Photochemistry and Photophysics of Coordination Compounds

56 N.A.P. Kane-Maguire
utilizing DNA as the potential quenching species are highlighted in the following
section.
7.1
DNA Interactions
The last 20 years have witnessed the emergence of a rich chemistry associated
with the non-covalent interaction of chiral [M(diimine)3] n+ complexes
with duplex DNA, with the ultimate goal of developing new diagnostic and
therapeutic agents [112, 113]. Of particular interest in the present context is
the potential utility of [M(diimine)3] n+ systems as DNA photocleavage agents
via excited state redox processes, which could lead to applications in the general
field of photodynamic therapy [111]. The most widely explored systems
have been [Ru(diimine)3] 2+ species. However, except for a few notable exceptions
[114], values for E 0 ( ∗ Ru 2+ /Ru + )fallwellshortofthe1.2 Vvalue
required for direct one-electron oxidation of guanine (the most readily oxidized
nucleobase [115]) via a reaction pathway analogous to Eq. 1 above.
Although [Ru(diimine)3] 2+ systems are known to photoinitiate DNA oxidation,
this damage normally occurs via the intermediacy of a singlet oxygen
pathway analogous to Eq. 2 [116, 117].
In view of the markedly higher oxidative power of the 2 Eg excited state
of Cr(III) polypyridyl complexes (approximately 1.4 V versus NHE), such
species would appear to be more attractive candidates for photoinitiated direct
oxidation of DNA via Eq. 1 (where Q = DNA). Another potential advantage
of these d 3 systems with regard to bimolecular redox activity is their longerlived
2 Eg → 4 A2g emission (often two orders of magnitude greater than that
for the analogous Ru(II) 3 MLCT emission signals [106]). These photoredox
expectations have been experimentally confirmed in several reports in the
present review period [118–120].
In the first of these studies, the interaction of the complexes [Cr(phen)3] 3+
and [Cr(bpy)3] 3+ with duplex DNA and a range of mononucleotides was explored
[118]. A key observation was that the Cr(III) emission signals were
strongly quenched in the presence of guanine-containing nucleotides, but not
by the mononucleotides of adenine, cytosine, or thymidine, nor by the synthetic
polynucleotide, poly(dA-dT) · poly(dA-dT). A representative example of
Cr(III) emission quenching in shown in Fig. 16 for the case of [Cr(phen)3] 3+ in
the presence of calf thymus B-DNA (which has 40%GCbasepairs).
Such behavior provides strong evidence for direct oxidation of the guanine
base of DNA via Eq. 1, since the corresponding oxidation of the other
nucleobases is thermodynamically more difficult [115]. Since guanine oxidation
has been shown in other systems to serve as a genesis point for DNA
strand scission [115], these Cr(III) complexes show potential as a new class
of DNA photocleavage agents (photonucleases). This expectation receives
support from our recent observation of permanent DNA damage (strand