Chemistry Reference
In-Depth Information
the dppz(11,11
)dppz-bridged species, AT-threading was faster than that for GC-rich
sequences due to the stronger base-pairing of the latter.
Kelly and coworkers have compared the binding behaviour of the dinuclear
complex
′
[(bpy)
2
Ru{bb
n
}Ru(bpy)
2
]
4+
(bb
n:
n
= 5
{1,5 - bis[4(4
′
- methyl - 2,2
′
-
bipyridyl)]pentane;
n
= 7 {1,7 - bis[4(4
- bipyridyl)]heptane}; Figure 11.2 k)
with its mononuclear analogues [Ru(bpy)
3
]
2+
and [Ru(bpy)
2
(Me
2
bpy)]
2+
(Me
2
bpy = 4,4
′
- methyl - 2,2
′
-bipyridine; Figure 11.1b) and found that the bimetallic
species had a much higher binding affi nity, was more effi cient at photosensitising
DNA strand breaks and its binding affi nity was less sensitive to ionic strength.
62,63
Additional investigations were undertaken with [(phen)
2
Ru{bb
n
}Ru(phen)
2
]
4+
(where
n
= 5, 7, or 10).
63
Again, the dinuclear complex was found to have a stronger
binding affi nity than its mononuclear counterpart, [Ru(phen)
2
(Me
2
bpy)]
2+
, and that
the binding affi nity was dependent upon the linker chain length. The most effective
binding was observed at
n
= 7, somewhat shorter than the optimal chain length
(
n
′
- dimethyl - 2,2
′
8) of the classical intercalators with polymethylene chains.
56
This shorter chain
length is consistent with partial intercalation of a phenanthroline ligand on each
metal centre.
54
Other intercalative bridging moieties used in the study of DNA-binding dinu-
clear ruthenium complexes include phenanthroline/imidazole derivatives
64,65
and
porphyrins,
66
each of which yielded only moderate binding affi nities akin to those
observed in mononuclear species which bind via intercalation. One particularly
interesting species utilized the intercalative bridge bipp (bipp = 2,9 - bis(2 - imidazo[4,5 -
f][1,10]phenanthroline) - 1,10 - phenanthroline; Figure 11.2 i) which features vacant
chelating sites well suited to the binding of a third metal ion, specifi cally Cu
2+
. The
emissive properties of the complex were quenched upon binding of the copper ion,
whereas the DNA-bound complex exhibited a large increase in emission. Subse-
quent addition of copper ions to the complex-DNA system had no effect because
the chelating site was blocked upon intercalation.
67,68
This is in contrast with the so-
called ' molecular nut and bolt ' system of [Ru(bpy)
2
(tpphz)]
2+
(tpphz = tetrapyrido[3,2 -
a
:2
>
-
j
]phenazine; Figure 11.2j), which possesses a similar chelating
site on the tpphz ligand. Upon intercalation with DNA, the chelating site projects
out of the opposite side of the helix where it can coordinate a copper ion, quenching
the emission of the complex and locking it in place.
69
Many multinuclear species
incorporating mixed-metal polypyridyl systems have shown considerable promise
as photoactivated drugs: trinuclear Ru-Rh species (essentially two Ru(L)
2
moieties
linked to a RhCl
2
core via 2,3 - dpp bridges (2,3 - dpp = 2,3 - bis(2 - pyridyl)pyrazine;
Figure 11.2e)), for instance, have demonstrated visible light-induced photocleavage
of DNA.
70
This has been attributed to a Ru
′
,3
′
-
c
:3
′
′
,2
′
′
-
h
:2
′
′
,3
′
′
Rh metal - to - metal charge transfer
excited state and is dependent upon the nature of the bridging ligand (the analogous
complex with 2,2
→
-bipyrimidine bridges (bpm; Figure 11.2a) was inactive).
71
Ru/Pt
mixed species, bridged by 2,3-dpp, dpq and dpb ligands, have been found to bind
DNA covalently via the square-planar platinum moiety.
72,73
Several nonintercalative, semi-rigid dinuclear complexes have also been inves-
tigated. Complexes bridged by the asymmetric phenanthroline derivative pztp ((3-
pyrazin - 2 - yl) -
as
- triazino[5,6 -
f
] - 1,10 - phenanthroline; Figure 11.2 f) have been found
to bind via electrostatic/groove binding interactions, whereas the mononuclear
′
