Search

Sodiated complexes of the aliphatic amino acids, Gly, Ala, Val, Leu, and Ile, were examined with infrared multiple-photon dissociation action spectroscopy utilizing light from a free-electron laser. To identify structures, the experimental spectra were compared to linear spectra calculated at the B3LYP/6-311+G(d,p) level of theory. Relative energetics of all complexes were calculated at B3LYP, B3P86, MP2(full), B3LYP-GD3BJ, and M06-2X levels using a 6-311+G(2d,2p) basis set. Spectral comparison for all complexes indicates that the dominant conformation, [N, CO], binds to the amino nitrogen and carbonyl oxygen. For all complexes except Gly, contributions are observed from [CO2-] structures, where the sodium cation binds to both oxygens of the carboxylate group in the zwitterionic form of the amino acid. The semiquantitative distribution between these two structures appears to be best-predicted by the B3LYP and MP2(full) levels of theory, with predictions from the other three levels inconsistent with the experiment.

We present the first experimental vibrational spectra of gaseous oxidized derivatives of C

Uranium trioxide, UO

The reactivity of the cysteine (Cys) and homocysteine (Hcy) radical cation was studied using ion molecule reactions. The radical cations were generated via collision-induced dissociation (CID) of their S-nitrosylated precursors. Cleavage of the S-NO bond led to the formation of the radical initially positioned on the sulfur atom. The reactions of the radical cations with dimethyl disulfide revealed that the cysteine radical cation reacts more quickly than the homocysteine radical cation. Infrared multiple photon dissociation (IRMPD) spectroscopy and density functional theory (DFT) calculations were used to determine the structure of the homocysteine radical cation, which was compared to the previously published structure of the cysteine radical cation (Sinha et al. Phys. Chem. Chem. Phys. 2010, 12, 9794-9800). IRMPD spectroscopy and DFT calculations revealed that this difference in radical reactivity was not a result of a radical rearrangement for the homocysteine radical cation but rather that the reactivity was modulated by stronger hydrogen bonding.

Flavins are a fundamental class of biomolecules, whose photochemical properties strongly depend on their environment and their redox and metalation state. Infrared multiphoton dissociation (IRMPD) spectra of mass-selected isolated metal–lumiflavin ionic complexes (M+LF) are analyzed in the fingerprint range (800–1830 cm–1) to determine the bonding of lumiflavin with alkali (M = Li, Na, K, Cs) and coinage (M = Cu, Ag) metal ions. The complexes are generated in an electrospray ionization source coupled to an ion cyclotron resonance mass spectrometer and the IR free electron laser FELIX. Vibrational and isomer assignments of the IRMPD spectra are accomplished by comparison to quantum chemical calculations at the B3LYP/cc-pVDZ level, yielding structure, binding energy, bonding mechanism, and spectral properties of the complexes. The most stable binding sites identified in the experiments involve metal bonding to the oxygen atoms of the two available CO groups of LF. Hence, CO stretching frequencies are a sens...

Conformational preferences have been surveyed for divalent metal cation complexes with the dipeptide ligands AlaPhe, PheAla, GlyHis, and HisGly. Density functional theory results for a full set of complexes are presented, and previous experimental infrared spectra, supplemented by a number of newly recorded spectra obtained with infrared multiple photon dissociation spectroscopy, provide experimental verification of the preferred conformations in most cases. The overall structural features of these complexes are shown, and attention is given to comparisons involving peptide sequence, nature of the metal ion, and nature of the side-chain anchor. A regular progression is observed as a function of binding strength, whereby the weakly binding metal ions (Ba(2+) to Ca(2+)) transition from carboxylate zwitterion (ZW) binding to charge-solvated (CS) binding, while the stronger binding metal ions (Ca(2+) to Mg(2+) to Ni(2+)) transition from CS binding to metal-ion-backbone binding (Iminol) by direct metal-nitrogen bonds to the deprotonated amide nitrogens. Two new sequence-dependent reversals are found between ZW and CS binding modes, such that Ba(2+) and Ca(2+) prefer ZW binding in the GlyHis case but prefer CS binding in the HisGly case. The overall binding strength for a given metal ion is not strongly dependent on the sequence, but the histidine peptides are significantly more strongly bound (by 50-100 kJ mol(-1)) than the phenylalanine peptides.

Structures of trivalent lanthanide metal cations La(3+), Ho(3+), and Eu(3+) with deprotonated Ala(n) (n = 2-5) or Leu-enk (Tyr-Gly-Gly-Phe-Leu) are investigated with infrared multiple photon dissociation (IRMPD) spectroscopy between 900 and 1850 cm(-1) and theory. In all of these complexes, a salt bridge is formed in which the metal cation coordinates to the carboxylate group of the peptide, resulting in a limited conformational space and many sharp IRMPD spectral bands. The IRMPD spectra clearly indicate that all carbonyl groups solvate the metal cation in each of the Ala(n) complexes. Due to strong vibrational coupling between the carbonyl groups, a sharp, high-energy amide I band due to in-phase stretching of all of the amide carbonyl groups bound to the metal cation is observed that is separated by approximately 50 cm(-1) from a strong, lower-energy amide I band. This extent of carbonyl coupling, which is sometimes observed in condensed-phase peptide and protein IR spectroscopy, has not been reported in IRMPD spectroscopy studies of other cationized peptide complexes. Intense bands due to carbonyl groups not associated with the metal cation are observed for Leu-enk complexes, indicating that a side chain group, such as the Tyr or Phe aromatic ring, prevents complete carbonyl coordination of the metal cation. Substitution of smaller lanthanide cations for La(3+) in these peptide complexes results only in minor structural changes consistent with the change in metal cation size. These are the first IRMPD spectra reported for lanthanide metal cationized peptides, and comparison to previously reported protonated and alkali metal or alkaline earth metal cationized peptide complexes reveals many trends consistent with the higher charge state of the lanthanide cations.