Chemistry Reference
In-Depth Information
between the two states. How, then, can we determine, from experimental data, the
interaction of a biomolecule in a single state with the surrounding solvent molecules?
The physical quantity responsible for answering the question is the preferential
hydration parameter (Timasheff 2002b). It is this parameter, which has first shown
that protein stabilizers, such as glycerol and trehalose, tend to be preferentially
excluded from the protein surface, whereas denaturants, such as urea, are usually
preferentially bound to protein surfaces (Timasheff 2002b). This parameter, how-
ever, cannot, by definition, give any information beyond preferential binding and
exclusion. Understanding how each individual component (i.e., water or cosolvent)
is solvating or distributing around the protein cannot be understood by preferential
hydration parameters alone (Timasheff 2002a). To go beyond this difficulty, FST
will again be indispensable. The theme here is again the FST-based reinterpretation
of well-established thermodynamic measurements.
The preferential hydration parameter, ν
21
, defined below, can be expressed by the
excess coordination numbers through FST (Shulgin and Ruckenstein 2005a),
∂
∂
c
c
(
)
1
∞
ν
=
=
cG
−
G
(11.12)
21
1 1
13
2
T
,,
µ
1
Equation 11.12, solved in combination to the following relationship (namely, single-
conformation equivalent of Equation 11.4) yields the microscopic information
regarding protein-solvent interactions expressed through the KB integrals,
∞
∞
∞
V
=−
NVNkT
BT
−
+
κ
(11.13)
2
1
21
3 3
where κ
T
, the isothermal compressibility of the solution, is negligibly small.
The difference between Equation 11.12 and the following equation, which is the
single-conformation equivalent of the cosolvent-induced equilibrium shift (which
has been the focus of all the preceding discussions) should be noted with caution
(Shulgin and Ruckenstein 2005a; Shimizu and Matubayasi 2006; Smith 2006a),
∂
∂
µ
µ
(
)
2
∞
∞
−
=
cG
−
G
(11.14)
1 1
23
1
Tpc
,,
→
0
2
We note here that prior to the introduction of FST, Timasheff and Xie have esti-
mated the parameters equivalent to
G
21
and
G
23
based upon an intuitive concept of
total site occupancy (Timasheff and Xie 2003). This has been a matter of debate, as
seen in Section 11.1, and it is only FST, which has brought a clearly defined relation-
ship (Equation 11.13) that is independent of Equation 11.14.
11.6.2 u
rea
and
T
rehalose
In order to understand the difference of action between protein denaturants and
stabilizers, FST-based analysis of experimental data has been performed for ribo-
nuclease in aqueous urea and trehalose solutions. Urea and trehalose are chosen
