  DNA and proteins are chiral: their three-dimensional structure cannot be
superimposed with its mirror image. Circular dichroism spectroscopy is widely
used to characterize chiral compounds, but data interpretation is difficult in
the case of mixtures. We recorded for the first time the electronic circular
dichroism spectra of DNA helices separated in a mass spectrometer. We
electrosprayed guanine-rich strands having various secondary structures as
negative ions, irradiated them with a laser, and measured the difference in
electron photodetachment efficiency between left and right circularly polarized
light. The reconstructed circular dichroism ion spectra resemble the solution
ones, thereby allowing us to assign the DNA helical topology. The ability to
measure circular dichroism directly on biomolecular ions expands the
capabilities of mass spectrometry for structural analysis.
