Induced chirality of binary aggregates of oppositely charged water-soluble porphyrins on DNA matrix

The induced chirality of achiral binary aggregates of meso-tetrakis(4-N-methylpyridyl)porphyrine (TMPyP) and meso-tetrakis(4-sulfonatophenyl)porphyrine (TPPS) on a deoxyribonucleic acid (DNA) matrix was investigated. Although the negatively charged TPPS did not show induced chirality in DNA solution due to the electrostatic repulsion, induced chirality was obtained through the addition of a positively charged TMPyP. It was confirmed that the induced chirality was due to the binary complex formation between TPPS and TMPyP on the DNA matrix. Moreover, the induced chirality depended on the relative molar ratio of TPPS to TMPyP (r) and the binding modes of the complex to DNA. When r<1, induced circular dichroism (CD) spectrum of the ternary complex was similar to that of intercalated TMPyP into DNA. For r = 1, the induced CD spectrum showed a reversed biphasic signal due to the complex of TMPyP and TPPS stacking along the DNA surface. At a higher r value (>1), there was an induced CD signal at 482 nm attributed to a lateral shifted arrangement of heteroaggregate of TPPS and TMPyP on DNA matrix where TMPyP acted as a spacer to mediate the growth of heteroaggregates. Increasing the concentration of sodium chloride in the solution would favor the formation of the lateral shifted arrangement of heteroaggregate of TPPS and TMPyP. The resonance light scattering (RLS) spectra confirmed the above results. Analysis of the CD spectral changes in DNA conformation showed that during the binary complex formation of TPPS and TMPyP, the intercalated TMPyP could be 'pulled out' from the base pairs of DNA, which might be useful in gene therapy. A model was proposed to account for these observations.