Photochemistry and Photophysics of Coordination Compounds

248 M.T. Indelli et al.
strated the failure of long-distance electron transfer model to account for the
data. Concurrently, Tuite and coworkers [162] arrived at a similar conclusion
for the ET quenching of Ru(phen)2dppz 2+ emission in a very similar untethered
DNA/metallointercalator system. In summary, considerable controversy
persists in the estimates of the distances over which fast ET may occur in such
type of untethered systems [144].
4.2.2
Long Range Oxidative DNA Damage by Excited Rh(III) Complexes
It is well known from a large variety of experimental studies and calculations
that guanine (G) is the most easily oxidized of the nucleic acid
bases [144, 163, 164]. Barton and coworkers have extensively exploited the
ability of Rh(III)-phi complexes to induce oxidative damage specifically at
the 5 ′ -G of the 5 ′ -GG-3 ′ doublets, when irradiated with low-energy light.
A first investigation was carried out using a 15-base duplex (Rh-DNA) which
possesses an end-tethered Rh(phi)2bpy 3+ complex in one strand and two
5 ′ -GG-3 ′ sites in the complementary strand. (Fig. 15). The peculiarity of
this Rh-DNA assembly is that the rhodium complex is spatially separated
in a well-defined manner from the potential sites of oxidation. Damage to
DNA was demonstrated to occurred as a result of excitation of the intercalated
rhodium complex, followed by long-range (30–40 ˚A) electron transfer
through the DNA base pair stack [165]. The strategy used to analyze the
mechanism is illustrated in Fig. 16.
Fig. 15 A15-base duplex with an end-tethered Rh(phi)2bpy 3+ complex in one strand and
two 5 ′ -GG-3 ′ sites in the complementary strand [165]
The Rh-DNA assembly was first irradiated at 313 nm to induce direct
strand cleavage. This photocleavage step marks the site of intercalation, and
permits determination of the distance separating the rhodium complex from
potential sites of damage. Rh-DNA samples were then irradiated with low
energy light at 365 nm, treated with hot piperidine, which promotes strand
cleavage at the damaged sites, and examined by gel electrophoresis. This
treatment reveals the position and yield of damage. The results clearly in-