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 microM). 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 microM and 5/10/7.5 microM, 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.