PICOSECOND TO NANOSECOND DYNAMICS OF HYDRATED PROTEIN: COMBINING DIELECTRIC AND NEUTRON SPECTROSCOPY TECHNIQUES WITH MOLECULAR DYNAMICS SIMULATIONS

 

Sheila Khodadadi1, J.H. Roh2,3, V. Garcia Sakai4, E. Mamontov5, Alexei P. Sokolov6,7, Joseph E. Curtis1

1NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899

2Department of Materials Science & Engineering, University of Maryland, MD 20742

3Department of Biophysics, Johns Hopkins University, MD 21218, USA

4ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, UK

5Spallation Neutron Source, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6475

6Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831

7Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996

 

Microscopic picture of protein dynamics provides insights into the protein functionality. Combining dielectric spectroscopy and neutron scattering techniques, we are able to follow protein dynamics over an extremely broad frequency range. We identify several relaxation processes in dielectric spectra of the hydrated lysozyme. Traditionally, the interpretation of these processes observed in dielectric spectra has been ascribed to the relaxation behavior of hydration water bounded to a protein and not to the protein. Based on analysis of neutron scattering data we assign the “main” observed dielectric relaxation process (tmain ~ tens of ps) to the structural relaxation of the protein-water coupled motion. Detailed analysis of the MD simulations and comparison to dielectric data indicate that the slower observed relaxation process (tslow ~ ns) of hydrated protein spectra is mainly due to protein atoms.  The relaxation processes involve the entire structure of protein including atoms in the protein backbone, side chains and turns. Both surface and buried protein atoms contribute to the slow and main processes; however, protein surface atoms demonstrate slightly faster relaxation dynamics.   Analysis of the water molecule residence and dipolar relaxation correlation behavior indicates that the hydration water relaxes at much shorter time scales.