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Singlet Oxygen-Induced Oxidation Of 26 RADIATION CHEMISTRY AND PHYSICS, RADIATION TECHNOLOGIES sible. Besides TP•–, also cation radical and triplet tion with solvated and dry electrons thus eliminat- excited state TP* are known to absorb in the same ing one path of TP•– formation. Some TP•– are region. TEA is a scavenger for cation radical and formed by reaction of excited TP* states with TEA. •– •– excited states of TP. The occurrence of both species Direct reactions involving TP, TP , CO2 and CO2 were confirmed also by the experiments under O2 are too slow to be observed in pulse radiolysis time and N2O. Kinetics of the absorption decay is quite scale. The reactions involving metal complexes are complicated and the best fitting to the experimen- underway. tal results can be achieved taking an exponential decay equation with three different constants. In References Figure 2, there are presented experimental traces of absorbencies at 475 nm vs. time in logarithmic [1]. Welton T.: Chem. Rev., 99, 8, 2071-2083 (1999). [2]. Wasserscheid P., Keim W.: Angew. Chem. Int. Ed., scale for 14 mM TP in Ar-saturated samples in the 39, 21, 3772-3789 (2000). absence and presence of TEA. [3]. Ionic liquids: Industrial application to green chemistry. Eds. R.D. Rogers, K.R. Seddon. ACS Symp. Ser., 818 (2002). [4]. Grodkowski J., Neta P.: J. Phys. Chem. A, 106, 22, 5468-5473 (2002). [5]. Grodkowski J., Neta P.: J. Phys. Chem. A, 106, 39, 9030-9035 (2002). [6]. Grodkowski J., Neta P.: J. Phys. Chem. A, 106, 46, 11130-11134 (2002). [7]. Wishart J.F., Neta P.: J. Phys. Chem. B, 107, 30, 7261-7267 (2003). [8]. Grodkowski J., Neta P., Wishart J.F.: J. Phys. Chem. A, 107, 46, 9794-9799 (2003). [9]. Grodkowski J., Płusa M., Mirkowski J.: Nukleonika, 50, Suppl.2, s35-s38 (2005). [10]. Zimek Z., Dźwigalski Z.: Postępy Techniki Jądrowej, Fig.2. Experimental absorbance vs. time trace at 475 nm in 42, 9-17 (1999), in Polish. 14 mM TP deoxygenated R4NNTf2 solution with and [11]. Grodkowski J., Mirkowski J., Płusa M., Getoff N., without 3% TEA added. Decaying parts of traces Popov P.: Rad. Phys. Chem., 69, 379-386 (2004). were fitted using equation: A(t)=A1*exp(-t*k1)+ [12]. Grodkowski J.: Radiacyjna i fotochemiczna redukcja A2*exp(-t*k2) + A3*exp(-t*k3) + A0. Dose – 15 Gy. dwutlenku węgla w roztworach katalizowana przez kompleksy metali przejściowych z wybranymi układa- CO2 saturation of 14 mM TP solution cut ini- mi makrocyklicznymi. Instytut Chemii i Techniki tial absorbance by about 50% and eliminates ad- Jądrowej, Warszawa 2004, 56 p. Raporty IChTJ. Seria ditional absorbance formation after the electron A nr 1/2004 (in Polish). pulse. Only in the sample with TEA added some [13]. Shida T.: Electronic absorption spectra of radical ions. participation of TP•– in the spectra can be distin- Elsevier, Amsterdam 1988, p.446. guished. The effect can be explained by CO2 reac- SINGLET OXYGEN-INDUCED OXIDATION OF ALKYLTHIOCARBOXYLIC ACIDS Monika Celuch1/, Mirela Enache1,2/, Dariusz Pogocki1/ 1/ Institute of Nuclear Chemistry and Technology, Warszawa, Poland 2/ Institute of Physical Chemistry “I.G. Murgulescu”, Romanian Academy, Bucharest, Romania 1 Singlet oxygen ( O2) could be generated in biologi- However, the major pathway of persulphoxide de- cal systems by endogenous and exogenous pro- cay is bimolecular reaction with the second mol- cesses (e.g. enzymatic and chemical reactions, UV ecule of thioether that leads to the formation of or visible light in the presence of a sensitizer) [1]. respective methionine sulphoxide [2,3]: Numerous data show that proteins are the major >S(+)O-O(–) + >S → 2 >S=O (3) 1 targets of O2-induced damage in the living cells. In this work, we have investigated the mecha- The primarily reactions occur here preferentially nisms of deprotonation and decarboxylation of sul- with residues of aromatic and sulphur containing phur-centered radical-cation (>S•+) the irrevers- amino acids [1]. ible processes, which compete with the formation 1 In particular, reaction of O2 with thioether sul- of sulphoxide (reaction 3) by moving the equilib- phur of methionine (Met) leads to the formation rium (2) to the right hand side. Importantly, effi- of persulphoxide [2,3]: ciency of both decarboxylation and deprotonation 1 → (+) (–) O2 + >S >S O-O (1) could be influenced by various factors such as which is in equilibrium with superoxide radical-an- neighbouring group participation and environmen- •– ion (O2 ) and respective sulphur-centered radical- tal effects. These phenomena may be studied us- -cation: ing thioethers diverse by the number and positions (+) (–) •+ •– >S O-O = >S + O2 (2) of carboxylate groups. Therefore, the experiments RADIATION CHEMISTRY AND PHYSICS, RADIATION TECHNOLOGIES 27 tonated carboxylic functionality to sulphur-centered radical-cation [5], since its efficiency depends on pH (Fig.3). It suggests that the formation of carbon dioxide may be catalyzed by the presence of Levis bases such as hydroxyl or chloride anions. It seems that the reaction (drafted for TDEA in Scheme 1 in [6]) can be described by nucleophilic substitution at the thioether sulphur, in which a weak nucleo- • •– ≈ phile superoxide radical anion (pKA(HO2/O2 ) 4.8 [7]) is replaced by a much stronger nucleophile like hydroxide anion. In support, our DFT calcu- lations [8] predict the possibility of the formation of the tetravalent transient product of hydroxide anion addition to persulphoxide. Therefore, the reaction may occur via two-step mechanism of nu- 1 cleophilic addition – nucleophilic dissociation of Fig.1. Efficiency of O2-induced carbon dioxide formation vs. time of illumination in solutions containing 50 mM (AN+DN)-type [9]. The observed influence of car- µ boxylate groups in β-position relative to the sul- thioether, 22 M RB, 1.045 mM oxygen at pH 6. phur on the efficiency of decarboxylation suggests have been performed for the following model thio- furthermore that they may also catalyze decarboxy- ethers: 2,2’-thiodiethanoic acid (TDEA), 3,3’-thio- lation of α-positioned carboxylate in a manner dipropionic acid (TDPA), 2-(methylthio)ethanoic similar to hydroxide anion. acid (MTEA), 3-(carboxymethylthio)propionic acid (CMTPA), 2-(carboxymethylthio)succinic acid (CMTSA). Singlet oxygen has been produced in aqueous, oxygen-saturated solutions of thio- ethers, illuminated by visible light in the presence of rose bengal (RB) as a photosensybilizer [4]. For- mation of carbon dioxide and respective sulph- oxides has been monitored by means of head-space chromatography (GC) and high performance ion chromatography exclusion (HPICE), respectively. For all investigated alkylthiocarboxylic acids, 1 the O2-induced oxidation leads to the release of carbon dioxide, and simultaneously to the forma- tion of respective sulphoxide (Figs.1 and 2). How- ever, the higher yield of decarboxylation has been observed for alkylthiocarboxylic acids containing carboxylate functionality in the α-position relative to the thioether sulphur, where such process leads to the formation of resonance stabilized α-alkyl- thioalkyl radicals (see example in Fig.1). The pro- 1 Fig.3. Efficiency of O2-induced carbon dioxide and sulph- oxides formation vs. time of illumination in oxygen- -saturated solutions (1.045 mM oxygen) containing 50 mM TDEA, 22 µM RB at pH 9 (square points) and pH 6 (circle points). This work described herein was supported by the Research Training Network SULFRAD (HPRN- -CT-2002-00184) and the State Committee for Scien- tific Research (KBN) grant (No. 3 T09A 066 26). The computations were performed employing the computer resources of the Interdisciplinary Cen- Fig.2. Efficiency of 1O -induced sulphoxide formation vs. 2 tre for Mathematical and Computational Model- time of illumination in oxygen-saturated solutions µ ling, Warsaw University (ICM G24-13). (1.045 mM oxygen) containing 22 M RB and 50 mM of thioether at pH 6. References cess of decarboxylation occurs most probably due [1]. Davies M.J.: Photochem. Photobiol. Sci., 3, 17-26 to the intramolecular electron transfer from depro- (2003). 28 RADIATION CHEMISTRY AND PHYSICS, RADIATION TECHNOLOGIES [2]. Clennan E.L.: Acc. Chem. Res., 34, 875-884 (2001). Annual Report 2004. Institute of Nuclear Chemistry [3]. Jensen F., Greer A., Clennan E.L.: J. Am. Chem. Soc., and Technology, Warszawa 2005, pp.27-29. 120, 4439-4449 (1998). [7]. Bartosz G.: Druga twarz tlenu. Wolne rodniki w przyro- [4]. Nowakowska N., Kępczyński M., Dąbrowska M.: dzie. Wydawnictwo Naukowe PWN, Warszawa 2003, Macromol. Chem. Phys., 201, 1679-1688 (2001). pp.1-447 (in Polish). [5]. Bobrowski K., Pogocki D., Schöneich C.: J. Phys. [8]. Frisch M.J. et al.: Gaussian 03. (Rev. B.03). Gaussian Chem., 97, 13677-13684 (1993). Inc., Pittsburgh 2003. [6]. Celuch M., Pogocki D.: Singlet oxygen-induced decar- [9]. Williams A.: Concerted organic and bio-organic boxylation of carboxyl substituted thioethers. In: INCT mechanisms. CRC Press, Boca Raton 2000, pp.1-286. COMPUTATIONAL STUDY ON THE 1,2-HYDROGEN SHIFT IN THIYL, OXYL, AMINYL AND AMIDYL RADICALS Dariusz Pogocki, Monika Celuch, Arvi Rauk1/ 1/ Department of Chemistry, University of Calgary, Canada • The central issue of this paper are the reactions of leads to the formation of thiyl radicals RCH2S . • the 1,2-hydrogen shift from the carbon atom to the Therefore, RCH2S are important intermediates adjacent heteroatom in thiyl, oxyl, aminyl and in biological conditions of oxidative stress [1]. They amidyl radicals. From the physiological point of are moderately good oxidants, having potential to view, thiols are one of the most important classes abstract “activated” hydrogen from carbon atoms. of sulphur-containing compound. The protective This ability has been already experimentally dem- or antioxidant effects of thiols result from the fact onstrated for several groups of biologically impor- that they can act as hydrogen or electron donors tant compounds such as alcohols [2], carbohydrates (pH dependent). One electron oxidation of thiols [3], fatty acids [4-7], amino acids [8], and peptides Table.
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