The reaction of ethidium bromide with oxidising free radicals: pulse radiolysis in water and DNA

Pulse radiolysis has been used to study the reaction of ethidium bromide with Br2·- (SCN)2·- and N2, both in aqueous solution and in aqueous solutions of DNA. Rate constants of 1.9 ± 0.2, 1.8 ± 0.2 and 5.7 ± 0.6 x 109 M-1 s-1 were measured for Br2·- (SCN)2 and N2·- respectively in aqueous solution. The rate constant for reaction of Br2·- and (SCN)2· - with ethidium bromide is reduced by a factor of approximately 1000 when the dye is intercalated in DNA, whilst the reduction is only a factor of 50 for N2. The product of these reactions is one electron oxidised ethidium bromide, which is shown to undergo a deprotonation reaction in alkaline solution. A pK value of 8.2 ± 0.3 is measured for oxidised ethidium bromide in aqueous solution and this value does not change appreciably in the presence of DNA. Possible implications of these observations are discussed. (author)