Chemistry Reference
In-Depth Information
8
Carboxyl and imidazolyl groups are the catalytic moieties present in the side chains of
amino acids. However, protease-like catalysts could be designed with organic func-
tional groups unrelated to amino acids. Indeed, a protease-like catalyst was synthesized
by using aldehyde groups [45]. Silica gel covered densely with aldehyde groups man-
ifested catalytic activity toward albumin and
-globulin with half-lives as short as
1.5-3 h at pH 5-6 and 50
8
C. As proposed in
8
, aldehydes can act as binding sites
by forming imines with amino groups located on the protein surface. In addition,
the hydrates of aldehydes can act as general acids, general bases, or nucleophiles
to hydrolyze the peptide bonds of the bound protein substrate. Catalytic action of
an aldehyde hydrate in a transacylation reaction has been reported [46].
When the surface of silica gel was covered with organic functional groups such as
carboxyl, hydroxyl, phenolic, imidazolyl, or mercapto groups as well as aldehyde
groups, the proteolytic activity was considerably improved [47]. Silica gel covered
with histidines and aldehydes afforded the best activity: the half-life was as short
as 50 min at pH 6-9 and 25
8
C. Furthermore, broad substrate specificity was achieved
with various proteins (albumin, ovalbumin, hemoglobin, and
c
-globulin) being hydro-
lyzed. Here, the aldehyde groups would act as the binding site and imidazolyl, carbox-
yl, and/or aldehyde hydrates would provide catalytic groups in a mechanism analogous
to
8
.
c
3.3.2
Proximal Group Adducts
Instead of random attachment of functional groups to the solid supports, active sites
consisting of two or more organic catalytic groups have been created on polystyrene
backbones. For example, poly(aminomethylstyrene-co-divinylbenzene) (PAD), the
cross-linked polystyrene with styryl residues containing aminomethyl groups, pro-
vides a framework for attachment of an active site that consists of three salicylate
groups. A procedure similar to that shown in Eq. (3) has been used to build such
a heterogeneous catalyst [48]. The excess amino groups on the resulting polystyrene
were acetylated and then Fe(
III
) ion was removed with acid to obtain Sal
3
-PAD-Ac.
