Chemistry Reference
In-Depth Information
Design Aspects: Toward Photobinding DNA
Metal complexes can be designed to bind DNA after excitation. The variety of ES
and their photochemistry provide a range of mechanisms for DNA photobinding.
Photobinding DNA through a metal centre.
Ruthenium(II) complexes of the
form
cis -
[(TL)
2
RuX
2
]
n
+
, where X = halide (
n
= 0) and X = H
2
O, NH
3
or aliphatic
amine (
n
= 2), have been investigated for their DNA thermal binding properties
and more recently as DNA photobinding agents.
3
The complexes
cis -
[(bpy)
2
RuCl
2
]
and
cis -
[(bpy)
2
Ru(OH
2
)
2
]
2+
readily covalently bind to DNA under physiological
conditions. Photobinding to DNA of
cis -
[(bpy)
2
RuCl
2
] and
cis -
[(bpy)
2
Ru(OH
2
)
2
]
2+
shows little enhancement over thermal binding pathways.
63
Singh and Turro
reported the DNA photobinding properties of
cis
- [(bpy)
2
Ru(NH
3
)
2
]
2+
, but have
shown that the complex did not appreciably thermally bind DNA.
22
The mononu-
clear complex absorbs strongly at 290 nm and 243 nm, consistent with IL transitions.
MLCT transitions in the visible region occur at
λ
max
abs
=
490
nm
. The complex
undergoes photoaquation when irradiated with near-UV light (
λ
max
ex
=
350
nm
or
400 nm, F
aq
= 0.024
450 nm, however, did not
lead to observable chemical reaction. The authors indicate the photoaquation takes
place from a
3
LF state that is thermally inaccessible from the
3
MLCT state. A pho-
toproduct of 400 nm excitation of
cis
- [(bpy)
2
Ru(NH
3
)
2
]
2+
,
cis
- [(bpy)
2
Ru(OH
2
)
2
]
2+
,
has been demonstrated to undergo ligand substitution with mononucleotides and
was shown to bind readily to double stranded (ds) and single stranded (ss) DNA.
The authors predict that the interacting electronic excited states can be tuned by
modifying the complex structure to favour photoaquation following visible light
excitation.
Octahedral polyazine complexes of Rh(III) with a
cis
- dihalide moiety are also
known to undergo ligand substitution upon photolysis with near-UV light.
23,29
Mor-
rison and coworkers have studied the DNA photobinding properties of
cis
-
[(TL)
2
RhCl
2
]
+
.
30,64 - 66
They demonstrated that [(phen)
2
RhCl
2
]
+
, when irradiated with
308 nm light, photobound to ss- or dsDNA with F
b
±
2 or 0.018
±
2). Excitation with light
>
0.006 and 0.001 respectively.
30
Dark controls confi rm that electronic excitation of the metal complex is required
for [(phen)
2
RhCl
2
]
+
to bind DNA. The authors have also proposed a photooxidation
step is necessary to initiate the sequence giving the photosubstitution products.
64
Previous assignment of the photolabile state of [(phen)
2
RhCl
2
]
+
was to the lowest
3
LF state.
29
Exchanging phen for methyl substituted 4,7,7,8-tetramethyl-1,10-phen-
anthroline (Me
4
phen) or 5,6 - dimethyl - 1,10 - phenanthroline (5,6 - Me
2
phen) had
varying impact on the photochemistry.
66
The lowest absorption band of [(5,6-
Me
2
phen)
2
RhCl
2
]
+
is red shifted compared to both [(phen)
2
RhCl
2
]
+
and
[(Me
4
phen)
2
RhCl
2
]
+
. Enhanced photoaquation for [(Me
4
phen)
2
RhCl
2
]
+
( F
aq
= 0.63)
compared to [(phen)
2
RhCl
2
]
+
( F
aq
= 0.03) when photolysed with 347 nm light was
seen. A similar complex, [(phen)(dppz)RhCl
2
]
+
was shown to both photobind and
photocleave DNA when irradiated with 311 nm light. The planar dppz ligand is
known to facilitate intercalative binding of the metal complex to DNA. The F
aq
of
[(phen)(dppz)RhCl
2
]
+
, when irradiated with 355 nm light is reported to be 0.068,
reduced compared to [(phen)
2
RhCl
2
]
+
( F
aq
= 0.087). Competitive decay to a low
lying n
≈
→
p* state localized on the dppz ligand is possible in the dppz-containing
