical oxygen. At 285 GHz the type 2 Cu(II) signal in the
complex enzyme laccase is clearly resolved from the Hg(II)
containing laccase peroxide adduct. For simple metal sites, the
systems over S = 1/2 can be described by the spin Hamiltonian:
H-S = BgS + D[S-z(2) - S(S + 1)/3 + E/D (S-x(2)-S-y(2))]. From
the high frequency EPR the D-value can be determined directly
by, (1) shifts of gff for half-integer spin systems with large
D-values as observed at 345 GHz on an Fe(II)-NO-EDTA complex,
which is best described as S = 3/2 system with D = 11.5 cm(-1),
E = 0.1 cm(-1) and g(x) = g(y) = g(z) = 2.0; (II) measuring the
outermost signal, for systems with small D values, distant of
(2S - 1)*\D\ from the center of the spectrum as observed in S =
5/2 Fe(III)-EDTA. In Mn(II) substituted mouse RNR R2 protein
the weakly interacting Mn(II) at X-band could be observed as
decoupled Mn(II) at 285 GHz. (C) 2002 Elsevier Science B.V. All
rights reserved.