Reactions of α-carbon- and nitrogen-centered radicals of piperazine with lumiflavin and paranitroacetophenone at pH 7

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Abstract
Analysis of data from radiation chemical and photochemical studies shows that the reactions of hydroxyl radicals and flavin triplets (Fl3) with amines produce aminyl as well as .alpha.-amino radicals. This must be taken into account, since the former tend to oxidise .FlH and FlH-, while the latter are inherently reducing. In the reaction of .OH with piperazine the ratio of aminyl to .alpha.-amino radicals at pH 7 is about 60:40. The former undergo complexation to form a N.thrfore.N+ bonded species, which possess a broad absorption peaking at 520 nm and are inert to lumiflavin (Fl) and paranitroacetophenone on a 30 .mu.s time scale. However, they oxidise dihydrolumiflavin anion (FlH-) to flavin semiquinone radical (.FlH). The .alpha.-amino radicals of piperazine reduced lumiflavin to .FlH with a rate constant of 3.3 .+-. 0.3 .times. 109 MK-1 s-1. The rate constant is similar to that observed for the CH2S(CH2)2CHNH2 radical formed from methionine. The 40:60 mixture of .alpha.-amino and N.thrfore.N+ radicals produced in N2O-saturated piperazine solutions decayed by a second order process with a rate constant of 7.2 .times. 108 M-1 s-1. Similar mixtures of .FlH and the N.thrfore.N+ species also underwent second order decay (k = 2.7 .times. 109 M-1 s-1) with reoxidation of .FlH to Fl as the major process.

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