Identification of the tautomeric structure of H2TMpyP4–DNA complexes from resonance Raman excitation profiles

Abstract

Aspects of the electronic structure of tautomers of tetrakis(N-methyl-4-pyridyl)porphyrin (H2TMpyP4) complexed to DNA have been determined from a study of the resonance Raman excitation profiles of normal modes of H2TMpyP4 in buffer solutions containing monomeric and aggregated molecules and of a H2TMpyP4/DNA complex with r(c) ~ 25. Maxima of the excitation profiles of the complex have been assigned to electronic origins for transitions between the ground state and states in the Soret absorption region of intercalated and externally groove-bound tautomers of H2TMpyP4. The positions of the electronic origins support the assignment of the CD spectrum of the complex. From the shift of the Cβ—H bending mode of the selectively excited resonance Raman spectra of tautomers of the porphyrin molecules it can be concluded that the pyridyl rings of externally bound H2TMpyP4 molecules are twisted to a lesser degree than those rings for intercalated H2TMpyP4. Raman intensity studies also reveal that the resonance-enhanced Raman intensity of modes of aggregated H2TMpyP4 is greater than those of the monomer.