Oxidative degradation of triazine derivatives in aqueous medium: a radiation and photochemical study

Pulse and steady state radiolysis techniques have been used to determine the bimolecular rate constants and to investigate the spectral nature of the intermediates and the degradation induced by hydroxyl radicals ((*)OH) with 1,3,5-triazine (T), 2,4, 6-trimethoxy-1,3,5-triazine (TMT), and 2,4-dioxohexahydro-1,3, 5-triazine (DHT) in aqueous medium. A competitive kinetic method with KSCN as the (*)OH scavenger was used to determine the rate constants for the reaction of (*)OH with T, TMT, and DHT. The bimolecular rate constants are 3.4 x 10(9), 2.06 x 10(8), and 1.61 x 10(9) dm(3) mol(-)(1) s(-)(1) respectively, for T, TMT, and DHT at pH approximately 6. The transient absorption spectra obtained from the reaction of (*)OH with T, TMT, and DHT have single absorption maxima at 320, 300, and 300 nm, respectively, and were found to undergo a second-order decay. The formation of TOH(*) [C(6)OH-N(5)-yl radical], TMTOH(*) [N(5)OH-C(6)-yl radical], and DHT(*) [C(6)-yl radical] is proposed from the initial attack of (*)OH with T, TMT, and DHT, respectively. A complete degradation of TMT (10(-3) mol dm(-3)) was obtained after absorbed doses of 5 kGy in N(2)O-saturated solutions and 16 kGy in aerated solutions. A similar degradation pattern was obtained with DHT in N(2)O-saturated solutions. Complete degradation was observed with an absorbed dose of 7 kGy. On the basis of the results from both pulse and steady state radiolysis, a possible reaction mechanism involving (*)OH-mediated oxidative degradation is proposed. A complete photodecomposition of DHT was also observed in the presence of ferric perchlorate using ultraviolet light at low pH. Photoinduced electron transfer between Fe(III) and DHT in the Fe(III)-DHT complex and subsequent formation of DHT(*) are proposed to be the major processes that lead to the complete degradation of DHT at pH 3.

Mechanism of biochemical action of substituted 4-methylbenzopyran-2-ones. Part 5: Pulse radiolysis studies on the antioxidant action of 7,8-diacetoxy-4-methylcoumarin

7,8-Dihydroxy-4-methylcoumarin (1, DHMC) and 7,8-diacetoxy-4-methylcoumarin (2, DAMC) were shown to possess radical scavenging property and strongly inhibit membrane lipid peroxidation. Although free polyphenolic compounds are known to be antioxidants, the antioxidant action of the acetoxy compound DAMC was intriguing. Hence, pulse radiolysis studies were undertaken to explain the antioxidant action of DAMC. Accordingly, DAMC and DHMC were separately reacted with the system generating azide radicals and the resulting transient spectra were recorded. The spectra so obtained in both the cases demonstrated peak at 410 nm, characteristic of phenoxyl radical. The rate constants for the formation of phenoxyl radical from DHMC and DAMC were 34 x 10(8) M(-1) s(-1) and 6.2 x 10(8) M(-1) s(-1), respectively. We propose that the free radical mediated oxidation of DAMC initially produces a radical cation that loses an acetyl carbocation to yield the phenoxyl radical. It is possible to conclude that the mechanism of the antioxidant action of DAMC follows the pathway similar to that of DHMC involving the formation of a stable phenoxyl radical.

Oxidative damage induced by the fullerene C60 on photosensitization in rat liver microsomes

We have examined the ability of a commonly used fullerene, C60, to induce oxidative damage on photosensitization using rat liver microsomes as model membranes. When C60 was incorporated into rat liver microsomes in the form of its cyclodextrin complex and exposed to UV or visible light, it induced significant oxidative damage in terms of (1) lipid peroxidation as assayed by thiobarbituric acid reactive substances (TBARS), lipid hydroperoxides and conjugated dienes, and (2) damage to proteins as assessed by protein carbonyls and loss of the membrane-bound enzymes. The oxidative damage induced was both time- and concentration-dependent. C60 plus light-induced lipid peroxidation was significantly inhibited by the quenchers of singlet oxygen ((1)O2), beta-carotene and sodium azide, and deuteration of the buffer-enhanced peroxidation. These observations indicate that C60 is an efficient inducer of peroxidation and is predominantly due to (1)O2. Biological antioxidants such as glutathione, ascorbic acid and alpha-tocopherol significantly differ in their ability to inhibit peroxidation induced by C60. Our studies, hence, indicate that C60, on photosensitization, can induce significant lipid peroxidation and other forms of oxidative damage in biological membranes and that this phenomenon can be greatly modulated by endogenous antioxidants and scavengers of reactive oxygen species.