Effect of microscopic environment on the self-stacking binding of porphyrin to DNA

The interaction model of meso-tetrakis(4-N-ethylpyridiumyl) porphyrin (TEPyP) and calf thymus DNA (ctDNA) was assumed by the change of UV-vis spectroscopy, fluorescence and resonance light-scattering (RLS). The changed absorption spectral properties of TEPyP titrated with ctDNA were observed from red shifted (Δλ = 14 nm) and a large hypochromicity (42%). In fluorescence spectra, λex,max of TEPyP shifted to longer wavelength (Δλ = 13 nm) and a decrease around 50% of the emission intensity were shown. The signal of resonance light-scattering was enhanced by nucleic acid. The study on the inclusion interaction of TEPyP with β-cyclodextrin (β-CD) and its derivatives including hydroxypropyl-β-cyclodextrin (HP-β-CD), sulfobutylether-β-cyclodextrin (SBE-β-CD) shown that the formation constants for β-CD, HP-β-CD and SBE-β-CD with TEPyP were 550, 1.1 × 104 and 2.0 × 105 M−1, respectively, with 1:1 stoichiometry. The conformation confirmed by 1HNMR technique is that the alkyl and pyrrole cyclic moiety entered into the cavity of cyclodextrins. Comparative study on the interaction of TEPyP with ctDNA was further carried out in the presence of β-CD, HP-β-CD, and SBE-β-CD. The significant decrease of the binding constants and binding numbers were observed and the interaction of DNA-bound porphyrin has been inhibited. Moreover, the stronger inclusion ability of TEPyP and CDs, the bigger influence on the interaction of TEPyP with DNA, which was due to the fact that TEPyP enter into the cavity of CDs. It may influence binding affinity of porphyrin to DNA and weaken the static electronic forces in the outside self-stacking. Typically, the negative SBE-β-CD hampers directly the binding of the negative phosphate groups of the DNA backbone and the positive TEPyP.