Chemistry Reference
In-Depth Information
Figure 13.13
Artifi cial nucleases based on ruthenium and rhodium intercalators reported by
Burton and coworkers
In 1987, Barton and coworkers published the fi rst example of an artifi cial
system with DNA hydrolytic cleavage activity.
4
It was based on a ruthenium inter-
calator with two arms having the role of metal-binding moieties (Figure 13.13a).
The Zn(II) and the Cd(II) binuclear complexes of this ligand cleaved plasmid DNA
with high effi ciency. At 37 °C and pH 8.5, 40% of the supercoiled form was degraded
in the presence of a 7 mM concentration of the complex after 5 hours of incubation
(a fi rst - order rate constant of 3
10
− 5
s
− 1
can be roughly estimated). Religation
experiments showed that the hydrolysis occured randomly at the P-O3
×
bonds. However, the affi nity of the ligand for the metal ions seemed to be quite low
and a large excess of metal ion was required. Unfortunately, no evidence concerning
the role played by the intercalator was reported.
Later on, Barton and coworkers studied the reactivity of a mononuclear Zn(II)-
binding peptide, tethered to a rhodium complex as major groove intercalator (Figure
13.13 b).
38
The system promotes plasmid DNA cleavage with a rate constant of
2.5
′
and P - O5
′
10
− 5
s
− 1
at pH 6, 37 ° C and 5 mM complex concentration, and shows similar activ-
ity toward linear double-stranded DNA. Analysis of the fragments produced showed
that the cleavage occurs only at the P-O3
×
′
bonds with a modest sequence selectivity
for 5
sites (with cleavage at
Py
), presumably the result of the Rh
complex binding selectivity. The reactivity decreases as the pH approaches neutral-
ity. The peptide is scarcely structured and this is probably the source of the weak
binding of the Zn(II) ion. Although the presence of the rhodium complex intercala-
tor is of paramount importance for the activity of the complex, its substitution with
a different rhodium-based intercalator, which preferentially binds at mismatches, or
with the ethidium intercalator led to unreactive systems.
38b
Schneider and coworkers appended two naphthalene groups to an azacrown
ligand via C
6
alkyl spacers (Figure 13.14a) and used it as a cofactor for Eu(III)-
promoted DNA cleavage.
39
The results are puzzling because conjugation with the
intercalating units led to an increase in the intrinsic reactivity with regard to the
′
- Pu -
Py
- Pu - Py - 3
′
