Do garlic-derived allyl sulfides scavenge peroxyl radicals?
The chain-breaking antioxidant activities of two garlic-derived allyl sulfides, i.e. diallyl disulfide (1), the main component of steam-distilled garlic oil, and allyl methyl sulfide (3) were evaluated by studying the thermally initiated autoxidation of cumene or styrene in their presence. Although the rate of cumene oxidation was reduced by addition of both 1 and 3, the dependence on the concentration of the two sulfides could not be explained on the basis of the classic antioxidant mechanism as with phenolic antioxidants. The rate of oxidation of styrene, on the other hand, did not show significant changes upon addition of either 1 or 3. This unusual behaviour was explained in terms of the co-oxidant effect, consisting in the decrease of the autoxidation rate of a substrate forming tertiary peroxyl radicals (i.e. cumene) upon addition of little amounts of a second oxidizable substrate giving rise instead to secondary peroxyl radicals. The relevant rate constants for the reaction of ROO(.) with 1 and 3 were measured as 1.6 and 1.0 M(-1) s(-1), respectively, fully consistent with the H-atom abstraction from substituted sulfides. It is therefore concluded that sulfides 1 and 3 do not scavenge peroxyl radicals and therefore cannot be considered chain-breaking antioxidants.
Amorati
Riccardo
R
Dipartimento di Chimica Organica "A. Mangini", Bologna University, Via San Giacomo 11, 40126 Bologna, Italia. riccardo.amorati@unibo.it
Pedulli
Gian Franco
GF
D000498:Allyl Compounds / Q000737:chemistry* / Q000494:pharmacology; D001555:Benzene Derivatives / Q000737:chemistry; D016166:Free Radical Scavengers / Q000737:chemistry*; D005609:Free Radicals / Q000737:chemistry*; D005737:Garlic / Q000737:chemistry*; D007700:Kinetics; D015394:Molecular Structure; D010084:Oxidation-Reduction; D010545:Peroxides / Q000737:chemistry*; D020058:Styrene / Q000737:chemistry; D013440:Sulfides / Q000737:chemistry* / Q000494:pharmacology
D000498:Allyl Compounds; D001555:Benzene Derivatives; D016166:Free Radical Scavengers; D005609:Free Radicals; D010545:Peroxides; D013440:Sulfides; C049375:perhydroxyl radical; D020058:Styrene; C038491:allyl sulfide; C015763:cumene; C077207:allyl methyl sulfide