ABSTRACT
Introduction .......................................................................................................... 12
Qualitative Predictions of the Dynamic Exchange Model............................ 13
Solvation Dynamics............................................................................................. 15
Solvation Dynamics in Aqueous Protein Solutions ............................... 16
Solvation Dynamics in Aqueous Micellar Solutions.............................. 18
Dielectric Relaxation............................................................................................ 19
Dielectric Relaxation of Aqueous Protein Solutions .............................. 19
Dielectric Relaxation of Aqueous Micellar Solutions ............................ 20
Single Water Molecule Reorientational Dynamics ......................................... 22
Water Reorientation in Aqueous Protein Solutions ............................... 22
Water Reorientation in Aqueous Micellar Solution ............................... 25
Hydrogen Bond Lifetime Dynamics................................................................. 25
Protein-Water Hydrogen Bond................................................................. 26
Micelle-Water Hydrogen Bond................................................................. 28
Protein-Glass Transition .................................................................................... 28
Concluding Remarks........................................................................................... 33
Acknowledgments ............................................................................................... 34
Appendix A .......................................................................................................... 34
System and Simulation Details of CsPFO Micelle.................................. 34
System and Simulation Details of DTAB Micelle ................................... 34
System and Simulation Details of 1ETN Protein.................................... 34
System and Simulation Details of HP-36 Protein................................... 35
References ............................................................................................................. 36
Amolecular level understanding of the biological activity of a given protein
is a goal that is hard to achieve but highly sought after. To perform this
biological activity, such as ligand binding, the protein must undergo certain
critical motions or fluctuations. The timescales of these motions may
determine the reaction pathways and also the rate. Since biological activity is
connected to the hydration level of a protein (Rupley and Careri, 1991), it is
natural to enquire how these water molecules also participate in the
dynamical events that lead to activity. Thus a microscopic level under-
standing of the dynamical coupling between the protein and the interfacial
water molecules (hydration layer) is crucial to understand microscopic
aspects of biological processes. Because of the importance of the issues
involved, this area is currently a subject of intense research. Ultrafast laser
spectroscopic techniques and computer simulation studies have played a
crucial role in answering many of the detailed questions regarding the
timescale and the nature of the dynamics near a protein surface.
Experiments and simulation studies have shown that the protein side
chain motions that are essential for its functionality require the breaking and
making of protein-water hydrogen bonds (Gu and Schoenborn, 1995;
Doster and Settles, 1998). The bond dynamics, however, is determined by the
protein-water coupling and the hydration layer dynamics. The water
molecules in the hydration layer experiences a surface that is heterogeneous,
even on amolecular length scale, and the interaction with the surface is often
quite strong, leading to a disruption of the regular hydrogen bond network
of bulk water. Studies have shown that the hydration layer exhibits rich
dynamical properties that are quite distinct from those of the bulk
(Nandi et al., 2000; Balasubramanian et al., 2002, 2003; Bizzarri and
Cannistraro, 2002; Pal et al., 2002, 2004; Bagchi, 2003; Bandyopadhyay
et al., 2004). The dynamics involves multiple timescales, ranging from the
bulk-like timescale to at least an order of magnitude slower than that of the
bulk. As a prototype for simple globular proteins, the study of micelle is
important because it is considered a biomimetic system. This article deals
with the slow dynamics of water molecules at the surface of self-assemblies
and proteins in aqueous solution. Considering these systems together, we
obtain a broader view of water dynamics at an interacting surface. A
schematic representation of the hydration layer around a protein is given in
Figure 2.1.