This study focuses on the application of 193-nm excimer laser (ArF) photodissociation to tandem time-of-flight mass spectrometry. In particular, it focuses on identifying the optimal experimental conditions for peptide sequencing and applying the technology to interesting systems. The early focus is on optimizing the sample preparation conditions that define the initial internal energy state of MALDI-produced ions. Subsequent chapters investigate the effect of changing photodissociation laser conditions and define conditions under which the information content of the spectrum is maximized. Later chapters compare the photodissociation experiment to technologies that represent the current state of the art in tandem mass spectrometry, illustrating both the advantages and shortcoming of photodissociation TOF methodology. Finally, we apply photodissociation to the study of interesting systems of biological relevance, including (1) peptides derived from enzymatic digestion, (2) post-translationally modified peptides, and (3) peptide-transition metal ion complexes. In the final chapter we consider the analytical implications of the work as a whole and comment on the analytical viability of the methodology and look forward to new directions for the experiments.

The photodissociation dynamics of cyanogen iodide (ICN) in the A continuum was studied by the three-dimensional sliced fluorescence imaging (3DSFI) method. The fourth harmonic of a Nd:YAG laser (266 nm) was used to dissociate a low pressure (5 mTorr) sample of ICN, and the resulting CN photofragments were probed by optical-optical double resonance (OODR) detection scheme. A theoretical framework on the emitted fluorescence intensity has been developed to analyze the experimentally obtained image. The vector correlations among the ICN parent molecule transition dipole moment £g, the CN fragment recoil velocity v and the CN rotational angular moment J can be revealed.

The focus of this dissertation is the study of the photodissociation dynamics of halogen oxide species (XO, X = Cl, Br, I). These radical species are known to be important in stratospheric and tropospheric ozone depletion cycles. They are also useful benchmark systems for the comparison to current theoretical methods where they provide insight into the dynamics occurring beyond the Franck-Condon region. These systems are studied using velocity map ion imaging, a technique that measures velocity and angular information simultaneously. Photofragment species are state-selectively ionized for detection using 2+1 REMPI (Resonance Enhanced Multi-Photon Ionization). The instrumentation employs a molecular beam of the XO radicals formed using pyrolitic and photolytic methods. The current work involves the measurement of fundamental physical constants of the XO species. The bond dissociation energy of IO is measured. Vibrational level dependent correlated final state branching ratios of the predissociation of the A(^2 II_3/2) state of ClO and BrO are reported, and comparison to theoretical methods is discussed.

This study focuses on the application of 193-nm excimer laser (ArF) photodissociation to tandem time-of-flight mass spectrometry. In particular, it focuses on identifying the optimal experimental conditions for peptide sequencing and applying the technology to interesting systems. The early focus is on optimizing the sample preparation conditions that define the initial internal energy state of MALDI-produced ions. Subsequent chapters investigate the effect of changing photodissociation laser conditions and define conditions under which the information content of the spectrum is maximized. Later chapters compare the photodissociation experiment to technologies that represent the current state of the art in tandem mass spectrometry, illustrating both the advantages and shortcoming of photodissociation TOF methodology. Finally, we apply photodissociation to the study of interesting systems of biological relevance, including (1) peptides derived from enzymatic digestion, (2) post-translationally modified peptides, and (3) peptide-transition metal ion complexes. In the final chapter we consider the analytical implications of the work as a whole and comment on the analytical viability of the methodology and look forward to new directions for the experiments.

Thesis (Ph. D.)--University of Chicago, Dept. of Chemistry, August 2001. / Includes bibliographical references. Also available on the Internet.

Photodissociation methods have been explored for structural analysis of peptides and oligosaccharides. Ultraviolet photodissociation (UVPD) was applied to carboxylated derivatized peptides and reducing end derivatized oligosaccharides which offer selective dissociation and specific fragmentation pathways in comparison to CID. Upon UVPD of the modified peptides at carboxylate comprised of reduced y ions and increased immonium ions. The derivatized oligosaccharides via reductive amination and hydrazide conjugation can undergo highly efficient 355 nm UVPD and offer different fragmentation pathways. Both derivatization methods upon UVPD yielded [superscript 0,2] A-type ions, however reductive amination and hydrazide conjugation produced dominant [superscript 0,1] A and [superscript 2,4] A-type ions, respectively. Ultraviolet photodissociation at 193 nm (ArF laser, 6.4 eV / photon) has been applied to sialylated oligosaccharides and glycans which were analyzed in negative mode due to their acidic condition. Primarily, UVPD provides a greater array of fragment ions including cross-ring cleavages and dual cleavage internal ions in comparison to CID. In addition, the UVPD generates unique fragment ions which arise from site-specific cleavage of the trial substituent of the sialic acid residue. UVPD of doubly deprotonated sialylated oligosaccharides produced mostly singly deprotonated fragment ions, whereas the product ions in the CID spectra were overwhelmingly doubly charged ions, an outcome attributed to the more extensive cleavages of sialic acid residue upon UVPD. Although electron transfer dissociation (ETD) has shown superior capabilities for the characterization of post-translational modifications of peptides due to its non-eragodic property, ETD has intrinsic drawback arising from its significant dependence on the charge state of the selected precursor ion. Precursor ions in low charge states tend to undergo charge reduction, often preferentially relative to production of the informative cand z-type ions. In order to increase charge states of peptides and ETD efficiencies, peptides were derivatized at their carboxylate groups via attachment of amine with fixed charge or hydrophobic group. The carboxylate-derivatized peptides exhibited higher ETD efficiencies relative to underivatized peptides along with greater numbers of diagnostic fragment ions. The carboxylate derivatization strategy in combination with ETD for proteomics applications by the proteolytic digestion, the derivatization, and LC-MS purification was demonstrated with Cytochrome C.

Studying the intrinsic properties of molecules in the gas-phase is advantageous, since it reduces the complexity present in solution that arises from interactions between the molecule of interest and other species present in the local environment, including those with the solvent itself. In this report, the photophysical properties of gaseous cationic rhodamine B (RBH+) were determined and photodissociation reaction kinetics and power dependence of three prototropic forms of fluorescein; the cation ([F + H]+), monoanion ([F - H]-), and dianion ([F – 2H]-2), each of which possesses their own distinct spectral properties, were measured. The analyte ions of interest were formed via electrospray ionization, mass-selected and stored in a quadrupole ion trap mass spectrometer which has been customized to enable gas-phase spectroscopic studies. Knowledge of the intrinsic photophysical properties of such chromophores in the gas-phase will enable a better understanding of how the local environment of the molecule alters its properties.

Studying the intrinsic properties of molecules in the gas-phase is advantageous, since it reduces the complexity present in solution that arises from interactions between the molecule of interest and other species present in the local environment, including those with the solvent itself. In this report, the photophysical properties of gaseous cationic rhodamine B (RBH+) were determined and photodissociation reaction kinetics and power dependence of three prototropic forms of fluorescein; the cation ([F + H]+), monoanion ([F - H]-), and dianion ([F – 2H]-2), each of which possesses their own distinct spectral properties, were measured. The analyte ions of interest were formed via electrospray ionization, mass-selected and stored in a quadrupole ion trap mass spectrometer which has been customized to enable gas-phase spectroscopic studies. Knowledge of the intrinsic photophysical properties of such chromophores in the gas-phase will enable a better understanding of how the local environment of the molecule alters its properties.

Studies of gas-phase ionic clusters have become an integral component in understanding microsolvation and catalysis by transition metal cations. Further interest in this field is due to the possibility of bridging the gap between the condensed and gas phases by developing our understanding of clusters and the possibility that small clusters can have unique chemical and catalytic properties. Most gas phase studies have focused on singly charged ions. Electrospray allows for the production of multiply charged ions solvated by a few solvent molecules. Understanding smaller reactive species such as metal centered clusters with well-defined, gas phase conditions also allows for detailed comparison between theory and experiments. In these studies the main focus is to understand bond activation by transition metal cations and solvation of transition metal dications. The gas phase ions of interest are studied using an electrospray-ionization or laser-ablation dual time-of-flight mass spectrometer and are characterized using photofragment spectroscopy in the visible and ultraviolet regions of the spectrum. Photofragment spectroscopy is a powerful method that can be used in gas phase studies to gather a wealth of information on the ions' bond strengths, spectroscopic constants, and dissociation kinetics and dynamics. The study of TiO + (CO 2 ) spectroscopy (Chapter 3) was a result of study of CO 2 bond activation by Ti + that went on to provide a wealth of information on the spectroscopy and dissociation kinetics of this molecule. An electronic transition of the TiO + chromophore was observed, 2 Π[arrow left] 2 Δ, revealing new information about the excited state and the effect of TiO + electronic state on the metal- CO 2 ligand interaction. The photodissociation spectrum of this molecule is well resolved and shows progressions in the covalent Ti-O stretch and metal-ligand stretch and rock. The lifetime of electronically excited TiO + (CO 2 ) was measured, and depends strongly on vibrational energy. Calculations on TiO + and TiO + (CO 2 ) were combined with experimental results on TiO + (CO 2 ) to predict spectroscopic transitions of TiO + , an astrophysically interesting molecule. The photodissociation dynamics of M 2+ (CH 3 CN)n(H 2 O)m where M = Co and Ni, (Chapter 4) is important in understanding the gas phase microsolvation of metal dications. The coordination number and type of solvent affect the dissociation pathways. M 2+ (CH 3 CN)n (n>2) primarily lose a solvent molecule. Electron transfer is a minor channel for n=3 and is the only channel observed for n=2. Mixed clusters M 2+ (CH 3 CN)n(H 2 O)m preferentially lose water. Loss of acetonitrile is a minor channel, as is proton transfer. Water is the proton donor. Replacing acetonitrile with water increases the proton transfer channel. Nickel and cobalt complexes show similar dissociation dynamics, with proton transfer more likely for nickel complexes. Methane activation by transition metal catalysts is industrially important as it can be used to produce gasoline from natural gas. We studied the products and intermediates of the reaction of laser-ablated platinum atoms with methane (Chapter 5). Photoionization efficiency curves were measured for PtCH 2 and the [H-Pt-CH 3 ] insertion intermediate using tunable vacuum ultraviolet light. The resulting ionization energies were combined with bond strengths for the cations to derive bond strengths for the neutrals. These were used to construct a potential energy surface for methane activation by platinum atoms.

The factors affecting peptide fragmentation have been extensively studied in the literature in order to better predict the fragment ion spectra of peptides and proteins. While there are countless influences to consider, metal cation binding in the gas-phase is particularly interesting. Herein, a comparison of fragmentation patterns of a model peptide series with various charge carriers (H+, Li+, Na+, K+, and Cu+) will assist in determining the location of the preferred binding site of the metal cation and in assessing differences in the fragmentation pattern as a result of this binding site. An interesting observation from these studies reveals abundant x-type fragment ions occurring from the fragmentation of alkali-metal cationized peptides. As these fragment ions have been observed in previous studies by others but not addressed, the factors affecting the formation of these x-type fragment ions are explored. Additionally, a home-built 193-nm photodissociation tandem time-of-flight mass spectrometer is utilized to study how peptide fragmentation kinetics affect the fragmentation pattern observed. Initially, the fragmentation timescales of various peptides are investigated. Results indicate that longer fragmentation timescales (~10 microseconds) result in an increased number of identified peaks with internal and ammonia loss fragment ions being the most common in comparison to 'prompt' fragmentation timescales (~1 microsecond). Furthermore, b-type fragment ion formation is also favored at longer timescales for the arginine containing peptides investigated. The fragmentation pattern of several proline containing peptides is examined by collision-induced dissociation and 193-nm photodissociation. Unique fragment ions are observed with each occurring at a proline residue. Few differences are detected between CID and 193-nm photodissociation spectra, indicating that the proline residues direct fragmentation rather than the dissociation method. In an effort to improve the performance of the photodissociation tandem TOF instrument, the addition of a second source and a dual-stage reflectron are incorporated. The modifications result in improved mass range, signal-to-noise, and increased fragment ion collection efficiencies. High quality mass spectra are acquired across a range of mass-to-charge ratios from ~600 to 1900. Furthermore, the modifications continue to allow investigation of various fragmentation timescales with the addition of an additional timeframe of ~3 microseconds.