Chemistry Reference
In-Depth Information
Ribonuclease-PEG
10,000
Preferential hydration
0
0
2000 4000
MW of PEG (g/mol)
6000
FIGURE 11.9
Preferential hydration parameter (solid) and the excess coordination num-
ber of PEG (dotted) around ribonuclease. Consequently, the contribution from protein-water
interaction is negligibly small.
Thus, the KB analysis shows, at least for large PEGs, that the strong exclusion of
PEGs from protein surfaces is the dominant cause of protein stabilization. This sup-
ports the previous view proposed by the molecular crowding approach.
11.6.4 T
he
h
oFmeisTer
e
FFecT
It has long been known that cations and anions affect biomolecular processes with
widely varied effectiveness (Baldwin 1996). Both cations and anions have been
ranked separately in terms of their capacity to precipitate proteins. This ranking is
called the
Hofmeister series
(Baldwin 1996). The same ranking applies to the stabi-
lization of proteins. Since its initial discovery through the study of protein precipita-
tion over a century ago, the Hofmeister series has been observed in a wide range of
phenomena not only in protein chemistry but also in colloid, surface, membrane, and
polymer chemistry (Baldwin 1996).
What, at a molecular level, causes the Hofmeister series? Again, an FST-based
analysis of the experimental data provides useful insights (Shimizu, McLaren, and
Matubayasi 2006). Figure 11.10 shows the
G
21
and
G
23
of bovine serum albumin in
the presence of a number of Hofmeister salts as well as PEGs. The following obser-
vations are particularly noteworthy:
• A linear correlation between preferential hydration parameter and
G
23
, but
not with
G
21
. This suggests that the protein-cosolvent interaction is respon-
sible for the variation of preferential hydration phenomenon.
•
G
23
values for the Hofmeister salts and PEGs fall onto the same line, suggest-
ing a unifying principle underlying both the Hofmeister effect and molecu-
lar crowding: namely, the variation of the protein-cosolvent interaction.
