Chemistry Reference
In-Depth Information
characterize the binding of the metal derivative complexes of the “TMPyP4
porphyrin” family. It has been found that metal porphyrins which exist as (or can
easily be converted to) four-coordinate species such as Cu(II)TMPyP4, Au(III)
TMPyP4, Ni(II)TMPyP4, and PdTMPyP4 (Fig.
2
) are capable of intercalating, as is
the planar metal-free, free-base form. On the other hand, metal derivatives containing
one (Zn(II)TMPyP4 and Fe(III)TMPyP4) or two (Mn(III)TMPyP4, Fe(III)TMPyP4,
Ni(II)TMPyP4, Co(III)TMPyP4, and V(II)(
¼
O)TMPyP4) axial ligands (Fig.
2
)
are blocked from intercalation and instead form external complexes in which
the metalloporphyrin locates in a DNA groove [
24
,
28
-
30
]. These findings for
metalloporphyrins and synthetic DNAs established a very convenient UV and CD
spectroscopic signature for these interactions; a large red shift (
5 nm) and substan-
tial hypochromicity (
30%) of the absorption maximum and a negative induced
CD band in the Soret region are diagnostic of intercalation, whereas a small red shift
(
10%) or even hyperchromicity of the absorp-
tion maximum and a positive ICD feature indicate external, groove binding [
24
].
Other studies have shown that H
2
TMPyP4 exhibits sequence-selective DNA
interactions [
24
,
25
,
31
,
32
]. The binding to A-T regions has been considered to be
nonintercalative, whereas that to G-C sequences has been defined as intercalative.
These generalizations about porphyrin-DNA interactions provided a basis for
interpreting a variety of experimental results including the observation (based
upon supercoiled DNA unwinding studies) that, unlike H
2
TMPyP4, FeTMPyP4
does not intercalate into DNA [
17
]. The key factor in preventing intercalation of
this latter derivative according to the model, already reported, is the presence of
axial ligands at the iron center. For H
2
TMPyP4 interaction with natural DNAs, two
induced CD bands are observed under certain solution conditions, a negative band
at about 440 nm and a positive one at shorter wavelength about 430 nm (Fig.
4
).
Bisignate CD spectra of this type can arise from chromophore aggregation, but such
proves not to be the case here. The profile of the CD spectrum is roughly indepen-
dent of drug load and represents two distinct binding modes (intercalation and
external, electrostatic binding) with the distribution of the porphyrin between these
modes dependent on ionic strength [
17
,
21
]. At a fixed concentration of porphyrin
and DNA, H
2
TMPyP4 tends to intercalate at low salt concentration (providing a
negative induced CD feature near the Soret maximum at 446 nm), but as salt is
added, external binding becomes relatively more favorable with the appearance of a
positive induced CD feature [
21
].
Among the factors that must be considered to distinguish intercalation and
external binding is the position of the
N-
methyl group in the pyridyl substituent,
which may hinder the rotation of the
N-
methylpyridyl groups to a position more
nearly coplanar to the porphyrin core. The rotational barrier to coplanarity has been
measured for the indium, titanium, and ruthenium tetraphenylporphyrins as
8 nm) and little hypochromicity (
ʔG
*~
15 kcal/mol if the ortho position of the meso-substituent ring system is occupied
by hydrogen and coplanarity of the meso-substituents is required [
33
,
34
]. For
solutions of ortho substituted of H
2
TMPyP4 (H
2
TMPyP2, Fig.
5
) with poly
(dG-dC), poly(dA-dT), and DNA, there is virtually no change in the Soret maxi-
mum, indicating minimal interaction with the bases of these duplexes. A substantial
