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ItemAmino Acid Profiling by Reactive Desorption Electrospray Ionization Tandem Mass Spectrometry (DESI-MS/MS)(Georgia Institute of Technology, 2009-12) Rizzo, David G.Desorption electrospray ionization mass spectrometry (DESI MS) has gained significant recognition over the past few years because of its simplicity and rapid throughput capabilities, allowing for the direct analysis of samples with a wide variety of sizes, shapes, and chemistries. Addition of various reagents into the DESI spray solvent enables atmospheric pressure ion molecule reactions between these reagents in the charged micro droplets and analyte molecules on the sample surface affording improved selectivity and sensitivity in some cases. Presented is a rapid screening method for amino acids (aas) based on reactive DESI. Amino acids have been shown to play key roles in the regulation of cellular processes. They are also particularly vital in the determination of metabolic disorders such as phenylketonurea, homocystinuria, and tyrosinemia. The most specific and reliable methods for diagnosing these disorders are based on the determination of aas in body fluids using methodologies such as gas/liquid chromatography, tandem MS, and various combinations thereof. However, these methods are usually time-consuming, increasing the time physicians wait before administering treatment or regulating the diet of diseased infants. The reactive DESI approach presented here is based on the formation of stable noncovalent complexes between alpha-cyclodextrin (dissolved in the spray solvent) and amino acids present in the sample affording a selective method for their detection. However, the selectivity and sensitivity of screening for aas was improved by performing the MS analysis in the multiple reaction monitoring mode when using a quadrupole ion trap or by the precursor ion scan when using a triple quadrupole MS instrument, affording an average twenty-five times sensitivity improvement compared to analysis in full scan mode. The observation of similar complexes with various carboxylic acids including formic acid and acetic acid, and evidence from tandem MS experiments indicate that amino acid:alpha-cyclodextrin complexation reactions occur by hydrogen bonding interaction with carboxyl group of the aas. The specificity and sensitivity provided by this approach seems very promising for applications in the rapid screening of aas directly from body fluids including urine and plasma for amino acid disorders in a clinical setting.
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ItemDetection of Respiratory Syncytial Virus DNA with Gold Nanorod Surface Enhanced Raman Spectroscopy Active Substrates(Georgia Institute of Technology, 2009-05-04) Siemens, KatherineSurface enhanced Raman scattering (SERS) is a powerful spectroscopic tool that can be used to identify and characterize compounds at low concentrations. Recent literature reports suggest that SERS may be applicable for the detection and identification of viruses present in biofluids. In this investigation, gold nanorod arrays were evaluated as SERS substrates with the specific purpose of probing spectral differences between single stranded (ssDNA) and hybridized, or double stranded (dsDNA) DNA. This was deemed as the first step in developing a SERS-based hybridization assay for viral identification. Hybridization was carried out both off and on the substrate surface to determine whether there are observable spectral differences from the two different methods of hybridization. Studies were also carried out to determine if it was possible to detect mismatched DNA pairs after hybridization had been attempted. Gold SERS active substrates were utilized instead of silver giving the advantage that these substrate do not oxidize under ambient conditions. It has also been found that ozone cleaning gold substrates before sample application increases the hydrophilicity of the gold, making the use of smaller sample volumes possible. Ozone cleaning the gold substrate after ample binding time has passed increases the SERS signal as well. It is believed that this cleaning immediately prior to SERS acquisition cleans the gold surface to a point where the plasmon being formed is more likely to move up the sample which increases the intensity of the SERS signal. In addition to examining the possibility of using gold instead of silver for the SERS substrates, a method allowing for 36 separate DNA assays to be run at one time was investigated. This is accomplished by creating wells with polymer. Up to 36 wells fit on one glass microscope slide meaning that anywhere from 1 to 36 DNA probes can be attached within individual wells. This allows for either 36 different biological assays for the same virus or 1 biological sample to be tested for 36 different viruses or virus strains.
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ItemSeparation and Identification of Peptides by Supercritical Fluid Chromatography Coupled with Mass Spectrometry(Georgia Institute of Technology, 2008-05-05) Terrett, StuartThe presence of certain proteins in physiological fluids could be used as an early diagnostic tool for disease; however, because of the large concentration range of proteins and the number of distinct chemical species the detection and quantification of these proteins is problematic. This research focuses on the ability to separate proteins using supercritical fluid chromatography (SFC), a form of chromatography that uses supercritical carbon dioxide (scCO2) as the mobile phase. This project was divided into two parts. The first is the synthetic aspect that involves reacting an amino acid, in this case tyrosine, with dimethyl-tert-butyl-chlorosilane which substitutes onto the hydroxyl group to increase its solubility in scCO2. The synthesis, purification, and characterization of this novel molecule have been successfully completed. The second part of the project is the optimization of the chromatograph itself, necessitating a complete rebuild of an extant SFC. Much of the internal controls were bypassed or replaced; at this stage, the SFC is capable of injecting and detecting large organic compounds and amino acids. Research efforts are now focused on separating the silylated tyrosine from nonderivatized amino acids. Once achieved, the synthesis will be scaled up to include other amino acids and ultimately small peptides, which should separate more readily and provide identification of target proteins.
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ItemCompeting Excited State Intramolecular Proton Transfer and Photoinduced Electron Transfer in 2-(2 -Arylsulfonamidophenyl)benzimidazoles(Georgia Institute of Technology, 2008-05-05) Chaudhry, Aneese F.Cation-induced inhibition of excited-state intramolecular proton transfer (ESIPT) can be effectively utilized to design fluorescent probes for ratiometric Zn(II)-sensing. The 2-(2'-sulfonamidophenyl)benzimidazole family of fluorophores undergoes efficient ESIPT in protic solvents to yield a highly Stokes-shifted emission of the phototautomer. Upon coordination of Zn(II), ESIPT is disrupted and results in a strongly blue-shifted fluorescence emission. Because Zn(II)-binding competes with protonation of the sulfonamide nitrogen, the coordination equilibrium can principally be tuned by adjusting the sulfonamide pKa. In this study we specifically addressed the question to what extent tuning of the pKa influences the photophysical properties of this class of fluorophores. For this purpose, a series of compounds with varying substituents attached to the sulfonamide aryl-ring were characterized in terms of their pKa, their absorption and emission energies, as well as their quantum yields. Although the pKa varied over almost two orders of magnitude following closely Hammett s free energy relationship, the peak absorption and emission energies remained largely unaltered. Interestingly, the quantum yields of derivatives with strongly electron withdrawing substituents were significantly lower compared to all other fluorophores, suggesting a photoinduced electron transfer process as a possible non-radiative deactivation pathway. Electrochemical analysis revealed indeed an additional reduction wave at a less negative potential for the quenched derivatives. Estimates of the electron transfer driving force based on the Rehm-Weller formalism supported the PET pathway. Furthermore, the quenching mechanism was confirmed through quantum chemical calculations. The findings of this study are expected to aid in the rational design of ESIPT fluorophores for zinc-sensing applications.
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ItemHigh-Throughput Screening of Antimalarials via Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS)(Georgia Institute of Technology, 2007-12-17) Johnson, Kristin R.The counterfeiting of pharmaceuticals, especially antimalarials, is a well-recognized and growing public health problem. There have been an alarming number of reports of counterfeit antimalarials throughout the world and insufficient regulations and xactivity. Thus there is an urgent need for a rapid and sensitive authentication and screening tool for multiple antimalarials. While many methods have been developed using HPLC, MS, or LC-MS to screen individual antimalarials, no methods are available for the analysis of multiple antimalarial drugs within a single run. In this study, Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS) with multiple reaction monitoring (MRM) was used as a selective and rapid way to screen ten common antimalarials. The compounds were first individually analyzed using electrospray ionization mass-spectrometry (ESI-MS). Following this, a corresponding MS/MS spectrum was obtained to enable selection of the optimal unimolecular decay fragmentation ( transition ) of each antimalarial. Finally, these single reaction monitoring (SRM) transitions were combined into a method that utilizes HPLC separation followed by multiple reaction monitoring (MRM) in an ion trap mass analyzer allowing for sequential screening of a mixture of these compounds within a single LC-MS/MS run. This method has both pharmaceutical and medical applications with the capability of providing drug quality control measurements and the detection of many drugs and their metabolites in biological samples.
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ItemStern-Volmer Quenching of Conjugated Polymers: A Study of Fluorophore Concentration(Georgia Institute of Technology, 2007-12-17) Vaughns, Christopher FranklinThe purpose of this thesis is to re-examine the independence of Stern Volmer constants on the fluorophore concentration. Conventional theory suggests that Stern-Volmer constants are independent of fluorophore concentration. Using PPEs as fluorophore, the Stern-Volmer constants were obtained for different PPE concentrations quenched with TbCl3. Stern-Volmer constants increased with decreasing PPE concentration.