Chemistry Reference
In-Depth Information
As mentioned above, we first made the cyclodextrin bis-imidazole catalyst with the
imidazoles attached to primary carbons that were as far apart as possible (A,D glucose
residues) or almost that far (A,C residues). We then made all three isomers selectively
- A,D and A,C and A,B - to see how their geometry affected the catalyzed hydrolysis of
substrate
36
[129]. All three showed the bell-shaped pH vs. rate curves, indicating that
there was both base and acid catalysis, by an Im and an ImH
+
group. They all catalyzed
hydrolysis of the substrate but, remarkably, the best catalyst by far was the A,B isomer
46
, i.e., that with the acid and base groups right next to each other. This was completely
inconsistent with a mechanism (
42
) in which a water molecule (bound to an Im) at-
tacks the phosphate while the leaving group departs, assisted by the ImH
+
. Such an in-
line mechanism would require the Im and ImH
+
to be 180
8
apart, more or less, and so
the A,D isomer should have been the best catalyst. Clearly, the ImH
+
was playing a
different role, protonating the phosphate oxyanion to facilitate formation of a phos-
phorane intermediate (
47
). This is the mechanism we had deduced for simple Im/
ImH
+
buffer-catalyzed hydrolysis of UpU (vide supra).
By proton inventory, a technique that determines whether acid and base groups act
simultaneously, we found that hydrolysis of
36
by artificial enzyme
44
involves two
protons moving in the transition state [130]. Thus, ImH
+
of
46
is hydrogen bonded
to a phosphate oxyanion of bound substrate
36
; water hydrogen bonded to the Im then
attacks the phosphorus, and as the O-P bond forms the ImH
+
proton transfers (along
with the water proton) to produce the phosphorane monoanion
47
. This then goes on
to the cleaved product in later catalyzed steps before there is time for pseudo-rotation.
These general conclusions have been described and summarized in several publica-
tions [131-137].
1.3.4
Artificial Enolases and Aldolases
We examined the ability of our bis-imidazole cyclodextrin artificial enzymes to per-
form other bifunctionally-catalyzed reactions, where again the availability of the
A,B and A,C and A.D isomers let us learn mechanistic details. As an important ex-
ample, we examined three isomeric catalysts' ability to promote the enolization of sub-
strate
48
, which binds into the cyclodextrin cavity in water [138]. Here there was again a
strong preference among the isomers, but it was the A,D isomer
49
that was the ef-
fective catalyst! It was also more effective than a cyclodextrin mono-imidazole that
cannot use the bifunctional mechanism.
