TIME-RESOLVED FLUORESCENCE STUDIES OF TOMAYMYCIN BONDING TO DNA

Tomaymycin is an antibiotic that reacts at guanine N2 in the minor gro ove of the DNA helix. The number and type of tomaymycin-DNA adducts pr esent on natural sequence DNA were identified using time-resolved fluo rescence spectroscopy. At low bonding density, only two discrete speci es were observed with lifetimes of 4.3 and 7.1 ns and relative amplitu des of 40% and 60%. These two lifetime species are proposed to represe nt either R5' or S5' and S3' binding modes at the preferred bonding se quence 5'-AGA. R and S denote the configuration at C11 of tomaymycin, and 5' and 3' describe the orientation of the aromatic ring on the cov alently modified strand. These two species were present over st range of solution conditions, including pH, nucleotide to drug ratio, DNA co ncentration, and DNA size. They have the same emission spectra, but sl ightly shifted absorption spectra. The weak temperature dependence of the fluorescence lifetimes presumably is due to the excited-state prot on-transfer reaction that quenches tomaymycin fluorescence. The Fate o f formation of the longer lifetime species of DNA adduct is about twic e as fast as that of the shorter lifetime species. Under saturating co nditions, the fluorescence decay shows a bimodal lifetime distribution whether analyzed by least-squares assuming a Gaussian distribution mo del or by the maximum entropy method. The two groups of lifetimes are centered around 2-3 and 6-6.6 ns, reflecting multiple species on diffe rent bonding sequences.