Chemistry Reference
In-Depth Information
in water estimated using Chalikian's method (Chalikian 2003). It is noteworthy (see
Figure 11.8) that the excluded volume contribution to
G
21
as
c
3
→ 0 is −
V
E
(Shimizu
2004; Shimizu and Matubayasi 2006). Therefore, the difference between
G
21
and
V
E
as
c
3
→ 0 provides an estimate for the contribution from the first solvation shell. The
difference between
G
23
and
V
E
is also informative in characterizing the distribution
of a cosolvent around a protein molecule relative to that of water, as will be discussed
below (it is worth noting here that the excluded volume of a protein is larger with
urea or trehalose than with water). Figure 11.8 indicates that
G
21
is more positive than
V
E
, suggesting an increase in the water density in the first solvation shell. (As a first
approximation, the
c
3
dependence of
V
E
has not been considered here.)
G
23
is even
more positive than −
V
E
, implying, in spite of the increased size of urea compared
to water (which makes the volume that is inaccessible to urea larger than
V
E
), urea
molecules do accumulate around the protein. On the other hand, Figure 11.8 shows
that
G
23
for trehalose is more negative than −
V
E
, showing a depletion in the trehalose
density in the solvation shell and/or the effect of the larger size of trehalose. In any
case, the relative depletion of trehalose in comparison with the case of water is evi-
dent from
G
21
and
G
23
. The opposite behavior of urea and trehalose in the solvation
shell has been clarified by the FST (Shulgin and Ruckenstein 2005a; Shimizu and
Matubayasi 2006; Smith 2006a).
11.6.3 m
olecular
c
rowding
Large inert cosolvents, such as PEG, tend to stabilize the native structure of pro-
teins (Davis-Searles et al. 2001). This effect is called
molecular crowding
and is
considered to be an important difference between biochemical reactions
in vivo
and
in vitro
. Here, we present a FST analysis of the preferential hydration and volu-
metric data to help understand the molecular mechanism of crowding (Shulgin and
Ruckenstein 2005a; Shimizu and Matubayasi 2006; Shulgin and Ruckenstein 2006b;
Smith 2006a; Gee and Smith 2009).
Figure 11.9 shows the preferential hydration parameter ν
21
and −
c
1
G
23
for BSA
in aqueous PEG solutions of various molecular weights. It is observed that the
larger the molecular weight of PEG, the more dominant the relative contribu-
tion of −
c
1
G
23
to ν
21
becomes. However, for smaller PEGs and trehalose (data
not shown), the contribution from hydration,
G
21
, is not negligible (Shimizu and
Matubayasi 2006).
What is the mechanism of protein stabilization by PEG? We have seen from
Figure 11.9 that −
c
1
G
23
is the dominant contribution for large PEGs, which is large
and positive. It follows that
G
23
is large and negative. The large and negative
G
23
val-
ues show that PEG molecules are strongly excluded from protein surfaces (Equation
11.3). This can indeed be understood more strikingly by comparing −
c
1
G
23
to
c
1
V
E
,
the contribution to the former due to the exclusion of PEGs from the excluded vol-
ume of proteins in water. For both BSA and ribonuclease, the value of −
c
1
V
E
is much
smaller than −
c
1
G
23
for large PEGs (Shimizu and Matubayasi 2006). This suggests
that the volume from which PEGs are excluded is much larger than
V
E
(i.e., the vol-
ume from which water molecules are excluded).
