Chemistry Reference
In-Depth Information
With the increased use of enzymes in polymer chemistry, the enzymology
terminology to describe the reaction kinetics and the enantioselectivity of a reac-
tion has become more and more common in polymer literature. The parameter of
choice to describe the enantioselectivity of an enzyme-catalyzed kinetic resolution
is the enantiomeric ratio
E
. The enantiomeric ratio is defined as the ratio of the
specificity constants for the two enantiomers,
(
R
)
and
(
S
)
(1):
E
=(
k
cat
/
K
M
)
/
(
k
cat
/
K
M
)
(1)
R
S
where
k
cat
is the rate constant,
K
M
the Michaelis-Menten constant, and
k
cat
K
M
the
specificity constant. Sih et al. developed (1) in terms of the enantiomeric excess of
product or substrate and the conversion, both for reversible and irreversible reactions
[
65
]. For an irreversible reaction, which is preferably the case in a polymerization
reaction, (2) can be used to calculate
E
from either the substrate enantiomeric excess
(
ee
S
) or the product enantiomeric excess (
ee
p
) and the conversion
c
:
/
ln
[
1
−
c
(
1
+
ee
p
)]
ln
[(
1
−
c
)(
1
−
ee
S
)]
E
=
)]
=
ee
S
)]
.
(2)
ln
[
1
−
c
(
1
−
ee
p
ln
[(
1
−
c
)(
1
+
Because calculation of
E
on the basis of one conversion and
ee
measurement is
highly unreliable, curve fitting should be employed using as many data points as
At 50% conversion, the values of
ee
S
differ the most, indicating that measurements
at around 50% conversion provide more valuable data than measurements at low or
high conversion levels.
In order to get good results in a kinetic resolution,
E
must be high, preferably
can depend strongly on parameters such as temperature and solvent, so medium
engineering is often a fast and highly effective tool to increase
E
.
Fig. 9
Theoretical
conversion versus
ee
S
curves
for different
E
s
