Chemistry Reference
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catalyst. Later, we confirmed [60] the thiamine mechanism proposed earlier, for which
an alternative had subsequently been proposed [61, 62].
We synthesized artificial enzymes
5
and
6
that incorporated the thiazolium ring of
thiamine into a cyclodextrin binding unit [63, 64]. The cyclodextrin imitated the hy-
drophobic binding pocket typical of many enzymes, but these mimics did not incor-
porate the catalytic groups that enzymes also use. Thus
5
and
6
showed the substrate
selectivity that enzyme binding also achieves, and there was some rate acceleration
from binding the substrates in proximity to the catalytic coenzyme group, as in en-
zymes. However, the rate accelerations were not nearly as large as those in artificial
enzymes (vide infra) that incorporated more features of natural enzymes.
With
-cyclodextrin, consisting of seven glucose units in a ring, benzaldehyde
bound into the cyclodextrin cavity of
5
and was converted into thiazolium adduct
7
, similar to a cyanohydrin [63]. This readily formed the benzylic anion
8
that under-
went deuterium exchange and easy oxidation. However, this
b
-cyclodextrin ring was
too small to bind both benzaldehydes - so this artificial enzyme did not catalyze the
formation of benzoin
9
significantly better than did a simple thiazolium salt without
the attached cyclodextrin binding group.
When the thiazolium unit was attached to the larger
b
-cyclodextrin in
6
, with eight
glucose units in the ring, benzoin condensation of two benzaldehydes was indeed well
c
