A Circular Dichroism Study of DNA . Platinum Complexes. Differentiation between Monofunctional. cis-Bidentate and trans-Bidentate Platinum Fixation on a Series of DNAs

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Abstract
The circular dichroism (CD) spectra of a series of DNA.cntdot.Pt complexes are presented. The following Pt compounds, [Pt(dien)Cl]Cl [dien = bis(2-aminoethyl)amine], cis-Pt(NH3)2Cl2, cis-Pt(en)Cl2 [en = ethylenediamine], trans-Pt-(NH3)2Cl2, K[Pt(NH3)Cl3] and K2[PtCl4] were complexed with the DNA extracted from bacteria Micrococcus lysodeikticus (72% dG + dC), Escherichia coli (50% dG + dC), Clostridium perfringens (32% dG + dC) and salmon sperm (41% dG + dC). Strong differences were found between the different DNA.cntdot.Pt complexes. Three types of spectra clearly demonstrate the different Pt binding modes on DNA. In the 1st type, the Pt compound, i.e., [Pt(dien)Cl]Cl, is fixed to DNA with only 1 bond (monofunctional complex formation) and no significant change of the CD positive band of DNA is found. The main feature of the 2nd type is a continuous intensity decrease of the positive band as observed for trans-Pt(NH3)2Cl2 (trans-bidentate complex formation). The 3rd type concerns the cis-bidentate Pt fixation obtained with cis-Pt(NH3)2Cl2, cis-Pt(en)Cl2, K[Pt(NH3)Cl3] and K2[PtCl4]. The CD spectra are in this case characterized by an increase in the positive Cotton effect which is dG + dC-dependent up to an rb value around 0.10 (where rb = number of Pt bound/nucleotide), followed by a decrease until DNA saturation with Pt is reached. A linear decrease in the amplitude of the negative band is detected in all the complexes except in the case of the monofunctional DNA.cntdot.Pt complexes. For the cis-bidentate and trans-bidentate Pt fixation, a continuous bathochromic shift occurs.