4 research outputs found
Multiple Electron Ejection from Proteins Resulting from Single-Photon Excitation in the Valence Shell
One-photon
multiple ionization is a signature of dynamical electron
correlations in atoms and small molecules, as observed in the Auger
process when Auger electron emission follows core–shell ionization.
In such a process, the high energy needed to remove several electrons
is due to the strong Coulombic attraction between the last departing
electron(s) and the ionic core. Multiply negatively charged molecules
offer the possibility to overcome the Coulombic attraction, opening
the way for multielectron photodetachment following valence shell
excitation. Here photodetachment studies have been performed on electrosprayed
protein polyanions using vacuum ultraviolet synchrotron radiation
coupled to a radiofrequency ion trap. Double, triple, and quadruple
electron emissions from protein polyanions resulting from single-photon
excitation in the valence shell were observed with ionization thresholds
below 20 eV photon energy. This suggests the existence of large electronic
correlations in proteins between weakly bound electrons standing on
distant sites. Besides, the resulting multiradical polyanions appear
to be remarkably stable, an important issue in radiobiology
The Gas-Phase Photophysics of Eosin Y and its Maleimide Conjugate
The use of the xanthene
family of dyes as fluorescent probes in
a wide range of applications has provided impetus for the studying
of their photophysical properties. In particular, recent advances
in gas-phase techniques such as FRET that utilize such chromophores
have placed a greater importance on the characterization of these
properties in the gas phase. Additionally, the use of synthetic linker
chains to graft the chromophores in a site-specific manner to their
target system is ubiquitous. There is, however, often limited information
on how the addition of such a linker chain may affect the photophysical
properties of the chromophores, which is of fundamental importance
for interpretation of experimental data reliant on grafted chromophores.
Here, we present data on the optical spectroscopy of different protonation
states of Eosin Y, a fluorescein derivative. We compare the photophysics
of Eosin Y to its maleimide conjugate, and to the thioether product
of the reaction of this conjugate with cysteamine. Comparison of the
mass spectra following laser irradiation shows that very different
relaxation takes place upon addition of the maleimide moiety but that
the photophysics of the bare chromophore are restored upon addition
of cysteamine. This radical change in the photophysics is interpreted
in terms of charge-transfer states, whose energy relative to the S<sub>1</sub> ← S<sub>0</sub> transition of the chromophore is dependent
on the conjugation of the maleimide moiety. We also show that the
shape of the absorption band is unchanged in the gas-phase as compared
to the solution-phase, showing a maximum with a shoulder toward the
blue, and examination of isotope distributions of the isolated ions
show that this shoulder cannot be due to the presence of dimers. Consideration
of the fluorescence emission spectrum allows a tentative assignment
of the shoulder to be due to a vibrational progression with a high
Franck–Condon factor
Visible and Ultraviolet Spectroscopy of Gas Phase Rhodamine 575 Cations
The visible and ultraviolet spectroscopy
of gas phase rhodamine
575 cations has been studied experimentally by action-spectroscopy
in a modified linear ion trap between 220 and 590 nm and by time-dependent
density functional theory (TDDFT) calculations. Three bands are observed
that can be assigned to the electronic transitions S<sub>0</sub> →
S<sub>1</sub>, S<sub>0</sub> → S<sub>3</sub>, and S<sub>0</sub> → (S<sub>8</sub>,S<sub>9</sub>) according to the theoretical
prediction. While the agreement between theory and experiment is excellent
for the S<sub>3</sub> and S<sub>8</sub>/S<sub>9</sub> transitions,
a large shift in the value of the calculated S<sub>1</sub> transition
energy is observed. A theoretical analysis of thermochromism, potential
vibronic effects, and–qualitatively–electron correlation
revealed it is mainly the latter that is responsible for the failure
of TDDFT to accurately reproduce the S<sub>1</sub> transition energy,
and that a significant thermochromic shift is also present. Finally,
we investigated the nature of the excited states by analyzing the
excitations and discussed their different fragmentation behavior.
We hypothesize that different contributions of local versus charge
transfer excitations are responsible for 1-photon versus 2-photon
fragmentation observed experimentally
Action-FRET: Probing the Molecular Conformation of Mass-Selected Gas-Phase Peptides with Förster Resonance Energy Transfer Detected by Acceptor-Specific Fragmentation
The use of Förster resonance
energy transfer (FRET) as a
probe of the structure of biological molecules through fluorescence
measurements in solution is well-attested. The transposition of this
technique to the gas phase is appealing since it opens the perspective
of combining the structural accuracy of FRET with the specificity
and selectivity of mass spectrometry (MS). Here, we report FRET results
on gas-phase polyalanine ions obtained by measuring FRET efficiency
through specific photofragmentation rather than fluorescence. The
structural sensitivity of the method was tested using commercially
available chromophores (QSY 7 and carboxyrhodamine 575) grafted on
a series of small, alanine-based peptides of differing sizes. The
photofragmentation of these systems was investigated through action
spectroscopy, and their conformations were probed using ion mobility
spectrometry (IMS) and Monte Carlo minimization (MCM) simulations.
We show that specific excitation of the donor chromophore results
in the observation of fragments that are specific to the electronic
excitation of the acceptor chromophore. This shows that energy transfer
took place between the two chromophores and hence that the action-FRET
technique can be used as a new and sensitive probe of the structure
of gas-phase biomolecules, which opens perspectives as a new tool
in structural biology