A. Ivancich et al., Multifrequency high-field EPR study of the tryptophanyl and tyrosyl radical intermediates in wild-type and the W191G mutant of cytochrome c peroxidase, J AM CHEM S, 123(21), 2001, pp. 5050-5058
Multifrequency (95, 190, and 285 GHz) high-field electron paramagnetic reso
nance (EPR) spectroscopy has been used to characterize radical intermediate
s in wild-type and Trp191Gly mutant cytochrome c peroxidase (CcP). The high
-field EPR spectra of the exchange-coupled oxoferryl-trytophanyl radical pa
ir that constitutes the CcP compound I intermediate [(Fe(IV)=O) Trp(.+)] we
re analyzed using a spin Hamiltonian that incorporated a general anisotropi
c spin-spin interaction term. Perturbation expressions of this Hamiltonian
were derived, and their limitations under high-field conditions are discuss
ed. Using numerical solutions of the completely anisotropic Hamiltonian, it
s was possible to simulate accurately the experimental data from 9 to 285 G
Hz using a single set of spin parameters. The results are also consistent w
ith previous 9 GHz single-crystal studies. The inherent superior resolution
of high-field EPR spectroscopy permitted the unequivocal detection of a tr
ansient tyrosyl radical that was formed 60 s after the addition of 1 equiv
of hydrogen peroxide to the wild-type CcP at 0 degreesC and disappeared aft
er 1 h. High-field EPR was also used to characterize the radical intermedia
te that was generated by hydrogen peroxide addition to the W191G CcP mutant
. The g-values of this radical (g(x) = 2.00660, g(y) = 2.00425, and g(z) =
2.00208), as well as the wild-type transient tyrosyl radical, are essential
ly identical to those obtained from the high-field EPR spectra of the tyros
yl radical generated by y-irradiation of crystals of tyrosine hydrochloride
(g(x) = 2.00658, g(y) = 2.00404, and g(z) 2.00208). The low g(x)-value ind
icated that ail three of the tyrosyl radicals were in electropositive envir
onments. The broadening of the g(x) portion of the HF-EPR spectrum further
indicated that the electrostatic environment was distributed. On the basis
of these observations, possible sites for the tyrosyl radical(s) are discus
sed.