Silybin dihemisuccinate sodium salt, a flavonoid used in human therapy of liver dysfunction, has an inhibitory effect in vivo on radiation-induced deactivation of enzymes and peroxidation of membrane lipids in rat liver microsomes. The reactivity of silybin and its phenolic OH groups towards free radicals (OH, N3., (SCN)2.-, Cl3CO2.) in aqueous solution was studied by pulse radiolysis. Absorption spectra for the phenoxyl-type radicals were assigned using structurally similar models. The one-electron reduction potential for silybin at pH 7 (E07 = 0.76 V), determined using the p-methoxy-phenoxyl/phenolate redox couple as reference standard (E07 = 0.72 V, Lind et al. 1990), is related to the 3'-methoxy-4'-OH structure, the exclusive target for one-electron oxidation at pH 7, while the 7-OH and 5-OH groups are prevented from oxidation by 4-keto substitution and intramolecular H-bonding, respectively. The free radical reactivity of silybin compares favourably with poly-OH-substituted flavonoids; however, the latter compounds have been reported to generate potentially toxic oxygen species at a biologically relevant pH.

These observations suggest that amiodarone in vitro and in vivo generates free radicals that may play a role in the pathogenesis of amiodarone toxicity beside other well-established mechanisms, and antioxidants may have a partial protective effect against amiodarone toxicity.

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linear with a slope related to k,. By using eq VIII, a plot of should be linear with a slope of k3 and an intercept of k4, the rate coefficients for reactions 3 and 4, respectively. Figure 7 shows a plot of for data obtained in the temperature range 191-240 K. As can be seen, k, or the slope of the plot is independent of temperature. Values of (2.15 f 0.13) X and (1.7 f 3.6) X lo-" cm3 molecule-' s-I were obtained for k3 and k4, respectively. The reported errors are twice the standard deviation of the slope and intercept values obtained from a linear least-squares fit of the data. The large error in the value of k4 is due to the large extrapolation involved in calculating the intercept. Therefore, we quote an upper limit of k4 < 2 X lo-'' cm3 molecule-' s-I.The value of k3 obtained in this study is larger than the currently r e c~m m e n d e d~~ value of 1.48 X cm3 molecule-' s-' but is in reasonable agreement with the value of 2.7 X cm3 molecule-' s-I measured recently by Baer et a1.2z The studies summarized in ref 25 all involved the use of chemical models to obtain a value of k,. We also assume a reaction scheme; however, we have directly monitored ClOO and used an independent measurement of the ClOO absorption cross section. Hence, we believe our result to be accurate.Only Baer et a1.22 have reported a value for k4. In their study they were able to use high enough [O,] to measure ClOO loss rates at l/(Keqll[Oz]) values much closer to zero than we could. Therefore, they could convert almost all C1 to C100, and hence were able to precisely determine k4. Our upper limit for k4 (<2 X lo-'' cm3 molecule-' SKI) is consistent with their value of 1.6x lo-'' cm3 molecule-I s-l. ConclusionsThe shape of the UV absorption spectrum of ClOO measured here is in reasonable agreement with those of Johnston et al.7 and Cox et aL9 However, the peak cross section we measured is a factor of 2.2 larger than that reported by Johnston et al. and is in excellent agreement with the recent determination of Baer et aLz2The value of the equilibrium constant for reaction 2, measured at 191 K, is slightly higher than would be expected using the value of Nicovich et aLiO obtained at 185 K yet well within their reported uncertainty. Our value for AoHm is in excellent agreement with those measured by Nicovich et al. and Baer et al.The measurement of the rate coefficient for removal of ClOO indicates that the rate coefficient is independent of temperature as previously r e p~r t e d .~~.~~ The value measured here is larger by a factor of 1.5 from the currently recommended value25 and in reasonable agreement with that measured by Baer et aLz2 This larger value can partially account for the discrepancy between the cross section values measured in this study and those of Johnston et aL7 Acknowledgment.The intratrack reactions of cyclohexyl radicals produced in the radiolysis of cyclohexane with helium and carbon ions have been examined using iodine-scavenging methods. It is found that in contrast to electron radiolysis, where most of the cyclohex...