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Various aliphatic amino acids are thermally placed into the gas phase and expanded into a vacuum system. Time of flight mass spectrometry (TOFMS) and infrared (IR) spectroscopy in the 2500-4000 cm-1 range (CH, NH, OH, stretching vibrations) are employed to investigate their properties. 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 IR radiation prior to ionization, an IR 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.
Additional studies for sugars are underway with the same experimental and theoretical approach. Sugars have not only a very rich neutral chemistry, but also a very surprising ion chemistry: many sugar ions are not stable and their fragmentation patterns are a topic of much experimental and theoretical interest.
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