Chemistry Reference
In-Depth Information
+
Hydration
change
FIGURE 11.1
Changes of hydration accompany many biomolecular processes and reac-
tions. The enumeration of the number of water molecules released (or adsorbed) for such
reactions was a matter of debate.
(See color insert.)
Slope =
Hydration change
-
RT
ln
a
1
(osmolyte concentration)
FIGURE 11.2
The principle of osmotic stress analysis (OSA), whose aim is to estimate the
number of water molecules released during biomolecular processes and reactions by the use
of osmolytes.
removed; this can be a bottleneck to ligand binding (Figure 11.1). The enumeration
of the change in the number of water molecules is thus crucial for the understand-
ing of the biomolecular processes.
Osmotic stress analysis (OSA) aims to estimate the number of water molecules
adsorbed (or released) as a result of biomolecular processes purely from thermody-
namic measurements (Parsegian, Rand, and Rau 1995). The experimental procedure
is: (i) add an osmolyte (such as glycerol or PEG—known as
protein stabilizers
) to
the system; (ii) measure the Gibbs energy change, Δ
r
G
o
, that accompanies the bio-
molecular process as a function of osmolyte concentration; and (iii) infer the change
of hydration number from the slope of the resulting plot (Figure 11.2). OSA is thus
a simple and elegant method to obtain crucial information concerning hydration
changes (Parsegian, Rand, and Rau 1995).
The elegance, robustness, and simplicity of OSA have led to many striking dis-
coveries on the role of water in a number of biomolecular processes. The first success
was hemoglobin's taut (T) to relaxed (R) transition: as many as 65 water molecules
were inferred to be adsorbed upon this transition. This number obtained by OSA
was claimed to be comparable to an independent estimation based upon the change
of buried surface area. Encouraged by this consistency, OSA has been applied to a
