6 research outputs found
Colors for Molecular Masses: Fusion of Spectroscopy and Mass Spectrometry for Identification of Biomolecules
We present an approach that integrates
ultraviolet (UV) photofragmentation
spectroscopy of cold ions with high-resolution Orbitrap mass spectrometry
(MS) and uses mathematical analysis of the recorded 2D data arrays
for structural identification of biomolecules. The synergy of the
two orthogonal techniques makes these arrays unique fingerprints of
molecular ions, enabling their reliable identifications. Using preliminary
created libraries of fingerprints, the UV-MS approach was successfully
applied for quantitative identification of exact isobaric molecules
in their mixtures, which is one of the challenging cases for mass
spectrometry. We also demonstrate how the UV and fragmentation mass
spectra of unknown chemical components of a mixture can be recovered
from its fingerprint even without a use of library
Identification of Isomeric Ephedrines by Cold Ion UV Spectroscopy: Toward Practical Implementation
Ephedrine
and pseudoephedrine are stimulant drugs whose use is
prohibited in athletic competition by the World Anti-Doping Agency
(WADA) at very different threshold doping violation concentrations.
We use a recently developed universal approach that integrates UV
photofragmentation spectroscopy of cold ions with Orbitrap mass spectrometry
(MS) for highly selective and highly sensitive identification of these
diastereomers. Both species can be selectively detected at a solution
concentration of a few tens of ng/mL, which is almost 3 orders of
magnitude lower than the threshold concentration required by WADA.
Relative concentrations of the isomers in solutions have been determined
with the standard deviation of 3.1%, when the ions were cooled in
an ion trap maintained at <i>T</i> = 6 K. Considering practical
implementation of the method, we evaluated its performance for a simplified
instrumentation. At an affordable elevated temperature of ∼70
K and with a low-maintenance midbandwidth optical parametric oscillator,
a few second measurement should yield nearly the same selectivity
and only ten times lower sensitivity than with the current research
grade instrument
Identification of Tyrosine-Phosphorylated Peptides Using Cold Ion Spectroscopy
The accurate and unambiguous detection
of post-translational modifications
in proteins and peptides remains a challenging task. We report here
the use of cold ion spectroscopy for the identification of phosphorylated
tyrosine residues in peptides. This approach employs the wavelength-specific
UV fragmentation of cryogenically cooled protonated peptides in the
gas phase. In addition to the appearance of specific photofragments,
the phosphorylation of tyrosine induces large spectral shifts of the
peptide electronic band origins. Quantum chemical calculations and
experiments together suggest a certain generality of the use of such
shifts in the spectroscopic identification of phosphotyrosines. The
enhanced selectivity offered by the joint application of wavelength-specific
fragmentation and mass spectrometry of cold molecules can also be
used in the identifications of aromatic residues in protonated peptides
and, potentially, of other UV-absorbing groups in a variety of large
polyatomic ions
Microhydration Effects on the Encapsulation of Potassium Ion by Dibenzo-18-Crown‑6
We
have measured electronic and conformer-specific vibrational
spectra of hydrated dibenzo-18-crown-6 (DB18C6) complexes with potassium
ion, K<sup>+</sup>•DB18C6•(H<sub>2</sub>O)<sub><i>n</i></sub> (<i>n</i> = 1–5), in a cold, 22-pole
ion trap. We also present for comparison spectra of Rb<sup>+</sup>•DB18C6•(H<sub>2</sub>O)<sub>3</sub> and Cs<sup>+</sup>•DB18C6•(H<sub>2</sub>O)<sub>3</sub> complexes. We
determine the number and the structure of conformers by analyzing
the spectra with the aid of quantum chemical calculations. The K<sup>+</sup>•DB18C6•(H<sub>2</sub>O)<sub>1</sub> complex
has only one conformer under the conditions of our experiment. For
K<sup>+</sup>•DB18C6•(H<sub>2</sub>O)<sub><i>n</i></sub> with <i>n</i> = 2 and 3, there are at least two
conformers even under the cold conditions, whereas Rb<sup>+</sup>•DB18C6•(H<sub>2</sub>O)<sub>3</sub> and Cs<sup>+</sup>•DB18C6•(H<sub>2</sub>O)<sub>3</sub> each exhibit only one isomer. The difference
can be explained by the optimum matching in size between the K<sup>+</sup> ion and the crown cavity; because the K<sup>+</sup> ion can
be deeply encapsulated by DB18C6 and the interaction between the K<sup>+</sup> ion and the H<sub>2</sub>O molecules becomes weak, different
kinds of hydration geometries can occur for the K<sup>+</sup>•DB18C6
complex, giving multiple conformations in the experiment. For K<sup>+</sup>•DB18C6•(H<sub>2</sub>O)<sub><i>n</i></sub> (<i>n</i> = 4 and 5) complexes, only a single isomer
is found. This is attributed to a cooperative effect of the H<sub>2</sub>O molecules on the hydration of K<sup>+</sup>•DB18C6;
the H<sub>2</sub>O molecules form a ring, which is bound on top of
the K<sup>+</sup>•DB18C6 complex. According to the stable structure
determined in this study, the K<sup>+</sup> ion in the K<sup>+</sup>•DB18C6•(H<sub>2</sub>O)<sub><i>n</i></sub> complexes tends to be pulled largely out from the crown cavity by
the H<sub>2</sub>O molecules with increasing <i>n</i>. Multiple
conformations observed for the K<sup>+</sup> complexes will have an
advantage for the effective capture of the K<sup>+</sup> ion over
the other alkali metal ions by DB18C6 because of entropic effects
on the formation of hydrated complexes
A Decapeptide Hydrated by Two Waters: Conformers Determined by Theory and Validated by Cold Ion Spectroscopy
The
intrinsic structures of biomolecules in the gas phase may not
reflect their native solution geometries. Microsolvation of the molecules
bridges the two environments, enabling a tracking of molecular structural
changes upon hydration at the atomistic level. We employ density functional
calculations to compute a large pool of structures and vibrational
spectra for a gas-phase complex, in which a doubly protonated decapeptide,
gramicidin S, is solvated by two water molecules. Though most vibrations
of this large complex are treated in a harmonic approximation, the
water molecules and the vibrations of the host ion coupled to them
are locally described by a quantum mechanical vibrational self-consistent
field theory with second-order perturbation correction (VSCF-PT2).
Guided and validated by the available cold ion spectroscopy data,
the computational analysis identifies structures of the three experimentally
observed conformers of the complex. They, mainly, differ by the hydration
sites, of which the one at the Orn side chain is the most important
for reshaping the peptide toward its native structure. The study demonstrates
the ability of a quantum chemistry approach that intelligently combines
the semiempirical and <i>ab initio</i> computations to disentangle
a complex interplay of intra- and intermolecular hydrogen bonds in
large molecular systems
Ion Selectivity of Crown Ethers Investigated by UV and IR Spectroscopy in a Cold Ion Trap
Electronic and vibrational spectra of benzo-15-crown-5
(B15C5)
and benzo-18-crown-6 (B18C6) complexes with alkali metal ions, M<sup>+</sup>•B15C5 and M<sup>+</sup>•B18C6 (M = Li, Na,
K, Rb, and Cs), are measured using UV photodissociation (UVPD) and
IR–UV double resonance spectroscopy in a cold, 22-pole ion
trap. We determine the structure of conformers with the aid of density
functional theory calculations. In the Na<sup>+</sup>•B15C5
and K<sup>+</sup>•B18C6 complexes, the crown ethers open the
most and hold the metal ions at the center of the ether ring, demonstrating
an optimum matching in size between the cavity of the crown ethers
and the metal ions. For smaller ions, the crown ethers deform the
ether ring to decrease the distance and increase the interaction between
the metal ions and oxygen atoms; the metal ions are completely surrounded
by the ether ring. In the case of larger ions, the metal ions are
too large to enter the crown cavity and are positioned on it, leaving
one of its sides open for further solvation. Thermochemistry data
calculated on the basis of the stable conformers of the complexes
suggest that the ion selectivity of crown ethers is controlled primarily
by the enthalpy change for the complex formation in solution, which
depends strongly on the complex structure