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Biophys. J. BioFAST: First Published January 20, 2006. doi:10.1529/biophysj.105.069088
© 2006 by the Biophysical Society.

A more recent version of this article appeared on April 1, 2006.
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SPECTROSCOPY, IMAGING, OTHER TECHNIQUES

Hydration Dependence of Conformational Dielectric Relaxation of Lysozyme

Joseph R. Knab 1, Jing Yin Chen 1 and Andrea G. Markelz 1*

1 Physics Dept., University at Buffalo, SUNY

* To whom correspondence should be addressed. E-mail: amarkelz{at}buffalo.edu.

Submitted on June 19, 2005
Revised on July 20, 2005
Accepted on 29 December 2005


  Abstract
Dielectric response of hen egg white lysozyme (HEWL) is measured in the far infrared (5-65 cm-1, 0.15-1.95 THz, 0.6-8.1 meV) as a function of hydration. The frequency range is associated with collective vibrational modes of protein tertiary structure. The observed frequency dependence of the absorbance is broad and glass-like. For the entire frequency range there is a slight increase in both the absorbance and index of refraction with increasing hydration for < 0.27 h (mass of H2O per unit mass protein). At 0.27 h, the absorbance and index begin to increase more rapidly. This transition corresponds to the point where the first hydration shell is filled. The abrupt increase in dielectric response cannot be fully accounted for by the additional contribution to the dielectric response due to bulk water, suggesting that the protein has not yet achieved its fully hydrated state. The broad, glass-like response suggests that at low hydrations, the low frequency conformational HEWL dynamics can be described by a dielectric relaxation model where the protein relaxes to different local minima in the conformational energy landscape. However, the low frequency complex permittivity does not allow for a pure relaxational mechanism. The data can best be modeled with a single low frequency resonance ({nu}~ 120 GHz = 4 cm-1) and a single Debye relaxation process ({tau} ~ .03-.04 ps). Terahertz dielectric response is currently being considered as a possible biosensing technique and the results demonstrate the required hydration control necessary for reliable biosensor applications.

Key Words: conformational modes, dielectric relaxation, far infrared, hydration, lysozyme, terahertz




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Copyright © 2006 by the Biophysical Society.