Isomer-Selected Photoelectron
Spectroscopy of Isolated
DNA Oligonucleotides: Phosphate and Nucleobase Deprotonation at High
Negative Charge States
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Abstract
Fractionation according to ion mobility and mass-to-charge
ratio
has been used to select individual isomers of deprotonated DNA oligonucleotide
multianions for subsequent isomer-resolved photoelectron spectroscopy
(PES) in the gas phase. Isomer-resolved PE spectra have been recorded
for tetranucleotides, pentanucleotides, and hexanucleotides. These
were studied primarily in their highest accessible negative charge
states (3–, 4–, and 5–, respectively), as provided
by electrospraying from room temperature solutions. In particular,
the PE spectra obtained for pentanucleotide tetraanions show evidence
for two coexisting classes of gas-phase isomeric structures. We suggest
that these two classes comprise: (i) species with excess electrons
localized exclusively at deprotonated phosphate backbone sites and
(ii) species with at least one deprotonated base (in addition to several
deprotonated phosphates). By permuting the sequence of bases in various
[A<sub>5–<i>x</i></sub>T<sub><i>x</i></sub>]<sup>4–</sup> and [GT<sub>4</sub>]<sup>4–</sup> pentanucleotides,
we have established that the second type of isomer is most likely
to occur if the deprotonated base is located at the first or last
position in the sequence. We have used a combination of molecular
mechanics and semiempirical calculations together with a simple electrostatic
model to explore the photodetachment mechanism underlying our photoelectron
spectra. Comparison of predicted to measured photoelectron spectra
suggests that a significant fraction of the detected electrons originates
from the DNA bases (both deprotonated and neutral)