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

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 α-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 α-amino radicals at pH 7 is about 60:40. The former undergo complexation to form a [Formula: see text] bonded species, which possess a broad absorption peaking at 520 nm and are inert to lumiflavin (Fl) and paranitroacetophenone on a 30 µs time scale. However, they oxidise dihydrolumifiavin anion (FlH−) to flavin semiquinone radical (•FlH). The α-amino radicals of piperazine reduced lumiflavin to •FlH with a rate constant of 3.3 ± 0.3 × 109 M−1 s−1. This rate constant is similar to that observed for the CH3S(ĊH2)2HNH2 radical formed from methionine.The 40:60 mixture of α-amino and [Formula: see text] radicals produced in N2O-saturated piperazine solutions decayed by a second order process with a rate constant of 7.2 × 108 M−1s−1. Similar mixtures of •FlH and the [Formula: see text] species also underwent second order decay (k = 2.7 × 109 M−1 s−1) with reoxidation of •FlH to Fl as the major process.