Page 258 - MODERN ELECTROCHEMISTRY

194 CHAPTER 2

As expected, decreases near the ions concerned. However, the structure of water
is disturbed by the field from such charged bodies only 500–600 pm out into the
solvent.

2.24.2. Molecular Dynamics Approach to Protein Hydration
Internal water plays a role in the structure of proteins. It is difficult to detect and
measure these waters by means of X-rays and therefore statistical thermodynamic
calculations may be helpful.
An example of this kind of calculation, due to Wade et al., is the computation of
the hydration of two internal cavities in a sulfate-binding protein. The results are given
in Table 2.30. The main difference between having a “dry” cavity and having a wet
one is the hydration bond energy.

2.24.3. Protein Dynamics as a Function of Hydration

Proteins in the dry state are “frozen.” They only open up and start moving ifsome
water is added, as in nature. It turns out that protein movements in, e.g., lysozyme are
activated only when there is 0.15 g of water per gram of protein, a good example of
the effect of hydration on living processes. However, it is difficult to examine protein
dynamics in solution because to make a satisfactory interpretation ofthe observations,
one would have first to do the corresponding spectroscopy in the dry state; this is
difficult because of the “frozen” state referred to and a tendency to decompose.
To avoid this difficulty, one technique is to use reverse micelles. These materials
can host a protein in a small water pool. Reverse micelles are spherical aggregates
formed by dissolving amphiphiles in organic solvents. The polar head of the am-
phiphilic molecule is in the interior of the aggregate and the hydrophobic tail is in the
organic phase. The micellar suspension is transparent, and controlled amounts of water
can be added.