Chemistry Reference
In-Depth Information
TABLE 11.1
A Comparison of the Number of Water Molecules Released
for a Series of Processes
a
Reaction
Cosolvent
-∂
Δ
r
G
o
/∂
μ
1
Δ
V
2
/
V
1
Hexokinase- glucose dissociation
PEGs
326
-4.2
Cytochrome P450-camphor dissociation
Osmolytes
19
-1.6
Hemoglobin T to R transition
PEGs and osmolytes
65
0
Lysozyme denaturation
GuHCl
-233
-3.0
Tendamistat denaturation
GuHCl
-99.5
-2.3
a
Obtained using Equation 11.2 and Equation 11.7: The dominance of the former guarantees
that the molecular crowding approximation (Equation 11.7) is an excellent approximation.
Change in
cosolvent
exclusion
+
Exclusion = negative
N
23
∆
N
23
FIGURE 11. 5
Schematic diagram showing what the osmotic stress analysis actually mea-
sures: not the hydration change, but the change of cosolvent exclusion. The meshed area
represents the region into which the osmolytes cannot penetrate. This provides a negative
contribution to the excess coordination number.
dominant contribution to the cosolvent-induced equilibrium shift. FST thus provides
a clear endorsement to the molecular crowding picture (Figure 11.5).
The crowding approach, which has been based upon McMillan-Mayer solution
theory, has employed Equation 11.7 as a starting point (Davis-Searles et al. 2001;
Shimizu and Boon 2004). This is based upon a second virial approximation. FST
can even provide the condition upon which this approximation is accurate. Since
Equation 11.7 holds under the condition that Δ
N
21
is negligibly small, and that this
quantity is related to the partial molar volume via Equation 11.5, the proposed condi-
tion is
o
∂
∆
G
r
0
c
∆
V
(11.8)
1
2
∂
µ
1
This means that the osmolyte-induced equilibrium shift is much more significant
than the pressure-induced one. This is satisfied indeed by biomolecular processes
(see Table 11.1).
FST has thus established a clear relationship between OSA and the crowding
approaches, which had been a matter of serious debate in the literature.
