Binding Properties of Various Cationic Porphyrins to DNA in the Molecular Crowding Condition Induced by Poly(ethylene glycol).

The binding modes of various cationic porphyrins to DNA in an aqueous solution and under the molecular crowding condition induced by poly(ethylene glycol) (PEG) were compared by normal absorption, circular dichroism (CD), and linear dichroism (LD) spectroscopy techniques. Large negative CD and LD signals in the Soret absorption regions of the meta- and para-TMPyP [meso-tetrakis (n-N-methylpyridiniumyl) porphyrin (meta, n = 3) and (para, n = 4)] were apparent in the aqueous solution, indicating an intercalative-binding mode, while a positive CD spectrum and a less intense negative LD spectrum for the ortho-TMPyP (n = 2)-complexed DNA suggested a major-groove-binding mode. These binding modes are retained under a molecular crowding condition, suggesting that the PEG cluster cannot access the TMPyPs that are intercalated between the DNA base pairs or that bind at the major groove. The spectral properties of the ortho-, meta-, and para-trans-BMPyP [trans-bis(N-methylpyrodinium-n-yl)diphenyl porphyrin, n = 2,3,4]-bound DNA in an aqueous solution correspond to neither the intercalative-binding nor the groove-binding mode, which is in contrast with the TMPyP cases. The spectral properties under the molecular crowding condition are altered considerably for all of the three trans-BMPyPs compared to those in an aqueous solution, suggesting that the matted PEG cluster is in contact with the cationic trans-BMPyPs, causing a change in the polarity of the porphyrin environment. Consequently, trans-BMPyPs bind to the external side of the DNA.