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Internet Electronic Journal of Molecular Design - IEJMD, ISSN 1538-6414, CODEN IEJMAT
| ABSTRACT - Internet Electron. J. Mol. Des. November 2006, Volume 5, Number 11, 530-541 |
Theoretical Studies on the Structural Change in the N-Protonated β-Octamethylporphyrin
Yuting Liao and Siyu Ma
Internet Electron. J. Mol. Des. 2006, 5, 530-541
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Abstract:
The structural changes of four pyrrole-rings and various substituting
groups of porphyrin derivatives have great influence on the selectivity
to molecular aggregation in DNA helix; the aggregate can stabilize the
DNA helix and disable telomerase. It not only can develop new
anticancer drugs, but also can distinguish G-quadruplexes. Therefore,
researchers are showing increasing interest in searching selective
porphyrin compounds and understanding the structural change in
acidic medium. In this paper, we report a theoretical calculation of
β-octamethylporphyrin (β-OMPH2) as well as
its diacid (β-OMPH42+)
with an attempt to elucidate the changes of the configuration and
property in the protonation process. The results calculated by
B3LYP/6-1G* show that although β-OMPH2 has various
conformations, D2hL configuration is the only stable equilibrium
geometry. After protonation, the stable equilibrium configuration of
β-OMPH42+ presents D2d symmetry.
The configurational change is
determined by a molecular intrinsic attribute and the intermolecular
interaction is a secondary factor at least. The D4h configuration of
β-OMPH42+ guessed by experimenters is a saddle point of multi-order,
not a stable equilibrium configuration. In addition, a decrease
in E2u
separation occurs concomitantly with an increase of degeneracy
between a1u and a2u. These will lead to producing shift and
strengthening absorbance of B and Q absorption bands. By means of
B3LYP method combining Becke's three-parameter hybrid functional
method with Lee-Yang-Parr's correlation functional (LYP) and Berny
energy gradient method, the target molecules under reasonable
symmetry restriction were optimized at 6-31G* basis set level. All of
the stationary points were confirmed by vibrational analysis. In all
calculations GAUSSIAN 98 program was used on a P4 computer. The
change of the geometry between β-OMPH2
and β-OMPH42+ is obvious
that leads to a series of property changes, especially, the change of
FMO leads to a larger change of molecular absorption spectrum.
Although β-OMPH2 has various conformations,
D2hL configuration is
the only stable equilibrium geometry. After protonation, the stable
equilibrium configuration of β-OMPH42+
presents D2d symmetry. The
configurational change is determined by a molecular intrinsic attribute
and the intermolecular interaction is a secondary factor at least. The
D4h configuration of β-OMPH42+
guessed by experimenters is a saddle
point of multi-order, not a stable equilibrium configuration. In the
protonation process, along with the averaged bond lengths, the bond
charge populations are also averaged generally. Although the changes
are opposite with those of bond lengths in general, the exception
exists. As expected, the energies of both the LUMOs and HOMOs of
β-OMPH42+ are lower than those in their unprotonated form. In
addition, a decrease in E2u separation occurs concomitantly with an
increase of degeneracy between a1u and a2u. These will lead to
producing shift and strengthening absorbance of B and Q absorption
bands. After protonation, the vibrational Raman displacements of the
Cα-Cβ bonds are shifted to higher frequencies as the populations are
increased, while those of the Cβ-Cβ bonds are shifted to lower
frequencies as the populations are decreased; and the Raman
displacements of the Cα-Cm bonds do not change in general, and those
of the Cα-N bonds of various positions are shifted to higher or lower
frequencies with the population changes. With the equivalence of the
pyrrole rings, the IR spectrum of β-OMPH42+
is simpler than that of β-OMPH2
because of the higher symmetry of the former species.
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