Electron spin resonance study of the binding structures of cationic water-soluble metalloporphyrins on highly oriented deoxyribonucleic acid fibres

Abstract

The binding structures of several paramagnetic metal complexes of 5,10,15,20-tetrakis(1-methyl-4-or -2-pyridinio) porphyrin (H₂L¹ or H₂L²) on the highly oriented DNA fibres were investigated by means of ESR spectroscopy and computer simulations of the spectra. The structural parameters characteristic of the sterospecific groove binding were obtained for [CuL²]Cl₄, high-spin [FeCl(L¹)]Cl₄ and low-spin [CoL¹][ClO₄]₄ and the low-spin monoimidazole adduct [CoL¹(Him)][ClO₄]₄. They bind to the DNA with the normals of the porphyrin planes titled by about 45° from the double helix axes. The low-spin bis(imidazole) adduct [FeCl(L¹)(Him)₂]Cl₄ binds to A-form DNA with the porphyrin plane nearly parallel to the double helix axis. The complex [CoL¹][ClO₄]₄ binds to DNA as an intercalator as well as a groove binder. It was also revealed that some metalloporphyrins stabilize the tertiary structures of DNA around the binding sites.