Re: P57 Lipofuscin and carotenoidsFrom: Louise Mulroy (cha02@chem.keele.ac.uk) Comments(1) The CCl3OO radical experiments are still on going. The radical is generated by pulse radiolysis of neat chloroform as follows: CHCl3 /\/\/\/\/® CHCl3·+ + e- CHCl3·+ ® CCl3 + H+ e- + CHCl3 ® ·CHCl2 + Cl- ·CHCl2 + CHCl3 ® CH2Cl2 + CCl3· CCl3· + O2 ® CCl3OO· The mechanism proposed for the reaction of the radical with the carotenoid arises from the facts that the species at 760 nm is only formed in the presence of oxygen and so is suggested to be an adduct but could be a neutral radical formed following hydrogen abstraction by CCl3OO·. In argon saturated solution only a peak at 1000 nm is observed, assigned as the radical cation. Additionally, the first order kinetics of the adduct decay (with a rate constant of 4.0 x 104 s-1) and radical growth (with rate constant of 5.0 x 104 s-1) would support the latter growing from the decay of the former, although we accept that this is open to some debate. In relation to the shift of the near infra red absorption maximum with time we have of course noticed but futher work is yet to be performed in order to establish what is occuring here. No species is observed in the near infra red following the reaction of the carotenoid with the benzophenone ketyl peroxyl radical. Future experiments to resolve the mechanisms of these reactions would be to study the reaction of peroxyl radicals with polyenes lacking the methyl groups on the side chain and to perform transient conductivity experiments to establish whether charged species are involved. In relation to hydrogen atom transfer involving carotenoids, this has been referred to in the literature by Woodall, Britton and Jackson (Biochimica et biophysica acta, 1997, Vol 1336, No. 1, p33 and No. 3, p575). They have studied the reactions between carotenoids and peroxyl radicals as well as the ability of the carotenoids to protect against lipid peroxidation by peroxyl radicals and suggest with supporting arguments the involvement of hydrogen abstraction in the anti-oxidant role of carotenoids. (2) The two radical species produced following laser flash photolysis of lipofuscin are not quenched by oxygen impying that they are not neutral carbon centred radicals or radical anions. They may possibly be alkoxyl radicals or radical cations? |