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.

Graphical AbstractUpon increasing the Hg2+ concentration (0, 2.5, and 5 μM), the fraction of polyT–EthBr–Hg2+ complexes arose and observed by ESI-MS.Figure optionsDownload high-quality image (72 K)Download as PowerPoint slide