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Torsional Vibrational Modes of Tryptophan Studied by Terahertz Time-Domain Spectroscopy

B. Yu ^*, F. Zeng ^*, Y. Yang ^*, Q. Xing ^*, A. Chechin , X. Xin ^*, I. Zeylikovich ^* and R.R. Alfano ^*

^* Institute for Ultrafast Spectroscopy and Lasers, New York State Center for Advanced Technology for Ultrafast Photonics Materials and Applications, The City College and Graduate Center of the City University of New York, New York, New York 10031; and Stuyvesant High School, New York, New York 10031

Correspondence: Address reprint requests to R. R. Alfano, E-mail: alfano{at}sci.ccny.cuny.edu.

The low-frequency torsional modes, index of refraction, and absorption of a tryptophan film and pressed powders from 0.2 to 2.0 THz (6.6–66 cm^-1) were measured by terahertz time-domain spectroscopy at room temperature. It was found that there were two dominated torsional vibrational modes at around 1.435 and 1.842 THz. The associated relaxation lifetimes (1 ps) for these modes of the tryptophan molecule were measured. Using a density-functional calculation, the origins of the observed torsional vibrations were assigned to the chain and ring of the tryptophan molecule.

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