Absorption spectra, phenoxyl radicals

The formation of coordinated phenoxyls in the monocations and dications, [Fe(L )]+ and [Fe(L )]2+, is clearly demonstrated by their electronic spectra (142). Fig. 23 displays the spectra of [Fem(LBuMet)]°, [Fe(LBuMet )]+, and [Fem(LBuMet )]2+. Since the spectrum of the neutral tris(phenolato)iron(III) species shows an absorption minimum at -400 nm it is significant that the monocation and dication both display a new intense asymmetric maximum in this region. This intense maximum is the fingerprint of phenoxyl radicals. It is also remarkable that this maximum doubles in intensity on going from the monocation to the dication. On increasing the oxidation level stepwise, the phenolate-to-iron CT band experiences a batho-chromic shift from 513 nm in the neutral species to 562 nm in the monocation and... 

PCP-phenolic radical that possesses an absorption spectrum in H2O at 440 nm and decays via second-order kineties with k = 9. x 10 /M/s, as evidenced by pulse radiolytie studies. The PCP-phenolic radical can attach covalently to the C8-site of dG to generate the C8-OPCP adductIn the presence of GSH, redox cyeling of the phenoxyl radieal with thiyl radieal generation will yield a GSH disulfide anion radieal that ean reduetively aetivate O2 to generate the superoxide radieal anion (02 ) that ean generate free ferrous iron Free Fe " " may... 

From pulse radiolysis lifetimes of phenol radical cations between 300 and 500 ns are known [4, 9]. Laser photolysis (3 ns, 266nm up to 15 mJ) ofN2-purged solutions of up to 10 3 mol dm 3 phenols yields phenoxyl radicals as dominating products (Figure 2). In the spectrum only a little hint for the phenol radical cations exists. The inset shows that the phenoxyl radical formation does not depend linearly from the energy but appears by biphotonic absorption contradictory to the fs-experiments described above. 

Fig. 16. Spectroscopic characterization of the oxidized apogalactose oxidase free radical, (a) Optical absorption spectrum for the radical-containing apoprotein, (b) X-band EPR spectrum of the metal-free protein following Ir(IV) oxidation, (c) Expansion of the region near g = 2 comparing experimental data (Exp) with a theoretical simulation (Sim) based on coupling of the unpaired electron spin with one and one Hp proton of a tyrosine phenoxyl. Simulation parameters g = 2.0017, g2 = 2.0073 Ai Ha) = 8.4 G, A2(Hc,) = 8.8 G di(Hp) = 12.7 G, A2(Hp) = 13.8 G.

In a more recent study pulse radiolysis was utilized to produce the phenoxyl radical in the gas phase and to measure some reaction kinetics. The irradiated gas mixmre contained mainly SFe (at 980 to 1000 mbar), which served as a source of F atoms. Phenol was present at 0.1 mbar. The rate constant for reaction of phenol with F atoms was determined to be 1.9 X 10" M s. This reaction led to formation of the phenoxyl radical (45%) and other products, probably fluorine-adducts to the ring. When HCl (20 mbar) was added to the mixture, most fluorine atoms reacted with HCl to produce chlorine atoms and these reacted with phenol to produce the phenoxyl radical as the predominant product. The rate constant for reaction of chlorine atoms with phenol, derived from several competition kinetic experiments, was 1.2 x 10" M s, slightly lower than the value for fluorine atoms. The spectrum of the phenoxyl radical in the gas phase was very similar to that recorded in aqueous solutions. It exhibits several peaks between 350 nm and 400 nm and much more intense absorptions in the UV, the main peak being at 235 nm (molar absorption coefficient 2.3 x 10 M cm ). By following the decay... 

Absorption spectra of phenoxyl radicals derived from biologically important molecules were recorded in numerous cases. The tyrosyl radical was studied by many investigators and its spectrum was used to detect tyrosine oxidation in a protein and to follow intramolecular electron transfer from tyrosine to the tryptophan radical in dipeptides and polypeptides . A number of catecholamines, such as adrenaline and dopa, were also studied by kinetic spectrophotometric pulse radiolysis " ". The absorption spectra of most of these substituted o-semiquinone anion were similar to those of the unsubstituted... 

Obviously, more work is required to further substantiate the presence of the proposed radical intermediates in the p-hydroxybenzoate hydroxylase reaction, possibly via EPR and spin-trapping studies. Studies by Detmer and Massey 247) on phenol hydroxylase have indicated that the reaction rate constants for the conversion of meta-substituted substrates plotted versus the Hammett parameters yield a straight line of slope equal to 0.5. This is consistent with the mechanism proposed by Anderson, as the negative slope is expected for an electrophilic aromatic substitution reaction, while the small magnitude of the slope may be indicative of a radical mechanism. Furthermore, recent work by Massey and co-workers on p-hydroxybenzoate hydroxylase utilizing 6-hydroxy-FAD as cofactor and p-aminobenzoate as substrate indicated that the absorption spectrum of intermediate 67 exhibited a satellite band at 440 nm 248). Anderson et al. suggest that the satellite band may result from the formation of an aromatic phenoxyl radical at the C-6 position of the isoalloxazine ring of the flavin 244). This species would result from a shift of the initial peroxyl radical center from C(4a) to C-6 via N(5) 245). 

Fig. 6.22 IR detection of phenyl cation, (a) IR spectrum of iodobenzene after argon resonance irradiation (b) difference spectrum of the same matrix showing the bleaching of phenyl radical and cation. The bands pointing downwards disappear during irradiation with 2 > 400 nm, whereas absorptions of iodobenzene and phenoxyl radieal increase in intensity. See [45,46]...

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