Chemistry Reference
In-Depth Information
behind energy transfer is, at least in part, the F¨rster-type resonance energy transfer
(FRET). The minimum distance in base pairs between ethidium and porphyrin
required to permit the excited ethidium to emit a photon was the longest for
CoTMPyP4 being 17.6 bp and was the shortest for CuTMPyP4 and NiTMPyP4 at
8.0 bp. The variation in the distance was almost proportional to the extent of the
spectral overlap, the common area under emission spectrum of ethidium and
absorption spectrum of porphyrin, supporting the FRET mechanism, whereas the
effect of the orientation factor which was considered by relative binding geometry
was limited [
101
,
102
].
3 Substituted Cationic Porphyrins
In the previous paragraph, we analyzed the interaction of free base and metallo
derivatives of H
2
TMPyP4 and very similar cationic derivatives with DNA. One of
the approaches developed, after the characterization of the binding mode of model
porphyrin, H
2
TMPyP4, with DNA is that to use porphyrins with chemical modifi-
cation in the peripheral position or in the core ring in order to enhance the
DNA-binding affinity and DNA-photocleavage efficacy. In this paragraph we
explore the role of the length of peripheral substituent, the distance between
neighboring positive charges of porphyrinoids, the number of positive charge,
and the composition of the chemical modification.
3.1 Porphyrin Derivatives with Chemical Modifications
in the Peripheral Positions
The first studies concerning the influence of the length of peripheral substituent were
conducted with
N
-alkyl substituted porphyrins. From these studies arose that alkyl
substituents don't modulate the binding interactions. Dabrowiak [
103
] showed that
the DNA cleavage properties of MnTDEt
Est
PyP4 ethyl ester substituted (Fig.
8
) are
the same of those obtained with model compound, MnTMpyP4 [
104
], suggesting an
end-on bound via the minor groove binding in a melted or partially melted region of
DNA. Marzilli et al. [
105
] by RR, NMR, UV, viscosity, and equilibrium dialysis
measures with H
2
TPrPyP4, H
2
TEtOHPyP4, and Ni(II) derivatives (Fig.
8
) obtain
very similar results compared to those with H
2
TMPyP4 species. It is suggested that
both H
2
TPrPyP4 and H
2
TEtOHPyP4 bind to G-C regions of DNA in the same
intercalative manner as H
2
TMpyP4 with the
N
-alkyl substituent extended into the
solvent. For A-T regions of DNA, the binding of H
2
TPrPyP4 and H
2
TEtOHPyP4
is nonintercalative, as found previously for H
2
TMPyP4. The NiTPrPy4 and
NiTEtOHPyP4 cations bind to these polymers (poly(dG-dC), poly(dA-dT), and
CT-DNA) in a similar manner to the apo-porphyrins. The similar Raman spectral
