Using electrospray ionization mass spectrometry to explore the interactions among polythymine oligonucleotides, ethidium bromide, and mercury ions

We have used electrospray ionization mass spectrometry (ESI-MS) and fluorescence and circular dichroism (CD) spectroscopy to explore the binding of ethidium bromide (EthBr) to non-self-complementary polythymine (polyT) strands in the absence and presence of Hg²⁺ ions. In the gas phase, ESI-MS revealed that Hg²⁺ ions have greater affinity, through T-Hg²⁺-T coordination, toward polyT strands than do other metal ions. These findings are consistent with our fluorescence and CD results obtained in solution; they revealed that more T₃₃-EthBr-Hg²⁺ complexes existed upon increasing the concentrations of Hg²⁺ ions (from 0 to 50 μM). Surprisingly, the ESI-MS data indicated that the Hg²⁺ concentration dependence of the interaction between T₃₃ and EthBr is biphasic. Our ESI-MS data revealed that the T₃₃-EthBr-Hg²⁺ complexes formed with various stoichiometries depending on their relative concentrations of the components and the length of the DNA strand. When the concentrations of T₃₃/EthBr/Hg²⁺ were 5/5/2. 5 μM and 5/10/7. 5 μM, 1:1:1 and 1:1:2 T₃₃-EthBr-Hg²⁺ complexes were predominantly formed, respectively. Thus, Hg²⁺-induced DNA conformational changes clearly affect the interactions between DNA and EthBr.