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.