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NMR Investigations and Supporting ab initio Calculations on Oligonucleotide-Porphyrin Complexes

The aim of the study is to confirm the binding mode and obtain detailed structural information of cationic porphyrins to DNA. For combined NMR investigations and complementary quantum mechanical calculations, model compounds have been selected to investigate the DNA-porphyrin interaction. Quantum mechanical calculations have been performed on the model system 5, 10, 15, 20-tetrakis(pyridinium-4-yl)porphyrin (TPyP). The structure of the porphyrin was optimized with respect to the dihedral angles between the pyridyl groups and the porphyrin plane. The conformational energy minimum occurs close to a pyridyl torsion of ∂ =90º . After preliminary investigations using circular dichroism spectroscopy, indicated that the binding mode is by intercalation, further structural investigations of the oligonucleotide d(GCGCGC)2 and its complexes with 5, 10, 15, 20-tetrakis(N-methylpyridinium-4-yl)porphyrin tetrachloride (TMPyP-Cl4) by NMR spectroscopy were performed. With these experiments the intercalative binding mode of TMPyP-Cl4 was confirmed, and a significant change in the dihedral angles beta and epsilon of the phosphorous backbone upon TMPyP-Cl4 binding could be observed.

NMR; DNA; oligonucleotide-porphyrin complex

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