5 research outputs found
Structures and Unimolecular Reactivity of Gas-Phase [Zn(Proline-H)]<sup>+</sup> and [Zn(Proline-H)(H<sub>2</sub>O)]<sup>+</sup>
A combination
of IRMPD spectroscopy, collision-induced dissociation,
deuterium isotopic substitution, and computational chemistry was used
to determine the structure and unimolecular chemistry of [ZnÂ(Pro-H)]<sup>+</sup> and the singly hydrated complex in the gas phase. Five competing
dissociation channels were observed: loss of H<sub>2</sub>O, CO, CO<sub>2</sub>, and HCOOH and the main fragmentation pathway, loss of neutral
Zn. By comparing the IRMPD spectrum with the predicted IR spectra
of the lowest energy structures, it was confirmed that [ZnÂ(Pro-H)]<sup>+</sup> complex is deprotonated at the amine moiety, and a hydrogen
from either C2 or C5 migrated to Zn<sup>2+</sup>. In this H-type complex,
ZnH<sup>+</sup> was chelated between the amine nitrogen and the carbonyl
oxygen. Calculations of the potential energy surface revealed that
the loss of neutral zinc is energetically more favorable than the
loss of dehydrogenated proline leading to ZnH<sup>+</sup> product.
Furthermore, calculations on all five primary decomposition routes,
all beginning with the lowest energy structure, revealed that loss
of Zn has the lowest energy requirement, consistent with it being
the most abundant product of unimolecular dissociation following collisional
or IR multiphoton activation. For the singly hydrated complex, [ZnÂ(Pro-H)Â(H<sub>2</sub>O)]<sup>+</sup>, IRMPD spectroscopy confirms a structure with
water added to the H-type structure and intramolecularly hydrogen
bonded to the deprotonated amine site. This structure is not the lowest-energy
[ZnÂ(Pro-H)Â(H<sub>2</sub>O)]<sup>+</sup> isomer, but it is the one
where water is added to the lowest energy [ZnÂ(Pro-H)]<sup>+</sup> isomer
IRMPD Spectroscopic Study of Microsolvated [Na(GlyAla)]<sup>+</sup> and [Ca(GlyAla–H)]<sup>+</sup> and the Blue Shifting of the Hydrogen-Bonded Amide Stretch with Each Water Addition
In this study, the structures of
[NaÂ(GlyAla)Â(H<sub>2</sub>O)]<sup>+</sup> and [CaÂ(GlyAla–H)Â(H<sub>2</sub>O)<sub><i>n</i></sub>]<sup>+</sup>, (<i>n</i> = 1–3) solvated ion–molecule
complexes (as well as the AlaGly isomers) were investigated using
infrared multiple photon dissociation (IRMPD) spectroscopy and with
computational methods. Calculations showed that in the calcium clusters,
the lowest-energy complex is the one in which the peptide is deprotonated
at the carboxylic acid end and that Ca<sup>2+</sup> binds to both
carboxylate oxygen atoms as well as the amide carbonyl oxygen. For
the microsolvated structures, all three water molecules also bind
directly to Ca<sup>2+</sup>. For the singly, doubly, and triply solvated
complexes, these structures are supported by experimental IRMPD spectra.
For the [NaÂ(GlyAla)Â(H<sub>2</sub>O)]<sup>+</sup> complex, both carbonyl
oxygen atoms, one from the intact carboxylic acid and one from the
amide group, as well as the water molecule were found to be bound
to the Na<sup>+</sup>. In all of the spectra, a strong band is observed
between 3300 and 3400 cm<sup>–1</sup> and is assigned to the
amide N–H stretch, which is red-shifted due to hydrogen bonding
with the amine nitrogen. The position of the hydrogen-bonded amide
N–H stretch is experimentally and theoretically found to be
sensitive to the number of water molecules; it is shown to blue shift
upon successive hydrations
Gas-Phase Structures of Pb<sup>2+</sup>-Cationized Phenylalanine and Glutamic Acid Determined by Infrared Multiple Photon Dissociation Spectroscopy and Computational Chemistry
Infrared multiple photon dissociation (IRMPD) spectroscopy
in the
3200–3800 cm<sup>–1</sup> region was used to determine
the gas-phase structures of bare and monohydrated [PbÂ(Phe-H)]<sup>+</sup> and [PbÂ(Glu-H)]<sup>+</sup>. These experiments were supported
by infrared spectra calculated at the B3LYP/6-31+GÂ(d,p) level of theory
as well as 298 K enthalpies and Gibbs energies determined using the
MP2Â(full)/6-311++GÂ(2d,2p)//B3LYP/6-31+GÂ(d,p) method. The gas-phase
structure of [PbÂ(Phe-H)]<sup>+</sup> has Pb<sup>2+</sup> bound in
a tridentate fashion between Phe’s amine nitrogen, one oxygen
of the deprotonated carboxyl group, and the aromatic ring. The IRMPD
spectrum of [PbÂ(Glu-H)]<sup>+</sup> can be assigned to a structure
where the side chain carboxyl group is deprotonated. The structure
of [PbÂ(Phe-H)ÂH<sub>2</sub>O]<sup>+</sup> is simply the hydrated analogue
of [PbÂ(Phe-H)]<sup>+</sup> where water attaches to Pb<sup>2+</sup> in the same hemisphere as the ligated amino acid. The spectrum of
[PbÂ(Glu-H)ÂH<sub>2</sub>O]<sup>+</sup> could not be assigned a unique
structure. The IRMPD spectrum shows features attributed to symmetric
and antisymmetric O–H stretching of water and a broad band
characteristic of a hydrogen bonded O–H stretching vibration.
These features can only be explained by the presence of at least two
isomers and agree with the computational results that predict the
four lowest energy structures to be within 6 kJ mol<sup>–1</sup> of one another
Ammoniated Complexes of Uracil and Transition Metal Ions: Structures of [M(Ura-H)(Ura)(NH<sub>3</sub>)]<sup>+</sup> by IRMPD Spectroscopy and Computational Methods (M = Fe, Co, Ni, Cu, Zn, Cd)
The
structures of deprotonated d-block metal dication bound uracil
dimers, solvated by a single ammonia molecule, were explored in the
gas phase using infrared multiple photon dissociation (IRMPD) spectroscopy
in a Fourier transform ion cyclotron resonance–mass spectrometer.
The IRMPD spectra were then compared with computed IR spectra for
various isomers. Calculations were performed using B3LYP with the
6-31+GÂ(d,p) basis set for all atoms, with the exception of Cd, for
which the LANL2DZ basis set with relativistic core potentials was
used. The calculations were then repeated using the def2-TZVPP basis
set on all atoms and were compared to the first set of calculations.
The lowest-energy structures are those in which one uracil is deprotonated
at the N3 position and, aside from the Cu complex, the intact uracil
is a tautomer in which the N3 hydrogen is at the O4 carbonyl oxygen.
The metal displays a tetradentate interaction to the uracil moieties,
with the exception of Cu, which is tridentate, and the ammonia molecule
is bound directly to the metal center. In the Cu complex, a square
planar geometry is observed about the metal center, consistent with
Jahn–Teller distortions commonly observed in CuÂ(II) complexes,
and the intact uracil assumes its canonical tautomer. All other metal
cation complexes are five-coordinate, square pyramidal complexes,
with the intact uracil adopting a tautomer in which the N3 hydrogen
is on O4. The IRMPD spectroscopic data are consistent with the computed
infrared spectra for the lowest-energy structures in all cases
Distinguishing Isomeric Peptides: The Unimolecular Reactivity and Structures of (LeuPro)M<sup>+</sup> and (ProLeu)M<sup>+</sup> (M = Alkali Metal)
The
unimolecular chemistries and structures of gas-phase (ProLeu)ÂM<sup>+</sup> and (LeuPro)ÂM<sup>+</sup> complexes when M = Li, Na, Rb,
and Cs have been explored using a combination of SORI-CID, IRMPD spectroscopy,
and computational methods. CID of both (LeuPro)ÂM<sup>+</sup> and (ProLeu)ÂM<sup>+</sup> showed identical fragmentation pathways and could not be
differentiated. Two of the fragmentation routes of both peptides produced
ions at the same nominal mass as (Pro)ÂM<sup>+</sup> and (Leu)ÂM<sup>+</sup>, respectively. For the litiated peptides, experiments revealed
identical IRMPD spectra for each of the <i>m</i>/<i>z</i> 122 and 138 ions coming from both peptides. Comparison
with computed IR spectra identified them as the (Pro)ÂLi<sup>+</sup> and (Leu)ÂLi<sup>+</sup>, and it is concluded that both zwitterionic
and canonical forms of (Pro)ÂLi<sup>+</sup> exist in the ion population
from CID of both (ProLeu)ÂLi<sup>+</sup> and (LeuPro)ÂLi<sup>+</sup>. The two isomeric peptide complexes could be distinguished using
IRMPD spectroscopy in both the fingerprint and the CH/NH/OH regions.
The computed IR spectra for the lowest energy structures of each charge
solvated complexes are consistent with the IRMPD spectra in both regions
for all metal cation complexes. Through comparison between the experimental
spectra, it was determined that in lithiated and sodiated ProLeu,
metal cation is bound to both carbonyl oxygens and the amine nitrogen.
In contrast, the larger metal cations are bound to the two carbonyls,
while the amine nitrogen is hydrogen bonded to the amide hydrogen.
In the lithiated and sodiated LeuPro complexes, the metal cation is
bound to the amide carbonyl and the amine nitrogen while the amine
nitrogen is hydrogen bonded to the carboxylic acid carbonyl. However,
there is no hydrogen bond in the rubidiated and cesiated complexes;
the metal cation is bound to both carbonyl oxygens and the amine nitrogen.
Details of the position of the carboxylic acid Cî—»O stretch
were especially informative in the spectroscopic confirmation of the
lowest energy computed structures