Comparison of Enhancement Effect of DNA-Mediated Energy Transfer by Divalent Cations:Mg~(2+),Ca~(2+),Mn~(2+),Co~(2+),and Ni~(2+)

The effect of various metal ions on the DNA mediated energy transfer between simultaneously bound drugs was investigated using spectroscopic methods. It was found that addition of divalent metal ions (Mg2+, Ca2+, Mn2+, Co2+ and Ni2+) resulted in further decrease of the ethidium fluorescence intensity, while a small increase was observed in the TMPyP emission band, implying that the energy of excited ethidium was transferred to TMPyP. This DNA-mediated quenching efficiency between ethidium and TMPyP was significantly enhanced by the presence of all metal ions. Among the divalent metal ions, alkali earth metal ions and Mn2+ displayed higher quenching efficiencies than other transition metal ions. The distances required to permit the energy transfer between the two drugs in DNA were calculated as 68, 66, 62, 48 and 38  in the presence of 100 μmol·L-1 of Mg2+, Ca2+, Mn2+, Co2+ and Ni2+ ion, respectively. The disturbed binding conformation of TMPyP in DNA by metal ions presumably accounts for the difference.