| Heterogeneous Catalysis | |
| Energetic Materials | |
| Bio-active Molecules |
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Bio-active Molecules | |
| Vibrational and Photoionization Spectroscopy of Biomolecules - Structural Probe of Aliphatic amino Acids | |
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Various aliphatic amino acids are thermally placed into the gas phase and expanded into a vacuum system for access by time of flight mass spectroscopy (TOFMS) and infrared spectroscopy in the energy range 2500 to 4000 cm-1 (CH, NH, OH, stretching vibrations). The isolated neutral amino acids are ionized by a single photon of 10.5 eV energy (118 nm), which is within less than 2 eV of their reported ionization thresholds. Following ionization, which does not directly fragment the amino acid ion, the parent ions undergo an isomer dependent structural rearrangement, which causes the amino acid ions to fragment. These fragmentation patterns are isomer dependent and can be related to theoretically predicted structural isomers for the individual amino acids. If the neutral amino acids are exposed to infrared (IR) radiation prior to ionization, an infrared spectrum of the individual isomers for each amino acid can be determined. Both the IR spectrum and fragmentation patterns for individual isomers can be identified and related to a particular isomer in each instance. Thus, each fragment ion detected presents an IR spectrum of its particular parent, amino acid isomer. In some instances the absorption of infrared radiation by the neutral amino acid parent isomer enhances a particular fragmentation mass channel, while other fragmentation mass channels are suppressed by the absorbed vibrational excitation energy. This phenomenon can be rationalized by considering that, with added energy in the molecule, different fragmentation barriers can be surmounted and new fragmentation pathways opened. These data are consistent with theoretical predictions for isolated amino acid secondary structures and can be related to previous infrared spectra of amino acid conformers. Theoretical calculations have been utilized in identifying lowest-energy amino acid structures, in identifying likely fragmentation patterns, and in predicting/identifying infrared absorption frequencies. These calculations thus far have primarily focused on glycine, with the goal of utilizing similar methods to further aid in the identification/analyzing of other amino acid spectra obtained using the described method. | |
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| Mass spectra for valine ionized with 118 nm light with the infrared laser (ca. 3000 cm-1) on (red) and off (blue). Note that most mass channels increase in intensity with the IR laser on but mass channel 73 amu decreases in intensity with the IR laser on. The tiny peak located at m/z 118 in the mass spectrum is assigned as the protonated valine cation, which is the product of the photodissociation of the valine dimer. |
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The IR spectra for valine isomers observed in different fragment mass channels as indicated. The isomers identified in the figure are suggested based on the structures given below |
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