Differentiation of DNA · Platinum Complexes by Fluorescence

Ethidium bromide (EtdBr) was used to differentiate perturbations induced in salmon sperm DNA complexed with a series of platinum compounds. The DNA · Pt complexes were classified into two groups. First, those which show a DNA · Pt · EtdBr fluorescence almost identical to that of the DNA · EtdBr complex and second, those which decrease linearly the fluorescence of the EtdBr molecules upon platinum fixation. In the first group, the platinum compounds are fixed to DNA at only one site, like [Pt(dien)Cl]Cl and [Pt(NH 3 ) 3 Cl]Cl. In this case, the platinum fixation corresponding to r b = 0.20 induced a 15–20% fluorescence decrease ( r b = number of Pt atoms bound per nucleotide). On the other hand cis ‐Pt(NH 3 ) 2 Cl 2 , cis ‐[Pt(NH 3 ) 2 (H 2 O) 2 ] (NO 3 ) 2 , cis ‐Pt(en)Cl 2 , cis ‐[Pt(en)(H 2 O) 2 ](NO 3 ) 2 , K[Pt(NH 3 )‐Cl 3 ] and K 2 [PtCl 4 ] were found to be of the second group. These platinum compounds react with DNA through chelation; the fluorescence decrease was found to be 70% for r b = 0.20. trans ‐Pt‐(NH 3 ) 2 Cl 2 can fit into either the first group, when one chlorine atom is still covalently fixed on platinum (one DNA · Pt bond), or into the second one, when the chlorine atoms are displaced ( trans ‐bidentate fixation). Scatchard plots indicated that when one Pt compound is fixed to DNA by chelation it inhibited the intercalation of one EtdBr molecule, this relationship being verified for r b ≤ 0.10. The binding constant of EtdBr to the different DNA · Pt complexes was not altered. Since no fluorescence quenching was detected in the DNA · Pt · EtdBr complexes, the fluorescence decrease was attributed to the so‐called ‘chelation effect’ and interpreted in terms of local denaturations of the DNA molecule. The perturbations could be due to the rupture of the hydrogen bonds in dG · dC pairs since chelation should modify the deoxyguanosine residues in DNA.