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42678-AC6
Threshold Photoionization Spectroscopy of Gas Phase Nucleic Acid Compounds
By coupling laser desorption (LD) for vaporization of non-volatile molecules and zero energy kinetic electron spectroscopy (ZEKE), we have made progress in obtaining submillimeter and far infrared spectroscopy of cationic species. So far we have obtained ZEKE spectroscopy of tetracene and pentacene, and we have finished the initial survey of the resonantly enhanced multiphoton ionization (REMPI) spectroscopy of phenylalanine. We should complete the ZEKE experiment of phenylanaline within the next month. Our investigation of the polycyclic aromatic hydrocarbon (PAH) series is motivated by their conformational simplicity and by their importance in the interstellar medium and comet dusts. The skeletal modes of these compounds are believed to be the key in identifying the chemical composition of the interstellar medium. This study is therefore ideal for fine tuning the experimental apparatus. Biological molecules, on the other hand, have additional conformational flexibilities. Our work on phenylalanine has revealed seven conformers in the REMPI experiment. With the assistance of vibrational assignment of both the intermediate state and the final ionic state, we should be able to sort through all the relevant conformers and identify their different ionization thresholds. We will then further study water clusters of phenylalanine, other amino acids, and nucleic acid bases. This type of information is relevant to photostability, charge transfer, and a few other related biological processes.
A substantial amount of our effort has been devoted to optimization of the condition of LD to obtain internally cooled molecular species for spectroscopic investigation. Using para-aminobenzoic acid as a vibrational thermometer, we have determined that the LD source can reach vibrational temperatures of ~30 K. However, for some species, such as coronene and perylene, our repetitive effort did not result in any resolvable REMPI spectrum. We are still puzzled by the origin of this problem. Nevertheless, for tetracene and pentacene, the skeletal modes are clearly assigned, and the vibrational analysis agrees with the change in the molecular structure upon electronic excitation and further ionization.
For example, compared with the highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO) of tetracene has two additional nodal planes along the short axis of the molecule, and it has no nodal plane along the long axis. Consequently, in the REMPI spectrum, the ribbon in-plane stretching modes along both axes are activated. Removal of the excited electron, on the other hand, does not introduce any changes in the distribution of nodal planes, and the molecular frame retracts back somewhat to the structure of the initial ground state. Consequently, progressions of the stretching modes are observable in the ZEKE spectra. This result is different from our previous studies of substituted aromatic systems, where only one vibrational band dominates the ZEKE spectrum.
The situation of phenylalanine is quite different from those of the PAH system. Different conformers have different ionization thresholds and different vibrational excitations. In the past, we have obtained completely different vibrational spectra for different conformers, and rich information on the conformational interchange has been deduced. We hope to pursue the same type of investigation with phenylalanine and its water clusters and ultimately, polypeptides and polynucleotides.
