  In order to inquire the microscopic origin of observed multiple time scales
in solvation dynamics we carry out several computer experiments. We perform
atomistic molecular dynamics simulations on three protein-water systems namely,
Lysozyme, Myoglobin and sweet protein Monellin. In these experiments we mutate
the charges of the neighbouring amino acid side chains of certain natural
probes (Tryptophan) and also freeze the side chain motions. In order to
distinguish between different contributions, we decompose the total solvation
energy response in terms of various components present in the system. This
allows us to capture the interplay among different self and cross-energy
correlation terms. Freezing the protein motions removes the slowest component
that results from side chain fluctuations, but a part of slowness remains. This
leads to the conclusion that the slow component in the ~20-80 ps range arises
from slow water molecules present in the hydration layer. While the more than
100 ps component may arise from various sources namely, adjacent charges in
amino acid side chains, the water molecules that are hydrogen bonded to them
and a dynamically coupled motion between side chain and water. The charges, in
addition, enforce a structural ordering of nearby water molecules and helps to
form local long-lived hydrogen bonded network. Further separation of the
spatial and temporal responses in solvation dynamics reveals different roles of
hydration and bulk water. We find that the hydration layer water molecules are
largely responsible for the slow component whereas the initial ultrafast decay
arise predominantly (~80%) due to the bulk. This agrees with earlier
theoretical observations. We also attempt to rationalise our results with the
help of a molecular hydrodynamic theory that was developed using classical time
dependent density functional theory in a semi quantitative manner.
