Chemistry Reference
In-Depth Information
We have made several artificial enzymes that use cyclodextrin to bind a substrate and
then react with it by acylating a cyclodextrin hydroxyl group. This builds on earlier
work by Myron Bender, who first studied such acylations [83]. We added groups to
the cyclodextrin that produced a flexible floor, capping the ring [84]. The result
was to increase the relative rate of cyclodextrin acylation by m-t-butylphenyl acetate
from 365 relative to its hydrolysis rate in the buffer to a k
complex
/k
buffer
of 3300. We
changed the substrate to achieve better geometry for the intracomplex acylation reac-
tion, and with a p-nitrophenyl ester of ferroceneacrylic acid
10
we achieved a relative
rate for intracomplex acylation of ordinary
-cyclodextrin vs. hydrolysis of over 50 000
and a V
max
comparable to that for hydrolysis of p-nitrophenyl acetate by chymotrypsin
[85].
Our best combination of the flexible capped cyclodextrin with the well-fitting sub-
strate p-nitrophenyl ester
10
gave an acceleration - relative to hydrolysis in the same
medium - of over one-million fold, exceeding that achieved by chymotrypsin with
p-nitrophenyl acetate [86]. An even better fitting substrate (
11
) afforded an acceleration
of ca. 80 000 000-fold, and saw a 62-fold increase in enantioselectivity as well [87, 88].
This is an enantiomeric excess of 98.4%.
Substrate binding into the cyclodextrin cavity, which ordinarily is studied in water
solution, also occurs in highly polar organic solvents such as DMSO [89]. Furthermore,
kinetic studies of our reactions at high pressure were consistent with the geometries
proposed for these acylation processes [90]. Molecular modeling showed geometries of
the bound substrates and the tetrahedral intermediates that helped explain some of the
large rate effects [91].
In the acylation of a cyclodextrin hydroxyl group by a nitrophenyl ester, the preferred
geometry requires that the oxyanion of the cyclodextrin attack perpendicular to the
plane of the ester carbonyl, so as to form the tetrahedral intermediate. However,
the product cyclodextrin ester has the cyclodextrin oxygen in the plane of the carbonyl
group. Thus, a rapid reaction requires enough flexibility to be present to permit this
geometric change to occur rapidly. With very rigid substrates the conversion of the
tetrahedral intermediate into the product can be rate determining, and slow. In a study
of this question we used substrate
12
in which the ester carbonyl can freely rotate, and
saw that this made the formation of the tetrahedral intermediate rate-limiting, and
rapid [92].
b
