Chemistry Reference
In-Depth Information
Figure 3.13 Dependence on pH of k
o
for
cleavage of the linearized pUC18 DNA by 26
(
; C
o
= 0.48m
M
) or by CoCyc (
*
)(C
o
= 1.0m
M
)
at 25
8
C.
Plasmid pUC18 DNA (2686 base pairs) was used as the supercoiled DNA substrate,
whereas the linearized form of plasmid pUC18 DNA prepared by EcoR I digest was
used as the linear DNA substrate. The disappearance of the supercoiled and the linear
DNA substrates during incubation with several forms of PCD-supported Co(
III
)Cyc,
such as
26
, was monitored by agarose gel electrophoresis. Kinetic data were collected
in the presence of excess catalyst at 4
8
C for the supercoiled DNA and at 25
8
C for the
linear DNA. Supercoiled DNA exhibited faster rates than linear DNA for both Co(
III
)-
Cyc and PCD-supported Co(
III
)Cyc. Half-lives as short as 40 min at 4
C and 30 min at
8
25
C were observed for hydrolysis of the supercoiled and linear DNA, respectively,
when catalyzed by
26
(Figure 3.13). Comparison of the rate constants measured
for Co(
III
)Cyc and for the PCD-based Co(
III
)Cyc at the same catalyst concentration re-
vealed that the Co(
III
)Cyc reactivity is enhanced upon attachment to PCD by more than
200 times towards the supercoiled DNA [61] and by at least 150 times towards the
linear DNA [62]. Considering that only the Co(
III
)Cyc moieties exposed on the surface
of PCD can attack the DNA, the normalized degree of activation should be much great-
er than 150-200-fold. As indicated by
27
, even among those exposed on the resin sur-
face only a minor fraction of the Co(
III
)Cyc moieties can act as the catalytic groups.
Then, the normalized degree of activation should be considerably greater than that
estimated above.
Dicerium complex
24
took 24 h at 37
8
Cor5hat55
8
C to yield hydrolysis products of
a 192-base pair DNA [60]. Conversely, PCD-based Co(
III
)Cyc derivatives degraded the
linear DNA duplex into small fragments in a few hours at 25
8
C. The facile DNA hy-
drolysis by the PCD-based Co(
III
)Cyc was due to the activation of Co(
III
)Cyc upon at-
tachment to PCD rather than to cooperation among two or more catalytic centers or
facilitation of complex formation between DNA and the PCD-supported catalyst.
8
27
26
