Observation of fragile-to-strong dynamic crossover in protein hydration water

doi:10.1073/pnas.0602474103

Observation of fragile-to-strong dynamic crossover in protein hydration water

*Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139;
^‡Department of Chemistry and Consorzio Interuniversitario per lo Sviluppo dei Sistemi a Grande Interfase (CSGI), University of Florence, via della Lastruccia 3, 50019 Florence, Italy;
^§NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899-8562; and
^¶Department of Materials Science and Engineering, University of Maryland, College Park, MD 20742-2115

Communicated by H. Eugene Stanley, Boston University, Boston, MA, March 28, 2006 (received for review March 11, 2006)

Abstract

At low temperatures, proteins exist in a glassy state, a state that has no conformational flexibility and shows no biological functions. In a hydrated protein, at temperatures ≳220 K, this flexibility is restored, and the protein is able to sample more conformational substates, thus becoming biologically functional. This “dynamical” transition of protein is believed to be triggered by its strong coupling with the hydration water, which also shows a similar dynamic transition. Here we demonstrate experimentally that this sudden switch in dynamic behavior of the hydration water on lysozyme occurs precisely at 220 K and can be described as a fragile-to-strong dynamic crossover. At the fragile-to-strong dynamic crossover, the structure of hydration water makes a transition from predominantly high-density (more fluid state) to low-density (less fluid state) forms derived from the existence of the second critical point at an elevated pressure.

Footnotes

^†To whom correspondence should be addressed. E-mail: sowhsin{at}mit.edu
Author contributions: S.-H.C. designed research; S.-H.C., L.L., A.F., and E.M. performed research; E.F. and P.B. contributed new reagents/analytic tools; L.L. and A.F. analyzed data; and S.-H.C. and L.L. wrote the paper.
Conflict of interest statement: No conflicts declared.
Abbreviations:

Abbreviations:

FSC,

fragile-to-strong dynamic crossover;

ISF,

intermediate scattering function;

QENS,

quasi-elastic neutron scattering;

RCM,

relaxing-cage model.