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Collective Dynamics of Lysozyme in Water: Terahertz Absorption Spectroscopy and Comparison with Theory

Summary: Collective Dynamics of Lysozyme in Water: Terahertz Absorption Spectroscopy and
Comparison with Theory
Jing Xu,*, Kevin W. Plaxco, and S. James Allen
Department of Physics and Department of Chemistry and Biochemistry, UniVersity of California,
Santa Barbara, California 93106
ReceiVed: July 28, 2006; In Final Form: September 15, 2006
To directly measure the low-frequency vibrational modes of proteins in biologically relevant water environment
rather than previously explored dry or slightly hydrated phase, we have developed a broadband terahertz
spectrometer suitable for strongly attenuating protein solutions. Radiation is provided by harmonic multipliers
(up to 0.21 THz), a Gunn oscillator (at 0.139 THz), and the UCSB free-electron lasers (up to 4.8 THz). Our
spectrometer combines these intense sources with a sensitive cryogenic detector and a variable path length
sample cell to detect radiation after it is attenuated by more than 7 orders of magnitudes by the aqueous
sample. Using this spectrometer, we have measured the molar extinction of solvated lysozyme between 0.075
and 3.72 THz (2.5-124 cm-1
), and we made direct comparison to several published theoretical models based
on molecular dynamics simulations and normal-mode analysis. We confirm the existence of dense, overlapping
normal modes in the terahertz frequency range. Our observed spectrum, while in rough qualitative agreement
with these models, differs in detail. Further, we observe a low-frequency cutoff in terahertz dynamics between
0.2 and 0.3 THz, and we see no evidence of a predicted normal mode at 0.09 THz for the protein.


Source: Allen, S. James - Department of Physics, University of California at Santa Barbara


Collections: Physics; Materials Science