EPR detection of guanine radicals in a DNA duplex under biological conditions: Selective base oxidation by Ru(phen)(2)dppz(3+) using the flash-quenchtechnique

Continuous-wave X-band EPR spectroscopy has been employed in examining the guanine radical within a DNA duplex at ambient temperature using the flash- quench technique. Guanine was selectively oxidized by DNA-bound [Ru(phen)(2 )dppz](3+) (dppz = dipyridophenazine, phen = 1,10-phenanthroline) generated in situ by photolysis in the presence of [Co(NH3)(5)Cl](2+) as the oxidati ve quencher. An EPR signal centered at g(iso) = 2.0048 is observed in exper iments with poly(dG-dC) as substrate. Comparable signals are also detected with a 13-mer oligonucleotide duplex containing only one guanine base and w ith calf thymus DNA, but no signal is observed with poly(dA-dT) or poly(dI- dC). These observations reflect the base selectivity of the reaction in for ming the guanine radical. With ruthenium hexaammine as oxidative quencher, no signal is observed, while, with methyl viologen, a strong signal with hy perfine pattern is seen, characteristic of the reduced viologen radical and indicating that [Ru(phen)(2)dppz](3+) was generated. The guanine radical s ignal, once formed upon continuous irradiation in argon-saturated aqueous b uffer solution (pH 7), decays with a half-life of 30 s, but vanishes instan taneously in the dark or upon introduction of oxygen. Spin trapping experim ents with N-tert-butyl-alpha-phenylnitrone substantiate the selectivity in generating the guanine radical; in the presence of poly(dG-dC), calf thymus DNA, the 13-mer oligonucleotide but not with poly(dA-dT) and poly(dI-dC), the detected nitroxide EPR signals are the same with g(iso) = 2.0059, [a(N) ] = 15.05 G, and [a(H)] = 3.11 G. Upon titration of the ruthenium intercala tor into poly(dG-dC), the signal intensity increases smoothly as the [base pair]/[intercalator] ratio decreases from 100 to 25, at which point the sig nal intensity decreases markedly; this result may be an indication of an an tiferromagnetic exchange interaction between guanine radicals. indeed. usin g the flash-quench technique, EPR spectroscopy of guanine radicals within D NA now will permit the evaluation of how radicals within the DNA base stack may be coupled under biological conditions.