Chemistry Reference
In-Depth Information
10.6 SOLUBILITY OF PROTEIN IN MIXED SOLVENTS
The solvation behavior of a macromolecule such as a protein in a binary aqueous
solvent is important in the understanding of such solutions (Schachman and Lauffer
1949; Casassa and Eisenberg 1964; Wyman 1964; Kuntz Jr. and Kauzmann 1974;
Timasheff 1993). A macromolecule can be preferentially hydrated when the concen-
tration of water in the vicinity of the macromolecule (local concentration of water)
is higher than in bulk. The macromolecule can be preferentially solvated when the
concentration of the cosolvent in the vicinity of the macromolecule is higher than in
bulk. A measure of the solvation (or hydration) is provided by the preferential bind-
ing parameter (Casassa and Eisenberg 1964; Wyman 1964; Kuntz Jr. and Kauzmann
1974; Timasheff 1992, 1993), which can be defined using various concentration
scales (component 1 is water, component 2 is a protein, and component 3 is a cosol-
vent) in various ensembles.
1. In molal concentrations,
(
)
()
m
Γ
23
≡
lim
∂
mm
/
∂
(10.43)
3
2
Tp
,,
µ
3
m
→
0
2
where Γ
2
(
m
)
is the preferential binding parameter defined in molal concentra-
tions,
m
i
is the molality of component
i
, and μ
i
is the chemical potential of
component
i
. Only isothermal-isobaric conditions are considered.
2. In molar concentrations,
(
)
Γ
23
()
c
≡
lim
∂
cc
/
∂
(10.44)
3
2
Tp
,,
µ
c
→
0
3
2
where Γ
2
(
c
)
is the preferential binding parameter defined in molar concentra-
tions,
c
i
is the molar concentration of component
i
. It should be noted that
Γ
2
(
m
)
and Γ
2
(
c
)
are defined at infinite dilution of the protein.
Many characteristics of a protein in aqueous solvents are connected to its preferen-
tial solvation (or preferential hydration). The protein stability is a well-known example.
Indeed, the addition of certain compounds (such as urea) can cause protein denatur-
ation, whereas the addition of other cosolvents, such as glycerol, sucrose, and so forth,
can stabilize at high concentrations the protein structure and preserve its enzymatic
activity (Kuntz Jr. and Kauzmann 1974; Timasheff 1992, 1993, 1998a). Analysis of
literature data demonstrates that as a rule Γ
2
(
m
)
> 0 for the former and Γ
2
(
m
)
< 0 for the
latter compounds. Recently, we showed how the excess (or deficit) number of water (or
cosolvent) molecules in the vicinity of a protein molecule can be calculated in terms
of Γ
2
(
m
)
, the molar volume of the protein at infinite dilution, and the properties of the
protein-free mixed solvent (Shulgin and Ruckenstein 2005a). The protein solubility in
an aqueous mixed solvent is another important quantity which can be connected to
the preferential solvation (or hydration) (Arakawa and Timasheff 1984, 1985b, 1987;
Timasheff and Arakawa 1988; Arakawa, Bhat, and Timasheff 1990) and can help to
