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Infrared-induced conformational isomerization and vibrational relaxation dynamics in melatonin and 5-methoxy-N-acetyl tryptophan methyl amide

J. Chem. Phys. 120, 9033 (2004); doi:10.1063/1.1697389

Issue Date: 15 May 2004

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Brian C. Dian, Gina M. Florio, Jasper R. Clarkson, Asier Longarte, and Timothy S. Zwier
Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-2084
The conformational isomerization dynamics of melatonin and 5-methoxy N-acetyltryptophan methyl amide (5-methoxy NATMA) have been studied using the methods of IR-UV hole-filling spectroscopy and IR-induced population transfer spectroscopy. Using these techniques, single conformers of melatonin were excited via a well-defined NH stretch fundamental with an IR pump laser. This excess energy was used to drive conformational isomerization. By carrying out the infrared excitation early in a supersonic expansion, the excited molecules were re-cooled into their zero-point levels, partially re-filling the hole created in the ground state population of the excited conformer, and creating gains in population of the other conformers. These changes in population were detected using laser-induced fluorescence downstream in the expansion via an UV probe laser. The isomerization quantum yields for melatonin show some conformation specificity but no hint of vibrational mode specificity. In 5-methoxy NATMA, no isomerization was observed out of the single conformational well populated in the expansion in the absence of the infrared excitation. In order to study the dependence of the isomerization on the cooling rate, the experimental arrangement was modified so that faster cooling conditions could be studied. In this arrangement, the pump and probe lasers were overlapped in space in the high density region of the expansion, and the time dependence of the zero-point level populations of the conformers was probed following selective excitation of a single conformation. The analysis needed to extract isomerization quantum yields from the timing scans was developed and applied to the melatonin timing scans. Comparison between the frequency and time domain isomerization quantum yields under identical experimental conditions produced similar results. Under fast cooling conditions, the product quantum yields were shifted from their values under standard conditions. The results for melatonin are compared with those for N-acetyl tryptophan methyl amide. ©2004 American Institute of Physics.
History: Received 19 September 2003; accepted 17 February 2004
Permalink: http://link.aip.org/link/?JCPSA6/120/9033/1
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KEYWORDS and PACS

Keywords
PACS
  • 82.30.Qt
    Isomerization and rearrangement in chemical reactions
  • 36.20.Ey
    Macromolecular conformation (statistics and dynamics)
  • 36.20.Kd
    Electronic structure and spectra of macromolecules
  • 87.15.He
    Biomolecular dynamics and conformational changes
  • 87.15.Mi
    Spectra, photodissociation, and photoionization of biomolecules; bioluminescence
  • 33.15.Mt
    Molecular rotation, vibration, and vibration-rotation constants
  • 33.20.Tp
    Vibrational analysis (molecular spectra)
  • 33.15.Bh
    General molecular conformation and symmetry; stereochemistry
  • 33.20.Ea
    Infrared molecular spectra
  • 33.20.Lg
    Ultraviolet molecular spectra
  • 33.80.Be
    Molecular level crossing and optical pumping
  • 33.50.Dq
    Molecular fluorescence and phosphorescence spectra
  • YEAR: 2004

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PUBLICATION DATA

ISSN:
0021-9606 (print)   1089-7690 (online)
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