Novel supramolecular assemblies of repulsive DNA–anionic porphyrin complexes based on covalently modified multi-walled carbon nanotubes and cyclodextrins

Abstract

We have prepared the water-soluble anionic 5-(p-aminophenyl)-10,15,20-tri(p-sulfonatophenyl)porphyrin (ATSPP) for constructing novel supramolecular assemblies of DNA–anionic porphyrin complexes. ATSPP was covalently linked to multi-walled carbon nanotubes (MWCNTs) to give a suspension of a conjugate (ATSPP–MWCNTs) that was stable for >2 weeks. On the addition of cyclodextrins (CDs), the corresponding complex (ATSPP–MWCNTs–CDs) was formed. Absorption and fluorescence spectra displayed that 1 : 1 inclusion complexes were formed with either α-CD or β-CD, but that virtually no interaction occurred with γ-CD. A comparative study on the interactions of DNA with ATSPP–MWCNTs and the ATSPP–MWCNTs–CDs complexes was carried out using fluorescence spectroscopy, resonance light-scattering, and transmission electron microscopy. The results showed that negatively-charged ATSPP–MWCNTs can interact with the negatively-charged DNA, indicating that covalently modified CNTs contribute to the counterintuitive binding of anionic porphyrins with DNA. This can possibly be attributed to π-stacking interactions between the sidewalls of the CNTs and the bases in DNA. Secondly, ATSPP–MWCNTs–CDs complexes interact better with DNA than the ATSPP–MWCNTs, indicating that CDs can promote the binding of DNA to anionic porphyrin-covalently modified MWCNTs. This is probably due to the fact that the amphiphilic CDs can greatly improve the solubility and dispersity of ATSPP–MWCNTs by their effective inclusion complexation with ATSPP. The successful construction of the supramolecular assemblies ATSPP–MWCNTs–CDs–DNA provides a new approach to the binding anionic porphyrins to negatively-charged DNA.