Chemistry Reference
In-Depth Information
20
0
Exp-Wilson
Exp-NRTL
Exp-mM
MD-Verlet
15
-5
10
10
Exp-Wilson
Exp-NRTL
Exp-mM
MD-Verlet
5
-15
0
-5
-20
0
0.2
0.4
0.6
0.8
1
0
0.2 0.4
Mole Fraction Water,
x
1
0.6
0.8
1
Mole Fraction Water,
x
1
30
Exp-Wilson
Exp-NRTL
Exp-mM
MD-Verlet
20
10
0
-10
0
0.2 0.4
Mole Fraction Water,
x
1
0.6
0.8
1
FIGURE 6.11
TCFIs for (a) water/water, (b) water/t-butanol, and (c) t-butanol/t-butanol
obtained from simulation of water/t-butanol mixtures using the Verlet method (crosses) ver-
sus the water mole fraction
x
1
, compared with TCFIs obtained from experimental data using
the Wooley/O'Connell procedure, where either the Wilson (black line), NRTL (red line), or
mM (green line) models were employed for obtaining the activity coefficient derivatives.
Note that the NRTL and mM model approaches infinity since they predict a phase split.
(Calculated values from R. J. Wooley and J. P. O'Connell, 1991, A Database of Fluctuation
Thermodynamic Properties and Molecular Correlation-Function Integrals for a Variety of
Binary Liquids,
Fluid Phase Equilibria,
66, 233.)
(See color insert.)
6.5.1 e
nzyme
s
oluTions
Water activity is an important element of nonaqueous biocatalytic systems. Recently,
we have explored different approaches to this property via MD simulation (Wedberg,
Abildskov, and Peters 2012). Two main strategies to study how protein properties
depend on water activity are termed
real- time
control and
a posteriori
analysis. The
former comprises simulations of the protein in a nonaqueous medium in which
the number of water molecules is adjusted to maintain a desired water activity. In the
latter strategy, conventional MD simulations are carried out, but the water activity is
calculated through postanalysis of the simulations. The study of Branco et al. (2009)
is apparently the only work that explicitly considers water activity as a variable.
However, their medium was assumed to be an ideal mixture. The greater challenge
of nonideal media, such as aqueous organic solutions, has been addressed (Wedberg,
Abildskov, and Peters 2012). Much more work needs to be done before establishing
a standard method.
