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coordinates to the carbonyl oxygen of the transferring acetyl group, and then catalyzes
hydrolysis of intermediate
20
. It was our first example of such an enzyme-like process,
and we built on it further.
Catalyst
17
is effective only with substrates that can bind to the metal ion, so we
attached it - coordinated as its Ni
2+
derivative - to the secondary face of
-cyclodextrin
in catalyst
21
[102]. This was then able to use the metallo-oxime catalysis of our pre-
vious study, but with substrates that are not metal ligands, simply those that bind hy-
drophobically into the cyclodextrin cavity. As hoped, we saw a significant rate increase
in the hydrolysis of p-nitrophenyl acetate, well beyond that for hydrolysis without the
catalyst or for simple acetyl transfer to the cyclodextrin itself. Since there was full cat-
alytic turnover, we called compound
21
an “artificial enzyme“ - apparently the first use
of this term in the literature. The mechanism is related to that proposed earlier for the
enzyme alkaline phosphatase [103].
Nitriles can be hydrated enzymatically to form amides. In a model system we
showed [104] that
22
can be converted into the amide
23
when metal ions are coordi-
nated into the phenanthroline system. With Ni
2+
the rate acceleration was 10
7
, while
with Cu
2+
the hydration was accelerated by 10
9
. These are huge rate increases. Much of
the driving force is related to the fact that the cyano group is not itself a strong metal
ligand in
22
, but the transition state for the hydration is metal coordinated. Also, we
used a metal ion to organize the intracomplex reaction of a ligand-ligand reaction [105]
in which tris-hydroxymethylaminomethane (Tris) adds to 2-cyanopyridine to form the
adduct
24
. Again, the rate was very large and, more to the point, the addition of Tris
occurs even though the concentration of water is 10
4
times that of Tris. In the absence
a
