Circular dichroism study of the irreversibility of conformational changes induced by polyamine-linked dinuclear platinum compounds

In this work, the reversibility of both the B-->Z and B-->A conformational change in polymer DNA induced by polynuclear platinum compounds was studied. The compounds examined were: [[trans-PtCl(NH(3))(2)](2)[NH(2) (CH(2))(6)NH(2)]](2+) (BBR3005); [[trans-PtCl(NH(3))(2)](2)[mu-spermine-N1,N12]](4+) (BBR3535); [[trans-PtCl(NH(3))(2)](2)[mu-spermidine-N1,N8]](3+) (BBR3571); [[trans-PtCl(NH(3))(2)](2)[mu-BOC-spermidine]](2+) (BBR3537); and [[trans-PtCl(NH(3))(2)](2)[mu-trans-Pt(NH(3))(2)(H(2)N(CH(2))(6)NH(2))(2)]](4+) (BBR3464). The conformational changes were assessed by circular dichroism and the reversibility of the transitions was tested by subsequent titration with the DNA intercalator ethidium bromide (EtBr). Fluorescent quenching was also used to assess the ability of ethidium bromide to intercalate into A and/or Z-DNA induced by the compounds. The results were compared with those produced by the simple hexamminecobalt cation [Co(NH(3))(6)](3+). The data suggest that while conformational changes induced by electrostatic interactions are confirmed to be reversible, covalent binding induces irreversible changes in both the A and Z conformation. The relevance of these changes to the novel biological action of polynuclear platinum compounds is discussed.