Chemistry Reference
In-Depth Information
Fig. 18 Molecular structure of carboranylated porphyrins
small conservative ICD in the Soret region, and RLS measurements confirm the
interaction of p-H
4
TCP
2
with DNA. Working at pH 9.5, when only about 1% of
porphyrin is protonated, these spectroscopic evidences are no more detectable,
confirming the relevance of the protonation state to encourage the binding with
DNA. However, similar studies on m-H
2
TCP
4
having a lower p
K
a (~5) indicated
that this porphyrin, completely deprotonated at pH 7, is able to interact with DNA.
Spectroscopic features such as UV hypochromicity (ca. 20%) of the Soret band at
412 nm (monomeric form), increasing in the intensity of the band at 424 nm, which
is assigned to the self-aggregated form, and enhancing of RLS intensity, suggested
self-aggregation of m-H
2
TCP
4
onto the DNA matrix. The small initial CD signal,
due to chiral aggregates of m-H
2
TCP
4
, changes upon addition of DNA, suggesting
the preferential binding of one or more chiral aggregates of m-H
2
TCP
4
, able to fit
DNA template through enantioselective binding.
Phthalocyanines, as discussed above, represent another class of macromolecules
able to interact with DNA [
172
]. In particular, positively charged phthalocyanines
possess promising photosensitizing properties for the photodynamic therapy of
cancer [
173
]. Anionic copper tetrasulfonated phthalocyanines (H
2
TSPc Fig.
19
)
are able to interact with calf thymus DNA (CT-DNA). The binding process,
enthalpically driven, is mainly due to an axial ligation of copper (II) with carbonyl
group of thymine and/or the ring nitrogen of the base pair group. Thus, an outside
groove-binding mode is suggested. The binding constant ranges from 9.8
10
5
to
10
5
M
1
. The fluorescence of Cu(tspc) was efficiently quenched upon
addition of CT-DNA, with a K
SV
of 5.9
0.4
10
3
M
1
.
Very recently, Balaz et al. [
174
] presented a characterization of free-base
tetrasulfonato anionic porphyrin (H
2
TPPS, Fig.
19
) and two metalloderivatives
(NiTPPS, ZnTPPS) in the presence of poly(dG-dC) sequence. Based on UV-Vis
absorption (very small hypochromic effect and no shift) and CD (no induced CD
signal in the Soret absorption region) data, they have concluded that the electrostatic
repulsion between negative charges of porphyrin sulfonate groups and B-DNA
phosphate backbone effectively prevented interactions of anionic porphyrins.
