Chemistry Reference
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(2) Mg-N7 (guanine). The pigment distributions are 60% with the backbone PO
2
group and 20% with the G-C base pairs. The chlorophyll interaction is associated
with a major reduction of B-DNA structure in favor of A-DNA. At high chlorophyll
content (
r ¼
1/10), helix opening occurs, with major spectral alterations of the G-C
and A-T bases. At high chlorophyll concentration (1/5), pigment aggregation is
observed, which does not favor CHL-DNA complexation.
Unfortunately, the poor solubility or self-aggregation of the chlorophyll derivatives
in the buffer solution not only hinders their intercalation into the base pairs of the
double-helical DNA but also deactivates the photoexcited dye molecules [
154
,
155
].
To avoid this problem Inoue designed and synthesized three cationic water-soluble
chlorin e6 (Fig.
15
) derivatives, 23, 24,and25 (Fig.
15
). The DNA unwinding assay,
melting temperature experiments of double-stranded DNA, and the induced CD and
visible absorption spectra have revealed that 23 and 25 are intercalated into the base
pairs of the double-helical DNA, while 24 is bound to outside the minor groove of the
double-helical DNA. The cationic water-soluble chlorin e6 derivatives effectively
cleave the double-helical DNA under photoirradiation and the DNA-photocleavage
activity increases in the order 25
24 [
156
].
Due to the intense red shift observed in absorption spectrum, phthalocyanines
and analogues such as porphyrazine are among the porphyrinoids more studied as
photosensitizer. Interaction of Cu(2,3-tmtppa)4C and Cu(3,4-tmtppa)4C (Fig.
16
)
with calf thymus DNA was studied by UV-visible and CD spectroscopies and
viscometric method. The existence of extensive hypochromicity, large red shift,
and negative CD in the visible part of Cu(3,4-tmtppa)4C spectra suggested
an intercalation binding mode. Analysis of the moderate hypochromicity, red
shift, and bisignate CD in the Q-band absorption region of Cu(2,3-tmtppa)4C
spectra possibly led us to the coexistence of intercalation and outside binding
modes [
157
,
158
].
Then Hoffman synthetized a new class of cationic tetraazaporphyrins, the
octa-methyl-pyridyl porphyrazines, including CuPz8+ and ZnPz8+ (Fig.
16
)[
159
].
These octacationic macrocycles are directly functionalized at the
23
>
>
-pyrrole carbons
with pyridyl groups to provide a higher charge per macrocycle, to decrease aggre-
gation, and to avoid problems of isomer purification. The spectrum of Cupz8+ red
shifts as DNA is added, which is interpreted as indicative of
β
stacking of the
charged macrocycle as it binds electrostatically to the outside of the DNA duplex.
The spectrum of Znpz8+ does not shift, indicative of simple electrostatic surface
binding [
160
].
ˀ
4 Anionic Porphyrins with Duplex DNA
The formation of porphyrin-DNA complexes, which is believed to be an important
step leading to antitumor activity, as shown above, is known to be facilitated by
the electrostatic attractions between the periphery of cationic porphyrins and
the anionic phosphate backbone. On the other hand, the interaction of anionic
