Chemistry Reference
In-Depth Information
32,
34
33,
imidazole or SH coordinating to the metal ion. In electrical terms, they can feed elec-
trons to the metal ion only when there is bridging by a substrate group; the resulting
electron flow leads to catalysis. The bifunctional catalysis led to cyclization of substrate
34
.
As another example, we prepared disubstituted cyclodextrin
35
in which one sub-
stituent was a metal-binding tren group while the other was an imidazole [122]. Zn
2+
complexed to the tren group gave good rate acceleration in the hydrolysis of bound
catechol cyclic phosphate
36
, which was fastest when the two catalytic groups were
attached to opposite sides of the cyclodextrin so they could not bind each other.
The geometry of the complex led to the selective formation of product
37
rather
than
38
; both are formed equally by ordinary hydrolysis without the catalyst.
The next section describes further mimics of ribonuclease enzymes. However, we
mention here one additional study on the cleavage of ribonucleotides in which metal
ions were used [123]. Cyclization of uridyluridine
39
, and hydrolysis of the resulting
cyclic nucleotides
40
, was catalyzed by Eu
3+
much more effectively than by Zn
2+
, and
some added ligands increased the rates.
1.3.3
Artificial Ribonucleases
Ribonuclease A is a member of a group of enzymes that cleave RNA using general
acid-base catalysis without a metal ion in the enzyme. In ribonuclease A, such cat-
alysis is performed by two imidazoles of histidine units, one as the free base (Im) and
the other, protonated, as the acid (ImH
+
). To mimic this in an artificial enzyme, we
prepared
b
-cyclodextrin bis-imidazoles
41
[124]. The first one was a mixture of the
35
36
37
38
