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

摘要

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 approx=0.09 THz for the protein.