Chemistry Reference
In-Depth Information
a
b
40
20
0
-20
-40
-60
-80
-100
80
40
0
-40
-80
400
450
500
400
450
500
Wavelength(nm)
Wavelength(nm)
Fig. 7 CD spectra of H
2
TMPyP4 5
M(a) in the presence of poly(dA-dT) (
continuous line
)or
poly(dA) · poly(dT) (
dashed line
) and (b) in the presence of poly(dG-dC) (
continuous line
) or poly
(dG) · poly(dC) (
dashed line
). In all experiments, porphyrin/DNA ratio is 0.4. Modified from [
88
]
ʼ
on template composition; it is promoted by a template consisting entirely of G-C
base pairs as compared to one having fewer (CT-DNA) or no G-C units (poly
(dA-dT)) [
42
]. As part of studies on DNA-bound porphyrin assemblies, the utility of
a nonconventional light-scattering method (in which the radiation employed is
within absorption Soret maximum) was demonstrated [
40
,
89
]. For extended, elec-
tronically interacting chromophore arrays, a remarkable enhancement of light scat-
tering is observed within these absorption envelopes. These so-called enhanced RLS
signals are useful not only for identifying such assemblies but for characterizing
them as well [
89
,
90
].
In fact, this technique was used by Pasternack and coworker to control the extent
of porphyrin aggregation on a nucleic acid scaffold. When more affine intercalator
binder, Au(III)TMPyP4, is added to a preformed intercalated
trans
-H
2
Pagg/DNA
complex, a slight decrease in the magnitude of the ICD bands of the latter
porphyrins can be detected, concomitant with the appearance of a new negative
component around 410 nm due to intercalated AuTMPyP4. This observation
suggests that the gold(III) derivative, bearing five positive charges, competes
effectively with
trans
-H
2
Pagg (which is a dicationic species) for binding sites in
the double helix, partially displacing the latter species. Upon addition of NaCl,
aggregation is fostered, and large bisignated ICD signals are detected in the Soret
region. At a fixed concentration of
trans
-H
2
Pagg, the intensity of the ICD signal and
the corresponding RLS intensity, hence the extent of aggregate, depend linearly on
the concentration of the added Au(III)TMPyP4 [
91
].
An important aspect studied by Kim is the ability of the porphyrins to quench the
fluorescence of intercalated ethidium. Energy and charge transfer along DNA has
been a subject of intense study since the stacked
-orbitals of DNA base pairs were
found to be an effective medium for electron and hole transfer [
92
-
98
]. The
biological importance of charge transport has been highlighted by the discovery
of distance oxidative damage to DNA in the cell nucleus [
99
,
100
]. Strong overlap
between emission spectrum of ethidium and the absorption spectrum of porphyrins
when they simultaneously bind to DNA was found suggesting the mechanism
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