Majors in other disciplines often do honors research with Chemistry Faculty and those major departments are listed in parenthesis after their names.
Title: Oxidation of Atmospherically-Relevant Organic Aerosols and Related Phospholipids
Advisors: Laura Voss and Elizabeth Stemmler
Abstract: The oxidative effect of ozone on monounsaturated aerosol fatty acids and lung surfactant phosphatidylcholines was studied. Bulk and monolayer samples of oleic acid and bulk samples of lung surfactant phosophocholines POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) and DOPC (dioleoyl-sn-glycero-3-phosphocholine) were exposed to varying levels of ozone. Products were examined using gas chromatography-mass spectrometry (GC-MS) and matrix assisted laser desorption ionization- Fourier transform mass spectrometry (MALDI-FTMS). The GC-MS data identified the production of low mass products nonanal, nonanoic acid, azelaic acid, and 9-oxononanoic acid from the bulk oleic acid samples, verifying that oxidation did occur in both phosphatidylcholines and oleic acid. Aldehydic and carboxylic acid phosphatidylcholine products from POPC were detected using MALDI-FTMS. The only oxidation products from experiments with monolayers of oleic acid on water droplets that were detected with significant yield using GC-MS were nonanoic acid and 9-oxononanoic acid. Overall, no polymer or hydroperoxide products were detected.
Title: Mechanisms of Cationic Amine Sorption to Montmorillonite
Advisor: Dharni Vasudevan
Abstract: Numerous cationic amine-containing chemicals have been released and detected within the environmnet, and some of these pose potential health risks. Retention or sorption to soils plays a major role in determining the environmental transport and exposure of chemicals in the environment. Thus, understanding cationic amine sorption behavior is of great enviornmental relevance and importance. The goal of this study was to investigate the sorptive behavior of cationic amines to montmorillonite, a prevelant mineral in soil systems. To this end, sorption of p-phenylenediamine to montmorillonite was compared to previously studied sorption of aniline, a monoamine analog. This comparison elucidated sorption mechanisms and associated intermolecular interactions for sorption below and above the cation exchange capacity (CEC) of montmorillonite.
The influence of compound charge was evident for sorption below the CEC, with p-phenylenediamine sorbing to a much greater extent than aniline. Computational atomic charge analysis revealed that the +2 p-phenylenediamine cation possesses a greater positive charge on the amine fuctional group than do the +1 cations of p-phenylenediamine and anilinium. This along with a seemingly linear sorption isotherm suggested that sorption of p-phenylenediamine below the CEC is driven primarily by cation exchange and that this process exclusively involves the +2 cation. In addition, analysis of Kd (equilibrium sorption constant) values pointed to non-linearity in sorption behavior and suggested that favorable intermolecular interactions aid sorption below the CEC.
Title: The Thermal and Photochemical Stabilities and the Excited Electronic States of Natural and Synthetic Polyenes
Advisor: Ronald Christensen
Abstract: We studied the photochemical and thermal stabilities of all-trans β-carotene and a synthetic, methyl-terminated N=11 polyene. HPLC was used to analyze changes in teh samples after exposure to heat and light in the presence of iodine as a photocatalyst. A C30 reverse phase carotenoid column and diode array detector were used for hte separation and identification of β-carotene isomers. All-trans β-carotene displayed a significantly different distribution of cis isomers when exposed to light and to heat. A C18 reverse phase column was used for analysis of the synthetic polyene, which was found to be significantly more stable to thermal and photochemical change. We also examined the electronic energies of HPLC-purified synthetic polyenes using room temperature and 77 K absorption and fluorescence spectroscopy. Corrected fluorescence and fluorescence excitation spectra were obtained for both unsubstituted (N=5 and 7) and methyl-terminated (N=5) synthetic polyenes to detect the symmetry-forbidden S1 electronic state. The (0-0) bands of the S1→S0 transistion in room termperature 2-methyl-tetrahydrofuran were 420 nm for N=5 and 520 nm for N=7 unsubstituted. These S1 energies for N=5 and N=7 polyenes were compared with preliminary S1 assignments for the N=7-15 polyenes obtained at the University of Connecticut using transient absorption spectroscopy. Comparison of the S1 energies obtained using the two techniques indicated systematic discrepancies, which will require a reinterpretation of the transient absorption experiments, at least for the shorter polyenes.
Title: 8-Quinolyl-Tetramethylcyclopentadiene: A Tailored Ligand for Cobalt Catalysis
Advisor: Richard Broene
Abstract: The goal of this project was the synthesis of a novel cobalt catalyst, [ethene-η5 -[1-(8-quinolyl)-2,3,4,5-tetramethylcyclopentadiene]-cobalt(I)]BArf [BArf = (3,5-(CF3)2C6H3)4B- (Et2O)2], for linear α-olefin dimerization. Reported is an improved method to prepare the 8-quinolyl-tetramethyl-cyclopentadiene ligand, used in this catalyst, along with the successful synthesis of diodo-ethene-η5 -[1-(8-quinolyl)-2,3,4,5-tetramethyl-cyclopentadiene]-cobalt(III), the immediate precursor to the target catalyst complex.
Title: Peptide Fragmentations in Mass Spectrometry: The Role of C-terminal Basic Amino Acid Residues
Advisor: Elizabeth Stemmler
Abstract: Fragmentation energies and patterns of fragmentation of dibasic peptides containing a basic C-terminal residue were studied using sustained off-resonance irradiation with collision-induced dissociation (SORI-CID) following matrix assisted laser/desorption ionization (MALDI) in a Fourier transform mass spectrometer (FTMS) with the goal of better understanding peptide fragmentations. Understanding peptide fragmentation mechanisms is important for the effective use of mass spectrometry as a peptide-sequencing tool. In our study, two sets of model peptides were analyzed to determine factors influencing the low energy production of [bn-1+H2O]+ ions. In one set of model peptides, the location of the basic arginine residue (R) was varied (RAAAAK, ARAAAK, AARAAK, AAARAK and AAAARK); in the second set the basicity of residue X was varied (X = R, K, H, F, acetylated-K and homoarginine) in the sequence RAAAAX. Model peptides of the first set were synthesized by solid-phase synthesis techniques and all peptides were characterized by energy-resolved SORI-CID. Peptides with a basic C-terminal residue showed selective fragmentation, often yielding one dominant [bn-1+H2O]+ fragment ion in what is presumed to be a rearrangement reaction. Model peptides of the first set did not show any significant difference in fragmentation or energy for dissociation as the location of the basic arginine residue was varied. Results for the basicity variance demonstrated that RAAAAK and RAAAAH produce the most selective cleavage, whereas acetylated RAAAAK acts as a monobasic peptide producing more sequence-informative fragment ions, similar to results observed for RAAAAAF. RAAAAR and guanidinated RAAAAK, the two peptides with the most basic C-termini, undergo selective fragmentation that is different from that observed for RAAAAK and RAAAAH, likely due to an alternative mechanism.
Title: Characterization of Pyrolysis Oil: Fractionation and Analysis by GC/MS and MALDI-FTMS
Advisor: Elizabeth Stemmler
Abstract: To provide information on how the properties of pyrolysis oil are influenced by pyrolyzing conditions, oil and pyrolyzed lignin samples from Virginia Polytech (VTT) and University of Maine at Orono (UMaine) were analyzed using selective solvent extractions and mass spectrometric techniques (GC/MS and MALDI-FTMS). Selective solvent extractions were used to fractionate pyrolysis oils to form solutions of related compounds. Two extraction techniques were used: a multi-step scheme was used to exhaustively fractionate the components of VTT pyrolysis oils, and a two-step scheme that utilized toluene and ethyl acetate extractions, which separated compounds based on polarity in UMaine pyrolysis oil and pyrolyzed lignin samples. Gas chromatography-mass spectrometry (GC/MS) analysis of the fractions was able to distinguish between pyrolyzed hardwood lignin (substituted syringols) and pyrolyzed softwood lignin (substituted guaiacols). Results also indicated a conversion from substituted guaiacols to substituted catechols with increasing pyrolysis temperature. To increase the number of chemicals detectable using GC/MS, derivatization with N-methyl-N-trimethylsilyl-trifluoroacetamide (MSTFA) was used to improve the chromatographic performance of polar compounds by creating trimethylsilyl derivatives. High resolution mass measurements were conducted using matrix assisted laser desorption ionization-Fourier transform mass spectrometry (MALDI-FTMS). For UMaine pyrolysis oil samples, high resolution mass data were converted to Kendrick mass units to obtain information about relative oxygen content and degree of alkylation. The toluene fraction showed decreased oxygen content and degree of alkylation with higher pyrolysis temperature and the reverse trend was observed for the ethyl acetate fraction.