Metal Coordinated Phenoxyl Radicals - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds - page 285

0.6 0.2

0.5

0.15

0.4

0.3 0.1

A

0.2

0.05

0.1

0.0

400 500 600

l (nm)

700 800 900

Figure 8.4 UV-Vis spectra of: solid lines: complex (b) depicted in Figure 8.2 (0.11mM in acetonitrile)
(see Table 8.6 for references and nomenclature). Dotted lines: the radical of the free ligand (0.24mM in
dichloromethane).Forthislatterspeciestheabsorbanceisunderestimatedbecauseofitsinstability(unpublished
results).

achieved by increasing the frequency (higher than 95 GHz). 41,42 As an example, the three principal g-tensor
components of the tri- tert -butylphenoxyl radical are 2.0072, 2.0043 and 2.0024 (g average = 0046). While

the g y and g z values are usually similar in all the phenoxyls (and close to the free electron g e value of
2.0023 for g z ), the g x values are strongly dependent on the electrostatic environment around the oxygen
atom; when it is hydrogen bonded to a positively charged ammonium group, the g x component can be as
low as 2.0064. 43,44
Coordination to a metal ion strongly affects the EPR spectrum of phenoxyls. Diamagnetic metals lead to

2 .

(S / 2 ) systems. For phenoxyl radicals containing diastereotopic benzyl hydrogen atoms, strong hyperfine

coupling to one hydrogen atom is usually observed, together with an additional interaction with the metal
nuclear spin if this latter is different from zero. In the presence of a paramagnetic metal ion, the system
has to be described by a total spin state, S t , arising from exchange and dipolar interactions between the
metal and radical electron spins. That results in profound changes in the EPR spectra, as shown below.

=

1

8.3 Occurrence of tyrosyl radicals in proteins

Since the 1970s it has been recognized that stable radicals exist in enzymes and that they are involved
in numbers of biological processes. Cysteinyl (RNR class I and II), glycyl (anaerobic RNR class III) and
tryptophanyl (cyt c peroxydases) radicals are all known, but the most representative family is that of the
tyrosyl radicals (photosystem II, Prostaglandine synthase, galactose oxidase, glyoxal oxidase etc.). 45 It is
noteworthy that nearly all of these proteins are metalloenzymes, and so the organic cofactor is believed to
complete or assist the metal-driven electron transfer. While most of the above mentioned enzymes involve
a free or hydrogen bonded radical, galactose oxidase (GO) and glyoxal oxidase are unique in that the
radical is coordinated to a metal ion. 46
Galactose oxidase catalyses the oxidation of a wide range of primary alcohols to their corresponding
aldehydes, with concomitant reduction of molecular oxygen to hydrogen peroxide. The active site of
GO comprises a copper atom in a distorted square pyramidal geometry with Tyr495 occupying the axial