J. Gallagher et al., OPTIMIZING THE TARGETED CHEMICAL NUCLEASE ACTIVITY OF 1,10-PHENANTHROLINE-COPPER BY LIGAND MODIFICATION, Bioconjugate chemistry, 7(4), 1996, pp. 413-420
Our interest in improving the efficiency of targeted scission reagents
has prompted us to study the influence of ring substituents on the nu
clease activity of 1,10-phenanthroline -copper conjugated to oligonucl
eotides and DNA-binding proteins. Since methyl substitution at all but
the 2 and 9 positions enhances the copper-dependent chemical nuclease
activity of 1, 10-phenanthroline, we have compared the reactivity of
conjugates prepared from 5-(aminomethyl)-1,10-phenanthroline (MOP) to
those of conjugates prepared from 5-amino-1,10-phenanthroline (amino-O
P). Tethering MOP derivatives to the Escherichia coli Fis protein enha
nces DNA scission several-fold at the weaker cleavage sites initially
observed with conjugates prepared from amino-OP. However, scission eff
iciency is not increased at the stronger cleavage sites, or when sciss
ion is targeted to single-stranded DNA by a complementary oligonucleot
ide. These results are consistent with a change in the rate-determinin
g step for cleavage associated with the differential accessibility of
the DNA-bound coordination complex to solvent and reductant. Although
the free bis cuprous complex of 2,9-dimethyl-1,10-phenanthroline (neoc
uproine) is redox-inactive, an oligonucleotide tethered to neocuproine
through C5 of the phenanthroline ring efficiently cleaves a complemen
tary DNA sequence. These results establish that the nucleolytic specie
s in targeted scission is the 1:1 cuprous complex and suggest that the
oxidative reaction proceeds through a copper-ore intermediate rather
than a metal-coordinated peroxy species. However, substituents at the
2 and 9 positions of the ligand will often hinder close approach of th
e phenanthroline -copper moiety to the oxidatively sensitive ribose as
shown by the preference of the oligonucleotide-targeted chimera for c
leavage of single-stranded regions and the failure of neocuproine-DNA-
binding protein chimeras and a C2-tethered chimera to cleave DNA.