Photoelectron photofragment coincidence spectroscopy of aromatic carboxylates: benzoate and p-coumarate.

Photoelectron-photofragment coincidence spectroscopy was used to study the dissociation dynamics of the conjugate bases of benzoic acid and p-coumaric acid. Upon photodetachment at 266 nm (4.66 eV) both aromatic carboxylates undergo decarboxylation, as well as the formation of stable carboxyl radicals. The key energetics are computed using high-level electronic structure methods. The dissociation dynamics of benzoate were dominated by a two-body DPD channel resulting in CO2 + C6H5 + e-, with a very small amount of stable C6H5CO2 showing that the radical ground state is stable and the excited states are dissociative. For p-coumarate (p-CA-) the dominant channel is photodetachment resulting in a stable radical and a photoelectron with electron kinetic energy (eKE) <2 eV. We also observed a minor two-body dissociative photodetachment (DPD) channel resulting in CO2 + HOC6H4CHCH + e-, characterized by eKE <0.8 eV. Evidence was also found for a three-body ionic photodissociation channel producing HOC6H5 + HCC- + CO2. The ion beam contained both the phenolate and carboxylate isomers of p-CA-, but DPD only occurred from the carboxylate form. For both species DPD is seen from the first and second excited states of the radical, where vibrational excitation is required for decarboxylation from the first excited radical state.

Resonant Inelastic X-Ray Scattering Reveals Hidden Local Transitions of the Aqueous OH Radical.

Resonant inelastic x-ray scattering (RIXS) provides remarkable opportunities to interrogate ultrafast dynamics in liquids. Here we use RIXS to study the fundamentally and practically important hydroxyl radical in liquid water, OH(aq). Impulsive ionization of pure liquid water produced a short-lived population of OH(aq), which was probed using femtosecond x-rays from an x-ray free-electron laser. We find that RIXS reveals localized electronic transitions that are masked in the ultraviolet absorption spectrum by strong charge-transfer transitions-thus providing a means to investigate the evolving electronic structure and reactivity of the hydroxyl radical in aqueous and heterogeneous environments. First-principles calculations provide interpretation of the main spectral features.

Overtone-induced dissociation and isomerization dynamics of the hydroxymethyl radical (CH2OH and CD2OH). I. A theoretical study.

The dissociation of the hydroxymethyl radical, CH(2)OH, and its isotopolog, CD(2)OH, following the excitation of high OH stretch overtones is studied by quasi-classical molecular dynamics calculations using a global potential energy surface (PES) fitted to ab initio calculations. The PES includes CH(2)OH and CH(3)O minima, dissociation products, and all relevant barriers. Its analysis shows that the transition states for OH bond fission and isomerization are both very close in energy to the excited vibrational levels reached in recent experiments and involve significant geometry changes relative to the CH(2)OH equilibrium structure. The energies of key stationary points are refined using high-level electronic structure calculations. Vibrational energies and wavefunctions are computed by coupled anharmonic vibrational calculations. They show that high OH-stretch overtones are mixed with other modes. Consequently, trajectory calculations carried out at energies about ~3000 cm(-1) above the barriers reveal that despite initial excitation of the OH stretch, the direct OH bond fission is relatively slow (10 ps) and a considerable fraction of the radicals undergoes isomerization to the methoxy radical. The computed dissociation energies are: D(0)(CH(2)OH → CH(2)O + H) = 10,188 cm(-1), D(0)(CD(2)OH → CD(2)O + H) = 10,167 cm(-1), D(0)(CD(2)OH → CHDO + D) = 10,787 cm(-1). All are in excellent agreement with the experimental results. For CH(2)OH, the barriers for the direct OH bond fission and isomerization are: 14,205 and 13,839 cm(-1), respectively.

Electronically excited and ionized states of the CH2CH2OH radical: a theoretical study.

The low lying excited electronic states of the 2-hydroxyethyl radical, CH(2)CH(2)OH, have been investigated theoretically in the range 5-7 eV by using coupled-cluster and equation-of-motion coupled-cluster methods. Both dissociation and isomerization pathways are identified. On the ground electronic potential energy surface, two stable conformers and six saddle points at energies below approximately 900 cm(-1) are characterized. Vertical excitation energies and oscillator strengths for the lowest-lying excited valence state and the 3s, 3p(x), 3p(y), and 3p(z) Rydberg states have been calculated and it is predicted that the absorption spectrum at approximately 270-200 nm should be featureless. The stable conformers and saddle points differ primarily in their two dihedral coordinates, labeled d(HOCC) (OH torsion around CO), and d(OCCH) (CH(2) torsion around CC). Vertical ionization from the ground-state conformers and saddle points leads to an unstable structure of the open-chain CH(2)CH(2)OH(+) cation. The ion isomerizes promptly either to the 1-hydroxyethyl ion, CH(3)CHOH(+), or to the cyclic oxirane ion, CH(2)(OH)CH(2) (+), and the Rydberg states are expected to display a similar behavior. The isomerization pathway depends on the d(OCCH) angle in the ground state. The lowest valence state is repulsive and its dissociation along the CC, CO, and CH bonds, which leads to CH(2)+CH(2)OH, CH(2)CH(2)+OH, and H+CH(2)CHOH, should be prompt. The branching ratio among these channels depends sensitively on the dihedral angles. Surface crossings among Rydberg and valence states and with the ground state are likely to affect dissociation as well. It is concluded that the proximity of several low-lying excited electronic states, which can either dissociate directly or via isomerization and predissociation pathways, would give rise to prompt dissociation leading to several simultaneous dissociation channels.

The effect of oxidation on the electronic structure of the green fluorescent protein chromophore.

Electronic structure calculations of the singly and doubly ionized states of deprotonated 4(')-hydroxybenzylidene-2,3-dimethylimidazolinone (HBDI anion) are presented. One-electron oxidation produces a doublet radical that has blueshifted absorption, whereas the detachment of two electrons yields a closed-shell cation with strongly redshifted (by about 0.6 eV) absorption relative to the HBDI anion. The results suggest that the doubly oxidized species may be responsible for oxidative redding of green fluorescent protein. The proposed mechanism involves two-step oxidation via electronically excited states and is consistent with the available experimental information [A. M. Bogdanov, A. S. Mishin, I. V. Yampolsky, et al., Nat. Chem. Biol. 5, 459 (2009)]. The spectroscopic signatures of the ionization-induced structural changes in the chromophore are also discussed.

Observation of the fastest chemical processes in the radiolysis of water.

Elementary processes associated with ionization of liquid water provide a framework for understanding radiation-matter interactions in chemistry and biology. Although numerous studies have been conducted on the dynamics of the hydrated electron, its partner arising from ionization of liquid water, H2O+, remains elusive. We used tunable femtosecond soft x-ray pulses from an x-ray free electron laser to reveal the dynamics of the valence hole created by strong-field ionization and to track the primary proton transfer reaction giving rise to the formation of OH. The isolated resonance associated with the valence hole (H2O+/OH) enabled straightforward detection. Molecular dynamics simulations revealed that the x-ray spectra are sensitive to structural dynamics at the ionization site. We found signatures of hydrated-electron dynamics in the x-ray spectrum.

Conversion of He(23 S) to He2( a3Σ u+) in Liquid Helium.

The report of an anomalously intense He4+ peak in electron impact mass spectra of large helium droplets created a stir 3 decades ago that continues to this day. When the electron kinetic energy exceeds 41 eV, an additional pathway opens that yields He4+ predominantly in an electronically excited metastable state. A pair of He*(23 S) atoms has been implicated based on the isolated He* energy of 19.82 eV and the 41 eV threshold, and the creation of He4+ has been conjectured to proceed via a pair of He2*( a3Σ u+) precursors. The mechanism whereby He* converts to He2* in liquid helium has remained a mystery, however. High level ab initio theory combined with classical molecular dynamics has been applied to systems comprising small numbers of He atoms. The conversion of He* to He2* in such systems is shown to be due to a simple many-body effect that yields He2* rapidly and efficiently.

[Quantitative gas chromatographic determination of 3-methylhistidine in biological fluids]. / Kolichestvennyi gazokhromatograficheskii analiz 3-metilgistidina v biologicheskikh zhidkostiakh.

The gas chromatographic procedure is suggested to determine 3-methylhistidine in biological fluids. The amino acid fraction containing 3-methylhistidine is separated by ion-exchange chromatography. Amino acids are transformed into N-trifluoroacetyl-O-isobutyl esters which are analyzed by the gas chromatography instrument with micropacked columns and ionization-resonance detector. The limit of the quantitative determination of 3-methylhistidine is 50 ng per a probe.

Quantum Chemistry Calculations Provide Support to the Mechanism of the Light-Induced Structural Changes in the Flavin-Binding Photoreceptor Proteins.

The proposed mechanisms of photoinduced reactions in the blue light using flavin chromophore photoreceptor proteins are primarily based on the results of X-ray crystallography and spectroscopy studies. Of particular value are the observed band shifts in optical and vibrational spectra upon formation of the signaling (light-induced) state. However, the same set of experimental data has given rise to contradictory interpretations suggesting different structures of the dark and signaling states. To verify the specific mechanism of light-induced changes involving the rotation/tautomerization transformations with the conserved Gln residue near the flavin chromophore, we performed accurate quantum chemical calculations of the equilibrium structures, vibrational and absorption bands of the model systems mimicking the BLUF domain of flavoprotein AppA. Geometry optimization and calculations of vibrational frequencies were carried out with the QM(B3LYP/cc-pVDZ)/MM(AMBER) approach starting from the representative molecular dynamics (MD) snapshots. The MD simulations were initiated from the available crystal structures of the AppA protein. Calculations of the vertical excitation energies were performed with the scaled opposite spin configuration interaction with single substitutions SOS-CIS(D) method that enables efficient treatment of excited states in large molecular systems. The computed molecular structures as well as the spectral shifts (the red shift by 12÷16 nm in absorption and the downshift by 25 cm(-1) for the C4═O flavin vibrational mode) are in excellent agreement with the experimental results, lending a strong support to the mechanism proposed by Domratcheva et al. (Biophys. J. 2008, 94, 3872).

Potential Energy Landscape of the Electronic States of the GFP Chromophore in Different Protonation Forms: Electronic Transition Energies and Conical Intersections.

We present the results of quantum chemical calculations of the transition energies and conical intersection points for the two lowest singlet electronic states of the green fluorescent protein chromophore, 4'-hydroxybenzylidene-2,3-dimethylimidazolinone, in the vicinity of its cis conformation in the gas phase. Four protonation states of the chromophore, i.e., anionic, neutral, cationic, and zwitterionic, were considered. Energy differences were computed by the perturbatively corrected complete active space self-consistent field (CASSCF)-based approaches at the corresponding potential energy minima optimized by density functional theory and CASSCF (for the ground and excited states, respectively). We also report the EOM-CCSD and SOS-CIS(D) results for the excitation energies. The minimum energy S0/S1 conical intersection points were located using analytic state-specific CASSCF gradients. The results reproduce essential features of previous ab initio calculations of the anionic form of the chromophore and provide an extension for the neutral, cationic, and zwitterionic forms, which are important in the protein environment. The S1 PES of the anion is fairly flat, and the barrier separating the planar bright conformation from the dark twisted one as well as the conical intersection point with the S0 surface is very small (less than 2 kcal/mol). On the cationic surface, the barrier is considerably higher (∼13 kcal/mol). The PES of the S1 state of the zwitterionic form does not have a planar minimum in the Franck-Condon region. The S1 surface of the neutral form possesses a bright planar minimum; the energy barrier of about 9 kcal/mol separates it from the dark twisted conformation as well as from the conical intersection point leading to the cis-trans chromophore isomerization.

Femtosecond multidimensional imaging of a molecular dissociation.

The coupled electronic and vibrational motions governing chemical processes are best viewed from the molecule's point of view-the molecular frame. Measurements made in the laboratory frame often conceal information because of the random orientations the molecule can take. We used a combination of time-resolved photoelectron spectroscopy, multidimensional coincidence imaging spectroscopy, and ab initio computation to trace a complete reactant-to-product pathway-the photodissociation of the nitric oxide dimer-from the molecule's point of view, on the femtosecond time scale. This method revealed an elusive photochemical process involving intermediate electronic configurations.

Gas chromatographic determination of 1- and 3-methylhistidine in biological fluids.

A gas chromatographic method for the determination of 1- and 3-methylhistidine in biological fluids was developed. The amino acid fraction containing 1- and 3-methylhistidine was isolated using ion-exchange chromatography. The amino acids were derivatized to N-trifluoroacetyl-O-isobutyl esters and analysed by gas chromatography with micropacked or capillary columns and an ionization-resonance detector. The conditions for the acylation of the histidines were studied and analyses of various derivatives such as N-trifluoroacetates, N-pentafluoropropionates and N-heptafluorobutyrates were tested. The detection limit of 1- and 3-methylhistidine was 50 ng per sample.