Journal of Photochemistry and Photobiology A: Chemistry
Laser flash photolysis of new water-soluble peroxyl radical precursor
Introduction
Peroxyl radicals are important intermediates in chemistry and biology. They are involved in atmospheric chemistry, combustion and many oxidation reactions. Also, they are the main radicals responsible for lipid peroxidation, which can lead to oxidative stress. For example, the hepatotoxicity of CCl4 has been attributed to the formation of trichloromethylperoxyl radical (CCl3O2) which triggers lipid peroxidation processes [1], [2].
Pulse radiolysis (PR) and laser flash photolysis (LFP) are two of the most important techniques to investigate fast reactions between peroxyl radicals and different compounds in aqueous and organic solutions [3]. Most of the reported peroxyl radical reactions in aqueous solutions have been investigated using pulse radiolysis. However, this technique is not easily accessible to many laboratories [4]. Peroxyl radical generation using pulse radiolysis has been described previously [4].
Laser flash photolysis is a far more available than pulse radiolysis. However, its use to investigate peroxyl radical reactions in aqueous solution is rare because of the poor solubility of most peroxyl radical precursors in water. Using laser flash photolysis technique, there are few examples for the reactions of peroxyl radicals with different substrates, using commercially unavailable cobalt or nickel complexes as precursors, in aqueous solutions [5], [6], [7], [8]. For example [5], [6], the generation of acetylperoxyl radicals using organo-cobalt complexes is shown in Scheme 1.
The use of water-soluble azo compounds as peroxyl radical precursors is not possible due to the very low quantum yield of the cleavage process in aqueous solutions [8], [9].
In this study, LFP of water-soluble and commercially available 4-acetyl-4-phenylpiperidine hydrochloride (APP), which has an analgesic activity [10], is reported in methanolic and aqueous solutions (see ground state spectra in supplementary material). In air-saturated solution, acetylperoxyl radicals, generated from photolysis of APP, are the main reactive peroxyl radicals. In addition, the reactivity of acetylperoxyl radical toward 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS2−), crocin and nitrite anion in aqueous solutions is reported (Fig. 1).
Section snippets
Materials
Methanol (Fisher Scientific, HPLC grade), sodium nitrite (Fisher Scientific), 4-acetyl-4-phenylpiperidine hydrochloride (Aldrich), and 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS2−) (Sigma) were used as received. Purified crocin was kindly supplied by Dr. V. Partali and Dr. H.R. Sliwka (NTNU, Inst. Kjemi, Trondheim, N-7491 Norway) and was used as received. Argon was supplied by the British Oxygen Company (BOC). For aqueous solutions, ultra pure water
Laser (266 nm) flash photolysis of 4-acetyl-4-phenylpiperidine hydrochloride (APP)
Laser flash photolysis of APP in oxygen-free aqueous solution gives a transient spectrum with λmax = 315 nm (Fig. 2).
The second-order decay of the 315 nm transient is accompanied by a transient growth at 340 nm. In addition, no other transient was observed at longer wavelengths (between 400 and 650 nm) in oxygen-free aqueous solutions. Moreover, no information about the triplet absorption spectrum and triplet lifetime of APP was previously reported. However, the very short triplet lifetime of some
Conclusions
In summary, this first time-resolved study of APP shows that this peroxyl radical precursor can easily be used to investigate fast peroxyl radical reactions in aqueous solutions using laser flash photolysis technique, which is more accessible than pulse radiolysis technique. This will facilitate the investigation of reactive peroxyl radical reactions with different substrates in aqueous solutions. In addition, by virtue of APP solubility in organic and aqueous solutions, it can be used to study
Acknowledgments
The author is grateful to Leverhulme Trust (Research Grant F/00130F) for financial support and to Dr. David J. McGarvey and T. George Truscott (School of Physical and Geographical Sciences, Keele University, Keele, Staffordshire ST5 5BG, UK) for their valuable comments on the manuscript and to Dr. V. Partali and Dr. H.R. Sliwka (NTNU, Inst. Kjemi, Trondheim, N-7491 Norway) for kindly providing the purified crocin.
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