Chemistry Reference
In-Depth Information
11.3.2 r
elaTionshiP
To
The
v
olumeTric
a
nalysis
Before concluding our long story of controversy concerning biomolecular hydration,
one must make a final mention of yet another facet of the same debate: volumetric
analysis versus osmotic stress analysis, which initially led to a quest for a sound
theoretical basis for studying biomolecular hydration.
Volumetric analysis has been established and formalized by Chalikian (2003,
2011), and is capable of estimating the number of water molecules involved in reac-
tion by measuring the changes in partial molar volumes of biomolecules. Partial
molar volumes are obtained from the change of equilibrium when hydrostatic pres-
sure is applied, in contrast to OSA's “volume of water” determined by the applica-
tion of osmotic pressure (Shimizu 2004). Partial molar volumes can be measured
from high-pressure experiments, as well as densitometry (Chalikian 2003, 2011).
Here, the numbers of waters estimated from OSA and from volumetric analyses were
often inconsistent. For instance, a fourfold difference was seen in the equilibrium
dissociation of human IFN-γ, and up to a threefold difference in the camphor binding
of cytochrome P-450 (Shimizu 2004).
Before the introduction of FST, the reason for these discrepancies was not under-
stood, although speculations were made concerning the different aspects of biomol-
eculer hydration that osmotic and hydrostatic pressures modulate (Robinson et al.
1995). Now we know that: (i) osmotic stress assumes exclusion as zero binding,
instead of negative excess coordination numbers; and (ii) volumetric analysis aims
at estimating the coordination numbers themself, instead of the excess coordination
numbers, based upon the assumption of a two-state model for water (Chalikian 2003,
2011), or through the estimated water density increment in the vicinity of biomol-
ecules (Kornblatt and Kornblatt 2002). However, the precise relationship between
FST and the approach of Chalikian has yet to be determined.
11.3.3 s
ummary
Prior to the application of FST, the lack of a rigorous theoretical framework had been
the major hindrance in clarifying how hydration changes could be estimated from
thermodynamic measurements. The FST provided a rigorous foundation, which
is capable of relating the effect of osmolytes and hydrostatic pressure to the excess
coordination numbers of the solvents surrounding the biomolecules. It has also
established a rigorous theoretical foundation for the molecular crowding picture.
11.4
COSOLVENT- INDUCED DENATURATION
AND STABILIZATION
11.4.1
h
yPoTheses
c
oncerning
The
o
rigin
oF
c
osolvenT
d
enaTuraTion
We have seen that FST has the power of clarifying the behavior of solvent molecules
underlying traditional thermodynamic measurements. It has also provided a unified
framework for the interpretation of experimental data under a single aegis of the excess
