Hexuronic Acid Stereochemistry Determination in Chondroitin Sulfate Glycosaminoglycan Oligosaccharides by Electron Detachment Dissociation

Electron detachment dissociation (EDD) has previously provided stereo-specific product ions that allow for the assignment of the acidic C-5stereochemistry in heparan sulfate glycosaminoglycans (GAGs), but application of the same methodology to an epimer pair in the chondroitin sulfate glycoform class does not provide the same result. A series of experiments have been conducted in which glycosaminoglycan precursor ions are independently activated by electron detachment dissociation (EDD), electron induced dissociation (EID), and negative electron transfer dissociation (NETD) to assign the stereochemistry in chondroitin sulfate (CS) epimers and investigate the mechanisms for product ion formation during EDD in CS glycoforms. This approach allows for the assignment of electronic excitation products formed by EID and detachment products to radical pathways in NETD, both of which occur simultaneously during EDD. The uronic acid stereochemistry in electron detachment spectra produces intensity differences when assigned glycosidic and cross-ring cleavages are compared. The variations in the intensities of the doubly deprotonated (0,2)X(3) and Y(3) ions have been shown to be indicative of CS-A/DS composition during the CID of binary mixtures. These ions can provide insight into the uronic acid composition of binary mixtures in EDD, but the relative abundances, although reproducible, are low compared with those in a CID spectrum acquired on an ion trap. The application of principal component analysis (PCA) presents a multivariate approach to determining the uronic acid stereochemistry spectra of these GAGs by taking advantage of the reproducible peak distributions produced by electron detachment.     

Electron photodetachment from gas phase peptide dianions. Relation with optical absorption properties

Electron detachment from peptide dianions is studied as a function of the laser wavelength. The first step for the detachment is a resonant electronic excitation of the dianions. Electronic excitation spectra are reported for three peptides, including gramicidin. A comparative study of the detachment yield for 13 peptides was performed at 260 nm and at 220 nm. At 260 nm, the detachment yield is mainly driven by the sum of the absorption coefficients of the aromatic amino acids that are contained in the peptide. At 220 nm, no direct relation is observed between the electron photodetachement yields and the sum of absorption efficiencies. At this wavelength, the sequence and the structure of the peptide may have an influence on the photodetachment process.     

Tautomeric forms of azolide anions: vertical electron detachment energies and Dyson orbitals

Several decouplings of the electron propagator, including the relatively new P3+ approximation for the self-energy, have been used to calculate vertical electron detachment energies of tautomeric forms of closed-shell, pentagonal, aromatic anions in which ring carbons without bonds to hydrogens appear. This study extends previous work in which the most stable forms of anionic, five-member rings with one to five nitrogens were considered. Whereas the lowest electron detachment energies sometimes are assigned by Koopmans's theorem results to pi orbital vacancies, electron propagator calculations always obtain sigma orbital vacancies for the ground states of the doublet radicals. Higher electron detachment energies that correspond to excited doublets with pi vacancies also are presented. The predicted transition energies are in good agreement with low-intensity peaks in recent anion photoelectron spectra that have been assigned to less stable, tautomeric forms of these anions.     

A theoretical study on structures, energetics, and spectra of Br-.nCO2 clusters: towards bridging the gap between micro-domain and macro-domain

Structures, energetics, and spectra of Br(-).nCO(2) (n = 1-8) clusters are studied based on ab initio electronic structure theory. The geometry of each size of clusters is evaluated by employing second-order Moller-Plesset (MP2) perturbation theory. It is observed that the solvent CO(2) molecules approach the bromide moiety from one side in an asymmetric fashion except for the Br(-).8CO(2) cluster. Simple electrostatic model for charge-quadrupole interactions is valid for the Br(-).nCO(2) clusters. Reduced variational space based energy decomposition method shows that the electrostatic interaction is the major component and polarization and charge transfer energies are the other significant components of the total interaction energy. Both adiabatic and vertical electron detachment energies and solvation energies are calculated at MP2 level of theory. We have observed an excellent agreement between theory and experiment for the vertical detachment and solvation energies. Calculated quantities based on the analytical expression which connects the finite domain to macroscopic one are found to be very good in agreement with the available experimental results. The present study reveals a 2.6 eV increase in the detachment energy of bromide anion due to the solvation effect of CO(2), which is relatively small compared to that of the corresponding 4.7 eV increase in detachment energy in water.     

Gas phase photo-formation and vacuum UV photofragmentation spectroscopy of tryptophan and tyrosine radical-containing peptides

Tryptophan (Trp(?)) and tyrosyl (Tyr(?)) radical containing peptides were produced by UV laser-induced electron detachment from a suitable precursor. Vacuum ultraviolet (VUV) action spectra of these radical peptides were recorded with synchrotron radiation in the 4.5-16 eV range, from which fragmentation pathways and yields are measured as a function of the VUV photon energy. An enhancement in photofragmentation yields of radical species by 1 order of magnitude with respect to nonradical peptides is demonstrated here for the first time. Photofragmentation spectra are compared with absorption spectra for model chromophores calculated in the frame of the time-dependent density functional theory (TDDFT). A qualitative agreement in the position of bands in the 6-8 eV region is observed between experimental photofragmentation and calculated absorption spectra. Photofragmentation spectra of peptide radicals can be useful to better assess the complex deactivation pathways that occur following the absorption of a VUV photon in biomolecular radical anions.     

Do anionic titanium dioxide nano‐clusters reach bulk band gap? A density functional theory study

The electronic properties of both neutral and anionic (TiO₂)_n (n = 1–10) clusters are investigated by extensive density functional theory calculations. The predicted electron detachment energies and excitation gaps of anionic clusters agree well with the original experimental anion photoelectron spectra (APES). It is shown that the old way to analyze APES tends to overestimate vertical excitation gaps (VGA) of large anionic clusters, due to the nature of multiple electronic origins for the higher APES bands. Moreover, the VGA of anionic TiO₂ clusters are evidently smaller than those of neutral clusters, which may also be the case for other metal oxide clusters with high electron affinity. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010     

Photoelectron spectroscopy and Ab initio study of the structure and bonding of Al7N- and Al7N

The electronic and geometrical structures of Al7N- are investigated using photoelectron spectroscopy and ab initio calculations. Photoelectron spectra of Al7N- have been obtained at three photon energies with six resolved spectral features at 193 nm. The spectral features of Al7N- are relatively broad, in particular for the ground state transition, indicating a large geometrical change from the ground state of Al7N- to that of Al7N. The ground state vertical detachment energy is measured to be 2.71 eV, whereas only an upper limit of approximately 1.9 eV can be estimated for the ground state adiabatic detachment energy due to the broad detachment band. Global minimum searches for A7N- and Al7N are performed using several theoretical methods. Vertical electron detachment energies are calculated using three different methods for the lowest energy structure and compared with the experimental data. Calculated results are in excellent agreement with the experimental data. The global minimum structure of Al7N- is found to possess C3v symmetry, which can be viewed as an Al atom capping a face of a N-centered Al6N octahedron. In the ground state of Al7N, however, the capping Al atom is pushed inward with the three adjacent Al-Al distances being stretched outward. Thus, even though Al7N still possesses C3v symmetry, it is better viewed as a N-coordinated by seven Al atoms in a cage-like structure. The chemical bonding in Al7N- is discussed on the basis of molecular orbital and natural bond analysis.     

I(-)·(CH3I)2 photoexcitation: the influence of dipole bound states on detachment and fragmentation

We present the results of a photoelectron imaging study of the I(-)·(CH(3)I)(2) cluster anion over excitation wavelengths 355-260 nm. The resulting spectra and photoelectron angular distributions (PADs) suggest extensive electron-molecule interaction following photoexcitation. Fragmentation channels are observed subsequent to excitation between 355 and 330 nm. The origin of these features, which begin 200 meV and peak 70 meV below the X band direct detachment threshold, is described in terms of a predissociative dipole bound state. The nature of the fragments detected and the energetics of the channel opening argue strongly in favor of an asymmetric, head to tail cluster anion geometry posited by Dessent et al. [Acc. Chem. Res. 31, 527 (1998)]. Above the direct detachment threshold, PADs display evidence of phenomena akin to electron-molecule scattering. The fragment anions disappear above the X band threshold but reappear some distance below the second (A) direct detachment band. At these energies there is also rapid variation of the X band PAD, an observation attributed to autodetachment via spin-orbit relaxation of the iodine core of the cluster.     

Photodetachment of zwitterions: probing intramolecular coulomb repulsion and attraction in the gas phase using pyridinium dicarboxylate anions

Zwitterions are critically important in many biological transformations and are used in numerous chemical processes. The consequences of electrostatic effects on reactivity and physical properties, however, are largely unknown. In this work, we report the results of negative ion photoelectron spectra of nine isomeric pyridinium dicarboxylate zwitterions and three nonzwitterionic methoxycarbonylpyridine carboxylate isomers (-O(2)CPyrCO(2)CH(3)). Information about the intramolecular electrostatic interactions was directly obtained from the photoelectron spectra. The adiabatic and vertical detachment energies were measured and understood in terms of intramolecular Coulombic forces. Calculations at the B3LYP and CCSD(T) level were performed and compared to the experimental electron binding energies. Structures, relative stabilities, and the electron detachment sites also were obtained from the calculations.     

Base and phosphate electron detachment energies of deoxyribonucleotide anions

Photoelectron spectra of deoxyribonucleotide anions are interpreted with ab initio, electron propagator calculations. Ground-state structures display hydrogen bonds which are not present in less stable minima that resemble Watson-Crick fragment geometries. For the adenosine and thymidine anions, there are two vertical electron detachment energies (VEDEs) within 0.1 eV of each other that correspond to phosphate- and base-centered Dyson orbitals (DOs). The first VEDE of the cytidine anion belongs to a phosphate-centered DO. The anomalously low VEDE of the guanosine anion is assigned to a base-centered, pi DO. Higher VEDEs of all four anions also are assigned.     

Geometric and electronic structures of multiple-decker one-end open sandwich clusters: Eu(n)(C8H8)n- (n = 1-4)

We have measured the photoelectron spectra of the multiple-decker 1:1 sandwich clusters of Eu(n)(COT)n- (n = 1-4; COT = 1,3,5,7-cyclooctatetraene), synthesized in the gas phase, and studied theoretically the bonding scheme, charge distribution, valence orbital energies, and photodetachment energies. We calculated the ground electronic state X- and the first excited electronic state A-, both of which have strong ionic bonding and a characteristic charge distribution. Moreover, we found that the valence orbital energies of Eu (6s) and COT (L delta) depend strongly on cluster size and their positions in the clusters. With the calculated vertical detachment energies for these valence orbitals, we assigned the peaks in the experimental photoelectron spectra and analyzed the origin of their interesting behavior by employing simple point charge models. From these analyses, it became clear that the characteristic behavior of the spectra is due to the strong ionic bonding and the charge distribution. In addition, using the point charge models, we estimated the vertical detachment energies of the one-dimensional polymer [Eu(COT)]infinity-.     

Near threshold Cl(-)·CH3I photodetachment: apparent 2P1/2 channel suppression and enhanced 2P3/2 channel vibrational excitation

Cl(-)·CH(3)I cluster anion photoelectron images are recorded over a range of detachment wavelengths in the immediate post threshold region. The photoelectron spectral features fall into two categories. A number of weak, photon energy dependent transitions are observed and attributed to atomic anion fragmentation products. Several more intense, higher electron binding energy transitions result from single photon cluster anion detachment. Comparison with I(-)·CH(3)I suggests that the detachment process is more complicated for Cl(-)·CH(3)I. The single photon transition spacing is consistent with CH(3)I ν(3) mode excitation, but the two distinct vibronic bands of I(-)·CH(3)I detachment are not easily distinguished in the Cl(-)·CH(3)I spectra. Similarly, while the spectral intensities for both cluster anions show non-Franck Condon behavior, the level of vibrational excitation appears greater for Cl(-)·CH(3)I detachment. These observations are discussed in terms of low lying electronic states of CH(3)I along the C-I coordinate, and the influence of the CH(3)I moiety on the neutral halogen atom states.     

Photoelectron imaging of negative ions: atomic anions to molecular clusters

The negative ion photoelectron imaging technique is illustrated using two relatively simple atomic and molecular anion systems, and then applied to the study of a cluster system. Photoelectron images of I- and CS2- at 267 nm and 800 nm respectively are presented. Photoelectron spectra and angular distributions are obtained from the images and the concepts underlying these and their interpretation are outlined. The imaging technique is then applied to (CS2)n - (n = 2-4) cluster anions, for which 400 nm images are presented. Features of these images are highlighted and discussed with reference to solvation effects and structural properties of the cluster anionic moiety. Photoelectron signatures of different forms of the cluster core are discussed. These core structures are anionic monomer units solvated by the remaining n - 1 CS2 molecules or covalent dimer units solvated by the remaining n - 2 molecules. Images of the n = 2 anion at 400, 530 and 800 nm reveal information about the electron detachment processes within the different cluster types and both direct detachment and autodetachment are seen. The direct transitions are seen from clusters with either core type, while autodetachment is only seen from clusters with the covalent dimer core. The imaging work also reveals evidence of a previously unreported electronic transition within the direct detachment band due to the covalently bound core type.     

Slow electron velocity-map imaging photoelectron spectra of the methoxide anion

High resolution anion photodetachment spectra are presented for the methoxide anion and its fully deuterated counterpart. The spectra were obtained with slow electron velocity-map imaging. Improved electron affinities are determined for CH3O as 1.5690+/-0.0019 eV and for CD3O as 1.5546+/-0.0019 eV. The spectra resolve many features associated with spin-orbit and vibronic coupling that were not seen in previous photodetachment studies. Photoelectron angular distributions taken as a function of detachment wavelength for the ground vibronic state transitions are recorded and are consistent with the removal of a nonbonding, p-type electron localized on the oxygen atom. Several hot bands and sequence bands are observed for the first time, providing insight into the vibrational structure of the methoxide anion. The results are compared to recent calculations of the anion photoelectron spectra that incorporate bilinear coupling terms among the methoxy vibrational modes and are found to be in reasonable agreement.     

Photoelectron spectroscopy of gramicidin polyanions: competition between delayed and direct emission

We present the first photoelectron (PE) spectra of polypeptide polyanions. Combining PE spectroscopy and mass spectrometry provides a direct measurement of the stability of the polyanions with respect to electron detachment and of the repulsive energy between excess charges. The second electron affinity of gramicidin was found to amount to 2.35 +/- 0.15 eV, and the value of the repulsive Coulomb barrier was estimated to be 0.5 +/- 0.15 eV. The spectra are interpreted as resulting from a competition between delayed and direct emission.     

Mass-spectrometric study of ring-chain tautomerism in a series of substituted 4-hydroxyhexahydropyrimidine-2-thiones

In order to investigate the ring-chain tautomerism of substituted 4-hydroxyhexahydropyrimidine-2-thiones the mass spectra of a series of compounds of this group were studied. It is shown that equilibrium exists between the cyclic hydroxy form and the acyclic oxo form, which belongs to the oxoalkylthiourea class, in a series of 3-alkyl(aryl)-4,6,6-trimethyl derivatives in the gas phase. The mass spectra of these compounds contain intense peaks of [M-18]⁺ and [M-33]⁺ ions, which are formed as a result of the successive elimination of a water molecule and a methyl radical by the molecular ions. The fragmentation of 3-alkyl-4,5-dimethyl derivatives takes place from the open oxo form of the molecular ion with detachment of the terminal groups.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1273–1278, September, 1983.     

Hydrated hydride anion clusters

On the basis of density functional theory (DFT) and high level ab initio theory, we report the structures, binding energies, thermodynamic quantities, IR spectra, and electronic properties of the hydride anion hydrated by up to six water molecules. Ground state DFT molecular dynamics simulations (based on the Born-Oppenheimer potential surface) show that as the temperature increases, the surface-bound hydride anion changes to the internally bound structure. Car-Parrinello molecular dynamics simulations are also carried out for the spectral analysis of the monohydrated hydride. Excited-state ab initio molecular dynamics simulations show that the photoinduced charge-transfer-to-solvent phenomena are accompanied by the formation of the excess electron-water clusters and the detachment of the H radical from the clusters. The dynamics of the detachment process of a hydrogen radical upon the excitation is discussed.     

Communication: Revised electron affinity of SF6 from kinetic data

Previously determined experimental data for thermal attachment of electrons to SF(6) and thermal detachment from SF(6)(-) over the range 590-670 K are reevaluated by a third-law analysis. Recent high precision calculations of SF(6)(-) harmonic frequences and anharmonicities (for several of the modes) lead to considerable changes in modeled vibrational partition functions which then have to be accommodated for by a smaller value of the derived adiabatic electron affinity EA of SF(6). The previously estimated value of EA = 1.20 (±0.05) eV in this way is reduced to a value of EA = 1.03 (±0.05) eV. In addition, the bond dissociation energy E(0,dis) for SF(6)(-) → SF(5)(-) + F is reduced to E(0,dis) = 1.44 (±0.05) eV. Finally, the consequences for modeled specific rate constants k(det)(E,J) of electron detachment from SF(6)(-) are discussed.     

Photoelectron spectroscopy of fullerene dianions C76(2-), C78(2-), and C84(2-)

We report laser photoelectron spectra of the doubly negatively charged fullerenes C(76) (2-), C(78) (2-), and C(84) (2-) at 2.33, 3.49, and 4.66 eV photon energy. From these spectra, second electron affinities and vertical detachment energies, as well as estimates for the repulsive Coulomb barriers are obtained. These results are discussed in the context of electrostatic models. They reveal that fullerenes are similar to conducting spheres, with electronic properties scaling with their size. The experimental spectra are compared with the accessible excited states of the respective singly charged product ions calculated in the framework of time dependent density functional theory.     

Influence of charge state and sodium cationization on the electron detachment dissociation and infrared multiphoton dissociation of glycosaminoglycan oligosaccharides

Electron detachment dissociation (EDD) Fourier transform mass spectrometry has recently been shown to be a useful method for tandem mass spectrometry analysis of sulfated glycosaminoglycans (GAGs). EDD produces abundant glycosidic and cross-ring fragmentations that are useful for localizing sites of sulfation in GAG oligosaccharides. Although EDD fragmentation can be used to characterize GAGs in a single tandem mass spectrometry experiment, SO3 loss accompanies many peaks and complicates the resulting mass spectra. In this work we demonstrate the ability to significantly decrease SO3 loss by selection of the proper ionized state of GAG precursor ions. When the degree of ionization is greater than the number of sulfate groups in an oligosaccharide, a significant reduction in SO3 loss is observed in the EDD mass spectra. These data suggested that SO3 loss is reduced when an electron is detached from carboxylate groups instead of sulfate. Electron detachment occurs preferentially from carboxylate versus sulfate for thermodynamic reasons, provided that carboxylate is in its ionized state. Ionization of the carboxylate group is achieved by selecting the appropriate precursor ion charge state, or by the replacement of protons with sodium cations. Increasing the ionization state by sodium cation addition decreases, but does not eliminate, SO3 loss from infrared multiphoton dissociation of the same GAG precursor ions.