Chiral Porphyrin Complexes and Modelling of their Electronic Circular Dichroism Spectra

J. Šebek,^a V. Král,^a M. Valík^a and P. Bouř^b

^aInstitute of Chemical Technology, Technická 2, ^bInstitute of Organic Chemistry and Biochemistry, AV ČR, Flemingovo nám. 6, 16610, Praha

Porphyrin core is an important component of biomolecules such as hemoglobine, myoglobine or chlorophyll. We study various porphyrin derivatives for applications in drug research, complexometry and biotechnologies. Many properties of these compounds are conveniently studied by optical (UV) spectroscopy because of the absorption properties of porphyrin chromophores. The spectra are influenced by substituents, solvent, and in some cases by bound metals. Since this variance and the relation of the spectra to molecular geometry is often poorly understood, we attempt to model spectral intensities of the electronic circular dichroism (ECD) by combined molecular mechanics/quantum mechanics (MM/QM) approach. Porphyrins have often no ECD signal because of their high symmetry; in complexes with chiral matrices, like peptides or nucleic acids, they may become chiral (optically active) and thus the spectra can reflect their interaction with the environment.

We model the geometry of porphyrins and their complexes using classical molecular mechanics models, while for spectra simulations we use semiempirical and ab initio computations. For the latter the time dependent density functional theory (TD DFT) is applied (typically at the B3LYP/6-31G* level). We plan to involve the influence of solvent by implicit and explicit solvent models. With ab initio parameters spectra of larger molecular complexes will be obtained by a semiempirical transition dipole coupling model (TDC).