2 research outputs found
Isomer-Selected Photoelectron Spectroscopy of Isolated DNA Oligonucleotides: Phosphate and Nucleobase Deprotonation at High Negative Charge States
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)
Ion Mobility Spectrometry, Infrared Dissociation Spectroscopy, and ab Initio Computations toward Structural Characterization of the Deprotonated Leucine-Enkephalin Peptide Anion in the Gas Phase
Although
the sequencing of protonated proteins and peptides with
tandem mass spectrometry has blossomed into a powerful means of characterizing
the proteome, much less effort has been directed at their deprotonated
analogues, which can offer complementary sequence information. We
present a unified approach to characterize the structure and intermolecular
interactions present in the gas-phase pentapeptide leucine-enkephalin
anion by several vibrational spectroscopy schemes as well as by ion-mobility
spectrometry, all of which are analyzed with the help of quantum-chemical
computations. The picture emerging from this study is that deprotonation
takes place at the C terminus. In this configuration, the excess charge
is stabilized by strong intramolecular hydrogen bonds to two backbone
amide groups and thus provides a detailed picture of a potentially
common charge accommodation motif in peptide anions