Effects of Protein Conformational Flexibilities and
Electrostatic Interactions on the Low-Frequency Vibrational Spectrum
of Hydration Water
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Abstract
The conformational flexibility of
a protein and its ability to
form hydrogen bonds with water are expected to influence the microscopic
properties of water layer hydrating the protein. Detailed molecular
dynamics simulations with an aqueous solution of the globular protein
barstar have been carried out to explore such influence on the low-frequency
vibrational spectrum of the hydration water molecules. The calculations
reveal that enhanced degree of confinement at the protein surface
on freezing its local motions leads to increasingly restricted oscillatory
motions of the hydration water molecules as evident from larger blue
shifts of the corresponding band. Interestingly, conformational fluctuations
of the protein and electrostatic component of its interaction with
the solvent have been found to affect the transverse and longitudinal
oscillations of hydration water molecules in a nonuniform manner.
It is further noticed that the distributions of the low-frequency
modes for the water molecules hydrogen bonded to the residues of different
segments of the protein are heterogeneously altered. The effect is
more around the frozen protein matrix and agrees well with slower
protein–water hydrogen bond relaxations