Chemistry Reference
In-Depth Information
Scheme 2.7
(Table 2.1), in which
5
with the longer chain was the fastest, supports our previous
contention that in transamination the catalytic group also performs the protonation at
the amino acid
-carbon. In the imidazole series (
7-9
) the shortest chain system
7
was
also fastest in HCl elimination. The striking contrast to the data for transamination
fully supports the proposition that in our transamination studies the catalysis was se-
quential, with proton transfer by the catalytic group to the remote position of the inter-
mediate.
We next synthesized the pyridoxal-bound
a
-CD catalyst
44
(Scheme 2.8) [43], which
produced 3-5 times more tryptophan when incubated with indole,
b
b
-chloroalanine,
and Al
2
(SO
4
)
3
(pH 5.2 and 100
8
C) than the reaction in which the pyridoxal derivative
was replaced by simple pyridoxal. However, tryptophan yield was still only a few per-
cent. As expected, this kinetic advantage disappeared at higher indole concentrations
due to saturation of the binding site. Furthermore,
L-
tryptophan was produced in ca.
10% excess relative to the
D-
enantiomer.
Murakami et al. utilized catalytic bilayer membranes to catalyze the
-replacement
reaction of serine with indoles [44]. The bilayer vesicle formed with
32
and
36
dras-
tically accelerated the
b
-replacement reaction by 51-fold (k
rel
) relative to pyridoxal in
homogeneous aqueous solution. They attributed this to the hydrophobic microenvir-
onmental effect provided by the bilayer vesicle, which affords effective incorporation of
indole molecules and elimination of water molecules in the reaction site. The imida-
zolyl group of
33
enhanced the reaction further, k
rel
being 130, possibly due to general
acid-base catalysis by the imidazolyl group. Copper(
II
) ions also improved the reaction.
b
