title: Collective dynamics of lysozyme in water: terahertz absorption spectroscopy and comparison with theory.
abstract: 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 approximately 0.09 THz for the protein.
authors.affiliation: Department of Physics and Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, USA. jingx@uci.edu
authors.forename: Jing
authors.initials: J
authors.lastname: Xu
authors.forename: Kevin W
authors.initials: KW
authors.lastname: Plaxco
authors.forename: S James
authors.initials: SJ
authors.lastname: Allen
mesh_terms: D001665:Binding Sites; D008962:Models, Theoretical; D009113:Muramidase / Q000737:chemistry*; D012710:Serum Albumin, Bovine / Q000737:chemistry; D012997:Solvents / Q000737:chemistry; D013054:Spectrophotometry, Atomic / Q000379:methods*; D014867:Water / Q000737:chemistry*
chemical_list: D012997:Solvents; D014867:Water; D012710:Serum Albumin, Bovine; D009113:Muramidase