Chemistry Reference
In-Depth Information
The ability of an excited state chromophore to oxidize DNA is a refl ection of
the redox properties of DNA bases and the excited chromophore. Guanine has been
identifi ed as the DNA nitrogenous base with the strongest reducing power [
E
(G/
G
+
)
0.92 V versus Ag/AgCl].
68 - 71
When incorporated in dsDNA, oxidation occurs
preferentially at the 5
≈
′
-G of a multi-G stack, suggested to result from a destabiliza-
-G donor orbitals, shifting the oxidation potential (
E
0
) of 5
tion of the 5
′
′
- G GG - 3
′
to ca. 0.42 V (versus Ag/AgCl).
62
Radical cations of the oxidized bases decompose
by multiple pathways, including cleavage of the sugar phosphate backbone and
base-labile nucleotide oxidation products.
52
The chromophore in its ES or an unstable redox state may also be involved in
cleavage schemes. Hydrogen atom abstraction from a sugar of DNA by
* [(phen)
2
Rh(phi)]
3+
has been proposed to initiate a reaction cascade ending with
single - strand scission.
58
Early studies have indicated that the
3
MLCT states of
[Ru(bpy)
3
]
2+
under anoxic and hypotonic conditions are quenched coincidentally
with single-strand cleavage of circular plasmid DNA, F
rxn
= 1.2
10
− 6
when [DNA]/
[Ru(bpy)
3
]
2+
= 9. Recently the photochemistry of the complex [(TAP)
2
Ru(dppz)]
2+
with DNA was reported, showing mixed photochemical pathways.
72
The authors
noted a mechanism by which the complex photobinds to DNA through a TAP ligand
as described above for [Ru(TAP)
3
]
2+
. Interestingly, the dppz complex has a strong
emission from a long-lived
3
MLCT state in room temperature and in aqueous solu-
tion (t
0
= 1.09
×
s). For other Ru(II)-polyazine complexes with the dppz ligand, emis-
sion in water is notably weak due to low lying n
µ
p *
3
IL state(s) that quenches
the emission from the
3
MLCT state, possibly through hydrogen bonding of the water
to the lone pairs on the nitrogens on the dppz ligand. Intercalation into DNA,
however, protects the nitrogens on dppz from water, decreasing quenching of the
3
MLCT state and greatly enhancing emission. Ru(II)-complexes with dppz are often
called 'DNA light switches' .
73
The emission of [(TAP)
2
Ru(dppz)]
2+
is effi ciently
' switched off ' upon binding to calf-thymus DNA (t = 60 ns), a very unusual result.
Oxygen mediated photocleavage of DNA.
Photocleavage of DNA represents a
major research effort in the study of DNA photomodifi cations with application to
the development of new PDT agents. Several mechanisms of DNA photocleavage
by platinum group metal polyazine complexes have been proposed that require
3
O
2
.
DNA photocleavage was reported for [Ru(TL)
3
]
2+
, TL = bpy, phen, in aerated solu-
tion .
74
Mongelli, Brewer and coworkers reported a series of mononuclear Ru(II)
complexes containing a potential molecular bridge, [(TL)
2
Ru(dpp)]
2+
, where
TL = bpy, phen or Ph
2
phen.
75
Enhancement of DNA photocleavage was noted for
complexes with more lipophilic terminal ligands, [(Ph
2
phen)
2
Ru(dpp)]
2+
being the
most active, consistent with earlier reports by Barton.
56
Hergueta - Bravo, Orellana
and coworkers investigated the
3
O
2
dependence of DNA photocleavage by a series
of Ru(II) complexes incorporating the intercalating ligand dibenzo[
h,j
]dipyrido[3,2 -
a
:2
→
-
c
]phenazine (ddz).
76
The
3
MLCT of [(bpy)
2
Ru(ddz)]
2+
state is a less effi cient
photosensitizer of
1
O
2
generation relative to [Ru(bpy)
3
]
2+
. Enhanced DNA photo-
cleavage by [(bpy)
2
Ru(ddz)]
2+
, however, was noted. The increase in DNA photo-
cleavage by [(bpy)
2
Ru(ddz)]
2+
was attributed to strong intercalative binding to the
DNA prior to photolysis. A similar observation was made for the complex
[(bpy)
2
Ru(DAP)]
+
, DAP = 1,12 - diazaperylene.
77
′
,3
′
