TIME-RESOLVED FLUORESCENCE STUDIES OF TOMAYMYCIN BONDING TO SYNTHETICDNAS

Tomaymycin reacts covalently with guanine in the DNA minor groove, exh ibiting considerable specificity for the flanking bases, The sequence dependence of tomaymycin bonding to DNA was investigated in synthetic DNA oligomers and polymers, The maximum extent of bonding to DNA is gr eater for homopurine and natural DNA sequences than for alternating pu rine-pyrimidine sequences. Saturation of DNA with tomaymycin has littl e effect on the melting temperature in the absence of unbound drug. Fl uorescence lifetimes were measured for DNA adducts at seven of the ten unique trinucleotide bonding sites, Most of the adducts had two fluor escence lifetimes, representing two of the four possible binding modes . The lifetimes cluster around 2-3 ns and 5-7 ns; the longer lifetime is the major component for most bonding sites, The two lifetime classe s were assigned to R and S diastereomeric adducts by comparison with p revious NMR results for oligomer adducts. The lifetime difference betw een binding modes is interpreted in terms of an anomeric effect on the excited-state proton transfer reaction that quenches tomaymycin fluor escence. Bonding kinetics of polymer adducts were monitored by fluores cence lifetime measurements. Rates of adduct formation vary by two ord ers of magnitude with poly(dA-dG) poly(dC-dT), reacting the fastest at 4 x 10(-2) M(-1) s(-1). The sequence specificity of tomaymycin is dis cussed in light of these findings and other reports in the literature.