  Proteins involved in signaling pathways represent an interesting target for
experimental analysis by ODNP (Overhauser Dynamic Nuclear Polarization), which
determines the translational mobility at the surface of proteins. They also
represent a challenge, since the hydration dynamics at all sites remains
relatively rapid, requiring sensitive measurements capable of drawing finer
distinctions. Targeting the protein K-Ras, we find ODNP cross-relaxivity values
that appear consistent within similar regions of 3D space, regardless of the
specific residue where the spin probe used to select the location has been
attached. The similar dynamics observed from nearby residues indicate a
persistence/uniformity of the translational dynamics of water on the nanometer
scale. This results makes sense, since it essentially means that the dynamics
of water remains consistent over a lengthscale (a nanometer) over which liquid
water exhibits structural persistence (i.e. its correlation length). This opens
up the possibility of strategically and comprehensively mapping out the
hydration layer in aqueous solution and identifying regions that contribute
significantly to the free energy of binding interactions -- for example, slow
water that might contribute significant entropy, or regions with strongly
temperature-dependent water mobility that might contribute significant
enthalpy.
