PHOTOINDUCED ELECTRON-TRANSFER REACTIONS BETWEEN COPPER IONS AND ETHIDIUM-BROMIDE IN POLYNUCLEOTIDES

Reaction of the first excited singlet state of ethidium with CU2+ ions results in electron transfer to form oxidized ethidium and Cu+. The d ecay of these species is via reverse electron transfer, leaving the sy stem unchanged. The reverse electron transfer reaction has been studie d in solutions containing polynucleotides, leading to information conc erning binding of Cu+ and electron transfer in these media. Cu+ binds strongly to calf thymus DNA, poly(deoxyadenylic-thymidylic acid), and poly(deoxyguanylic-deoxycytidylic acid), in agreement with previous wo rk. The rate of escape of Cu+ from the poly(deoxyadenylic-thymidylic a cid) helix is measured as (1.3 +/- 0.3) x 10(5) s(-1). The strongest c omplex is that with poly(deoxyguanylic-deoxycytidylic acid), and in th is case reverse electron transfer occurs over several decades of time. Fitting these data to a summation of exponentials, k(et) = Sigma nu e xp(-beta r), where r is the separation distance quantized in base pair s, leads to an attenuation factor, beta, of 0.73 +/- 0.05 Angstrom(-1) , for electron transfer in this medium.