Chemistry Reference
In-Depth Information
that they associated to binding of free-base and protonated form of H
2
T
OPP4,
ϑ
respectively [
112
,
113
]. Due to the relatively high p
K
a
of H
2
T
OPP4 (p
K
a
4.6),
it is possible for the porphyrin to be bound to CT-DNA as the protonated species
at physiological pH [
114
]. The protonated porphyrin exhibits DNA-binding
characteristics that are different from those of the free base, of the red shift in
fluorescence, and of the Soret absorption band [
114
]. This hypothesis is also
confirmed by CD data; in fact, conservative-type ICD spectrum, observed in the
presence of CT-DNA, changes to a simple positive CD band upon protonation of
H
2
T
ϑ
OPP4 indicating that the binding mode underwent a transition from outside
binding with high intense self-stacking to simple outside binding.
By comparing these results with those obtained using another tentacle porphyrin
(H
2
T
ϑ
OPP4, but bearing eight positive charges,
showing negative ICD signal with poly(dG-dC) indicating intercalation binding
[
115
], and another electron-deficient tentacle porphyrin, having bulky substituent,
showing external binding, (H
2
T
PyP4 Fig.
9
) similar in size to H
2
T
ϑ
ϑ
F4TAP, Fig.
9
)[
116
], they concluded that the
results presented provided clear evidence that porphyrin intercalation does require
both planar traversing groups and an electron-deficient porphyrin core. The studies
with tentacle porphyrins thus indicated that, while the influence of porphyrin
electron richness on the binding mode of relatively thin porphyrins was not
important, pyridinium groups appear to be necessary for intercalation. To solve
this doubt they designed a new porphyrin, expanding the size of the pyridinium-
containing porphyrins by placing a linking group between the porphyrin core and
the pyridinium groups. These groups are linked to the 4-position of the phenylene
group of the porphyrin by secondary (
ϑ
SO
2
NR2) sulfon-
amide groups (Fig.
9
)[
117
]. The authors designed these porphyrins to determine
whether the new porphyrins containing N-MPy groups (sulfonamide-1) would be
intercalators and whether the new porphyrins lacking N-MPy groups (sulfonamide-2)
would allow to gain some insight into factors that might influence outside binding
interactions. The DNA-induced changes in the porphyrin Soret region (a positive
induced CD feature and, at high DNA concentration, increases in the Soret band
and fluorescence intensities) indicate that the new porphyrins interact with DNA in
an outside, non-self-stacking binding mode. This finding allows the authors to
conclude that direct attachment of the
N
-alkylpyridinium groups to the porphyrin
ring in such a way that the
N
-alkylpyridinium group can become nearly coplanar
with the porphyrin ring is necessary for intercalation to occur [
118
].
The extensive exploration of cationic porphyrins has been mainly limited to
porphyrins with six-membered rings as meso-substituents. Inoue and coworker
report on the porphyrins meso-substituted by diazole rings such as imidazoles
(H
2
TDMImP4) and pyrazoles (H
2
TDMPzP4) (Fig.
10
) which have recently become
of interest [
119
-
122
]. These two compounds interact with CT-DNA in different
binding modes from that H
2
TMPyP4, in particular H
2
TDMImP4 outside binds to
the minor groove of CT-DNA while H
2
TDMPzP4 intercalates into CT-DNA [
54
].
Then they analyzed the interaction of the metallo derivatives (M
SO
2
NHR) or tertiary (
Mn(III), Ni(II),
Cu(II), or Zn(II)) of H
2
TDMPZP4 with CT-DNA, poly(dA-dT), and poly(dG-dC).
NiTDMPzP4 and CuTDMPzP4 intercalate into the 5
0
GC3
0
sequence of CT-DNA;
¼
