Intercalation metal-phenanthroline complexes

Tris(phenanthroline) complexes of ruthenium(II), cobalt(III), and rhodium(III) are octahedral, substitutionally inert complexes, and as a result of this coordina-tive saturation the complexes bind to double-helical DNA through a mixture of noncovalent interactions. Tris(phenanthroline) metal complexes bind to the double helix both by intercalation in the major groove and through hydrophobic association in the minor groove. " " Intercalation and minor groove-binding are, in fact, the two most common modes of noncovalent association of small molecules with nucleic acids. In addition, as with other small molecules, a nonspecific electrostatic interaction between the cationic complexes and the DNA polyanion serves to stabilize association. Overall binding of the tris(phenanthroline) complexes to DNA is moderate (log K = 4)." ... 

The intercalation of platinum complexes containing bidentate or tridentate aromatic amines was first proposed for binding to tRNA and then to DNA [53]. The structures of the relevant complexes of palladium and platinum are shown in Figure 1.5 and a requisite is the presence of planar ligands such as bipyridine, terpyridine, or o-phenanthroline, which are coplanar with the coordination plane formed by the four ligand donor atoms and the metal ion. The similar pyridine derivative, [Pt(en)py2], where the pyridine Ugands are forced by nonbonded steric constraints to be out of the coordination plane, does not intercalate. 

Typical OH-scavengers suppress this reaction of (OP)2Cu+ (Que et al. 1980) yet, acetate and benzoate seem to be equally efficient, despite the fact that acetate is nearly two orders of magnitude less reactive towards OH than benzoate (k=7 X 107 dm3 mol-1 s 1 vs k = 5 X 109 dm3 mol-1 s 1 Buxton et al. 1988), and obviously it is not a freely diffusing OH that is responsible for the reaction. The reaction is also suppressed by Cu-complexing compounds and by transition metal ions such as Zn2+, Co2+, Cd2+ and Ni2+ that form stable complexes with 1,10-phenanthroline (Que et al. 1980) and also by competitive intercalators such as ethidium bromide and 2,9-dimethyl-l,10-phenanthroline (Reich et al. 1981). Interestingly, compared to its parent, the latter is inactive. NADH may serve as a reductant, but 02, seems to be a salient intermediate in this cleavage reaction, because cleavage is fully suppressed in the presence of SOD (Reich et al. 1981). 

Researchers have also studied DNA-mediated electron transfer by using donor-acceptor pairs that bind DNA noncovalently (27-30). Early work in our laboratory used cationic tris(phenanthroline) metal complexes as donor-acceptor pairs (29, 30). These complexes, shown in Figure 1, associate with DNA through two modes, (i) intercalation and... 

Ru(TMP)3] A distinctive characteristic of the A conformation is its shallow and wide minor-groove surface. Tris(phenanthroline)metal complexes bind to DNA both through intercalation in the major groove and through a surface-bound interaction in the minor groove (18-20) (see above). It is this surface-bound interaction that has been exploited in the construction of a complex, a derivative of tris(phenanthroline)ruthenium(II) that selectively targets A-form helical structures (30, 75). 

The foregoing metal complexes have served as donor-acceptor pairs in other, related systems, and these also demonstrate some aspects of the preceding rapid electron transfer (128). In a related study, the electron transfer between PBAC as the donor and tris(9,10-phenanthroline)Co(III) + (abbreviated Co(-phen)s +) as the electron acceptor was investigated. Even when just micromolar concentrations of the acceptor were intercalated in the galleries of BAZrP, the donor fluorescence was quenched very efficiently. The estimated quenching rate constant ( 3 X lO" dm mol s ) far exceeded the expected diffusional rate (69,120). 

In 1993, Barton and coworkers investigated the electron transfer in DNA, of which 5 termini were tethered with bis(phenanthroline)(dipyridophenazine)ruthemium(II) (Ru(phen )2dppz +) and bis(9,10-phenanthrene diimine) (phenanthroline)rhodium(in) (Rh(phi)2phen +) as the donor and acceptor respectively. " These metal complexes were intercalated either one or two base steps from the termini of 15 base-pair DNA. The fluorescence lifetime was shorter than their instrumental limitation (300 ps). From... 

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