Chemistry Reference
In-Depth Information
(s) bbtb
N
N
Y
S
N
N
S
N
N
N
(t) PIP
N
N
N
NH
N
NH
N
(u) tactp
(v) chrysi
Figure 11.1
Continued
The controversy surrounding the exact mode of binding adopted by [Ru(phen)
3
]
2+
,
heightened by the relatively low binding affi nity of the complex (
10
3
M
− 1
,
dependent upon DNA sequence),
3,12
was exacerbated by debate over whether the
complex was binding in the major or the minor groove. Two-dimensional NMR
experiments have revealed that both enantiomers of [Ru(phen)
3
]
2+
bind in the
minor groove of DNA.
13
The numerous confl icting results regarding the site and orientation of binding
have been attributed to variations in technique, DNA sequence and even metal
complex/DNA/salt concentrations.
14,15
A minor-groove binding mode was consistent
with the trend observed amongst the bulk of other polypyridylruthenium(II) com-
plexes studied.
16 - 18
The binding nature of the related complex [Ru(phen)
2
(dppz)]
2+
(dppz = -
dipyrido[3,2 -
a
:2
∼
6
×
-
c
] phenazine; Figure 11.1j) has also invoked vigorous debate.
19
Complexes based on the dppz ligand function as 'molecular light switches': in
aqueous solution [Ru(bpy)
2
(dppz)]
2+
and [Ru(phen)
2
(dppz)]
2+
are effectively
quenched by the proton-donating solvent and show no luminescence, but when
intercalated to DNA the hydrophobic environment allows them to luminesce
strongly.
20,21
Additionally, the facile electron transfer capabilities of intercalated
ruthenium dppz complexes has seen their frequent use in studies of long-range
DNA - mediated electron transfer.
20,22 - 26
Unlike phen, the intercalative ability of the
dppz ligand was unambiguous and undisputed, having been proven using a variety
of techniques and methodologies.
16,21,27,28
[Ru(phen)
2
(dppz)]
2+
has been found to
bind to DNA with an association constant in excess of 10
6
M
− 1
:
27,28
the extended
planar aromatic ring system of the dppz ligand lends itself to intercalative stacking
between base pairs much more readily than its less-extended phen counterpart.
29 - 31
Nevertheless, there still exists a signifi cant electrostatic contribution to the binding
of dppz complexes.
27
′
,3
′
