The intrinsic photophysics of gaseous ethidium ions

• Mass spectrometry and gas-phase laser-induced fluorescence probe gaseous ethidium.
• Fluorescence and electronic action spectra of gaseous ethidium ions are presented.
• Time-resolved fluorescence shows two components with lifetimes of ∼5.1 and ∼21.4 ns.
• Brightness indicates substantial quantum yield enhancement over aqueous ethidium.
• In some ways, photophysics of gaseous ethidium resemble ethidium-DNA complex.

Ethidium is a cationic dye with fluorescence that is enhanced ∼9-fold upon binding DNA. In order to better understand how the local environment modulates the behavior of this dye, we measured the photophysical properties of gaseous ethidium ions, using a quadrupole ion trap mass spectrometer that has been modified for fluorescence spectroscopy. The photodissociation maximum of gaseous ethidium measured through action spectroscopy is 485 nm and the emission maximum is 548 nm. The Stokes shift (2370 cm−1) of gaseous ethidium is marginally larger than that of ethidium in non-polar solvents, and significantly less than that in polar solvents. Time-resolved fluorescence measurements of gaseous ethidium ions show two components with lifetimes of 21.4 ± 1.5 and 5.1 ± 0.7 ns, which suggest the presence of multiple conformations in the gas phase. Both lifetimes are significantly longer than that of aqueous ethidium, while the longer of the two lifetimes is remarkably similar to that of ethidium in complex with double-stranded DNA in solution. In line with this, the estimated quantum yield of gaseous ethidium is ∼30% lower than that of ethidium in complex with DNA in solution, and ∼10-fold higher than that of aqueous ethidium. These benchmark results provide a reference from which to better understand the factors that modulate the fluorescence of phenanthridine-based dyes by the local environment.

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