N-fused porphyrin (NFP) is a porphyrin analogue with an 18π tetrapyrrolic macrocycle, in which a unique tripentacyclic ring is embedded. While the optical properties of NFP of absorbing and emitting near-infrared (NIR) light around 1000 nm are promising for its application to NIR technology, its unique structure is also attractive as a platform to construct a novel class of DNA-binding ligands. Herein, we have synthesized a water-soluble derivative of NFP (pPyNFP) possessing four cationic pyridinium substituents and examined its acid/base behaviors and interactions with various forms of DNAs in aqueous solution. pPyNFP interacts with ssDNA and dsDNA electrostatically. pPyNFP also interacts with a G-quadruplex DNA derived from the human telomeric sequence and causes a characteristic spectral change of the G-quadruplex DNA, which suggests that pPyNFP modulates the Na +-induced (2 + 2) antiparallel G-quadruplex to the all-parallel structure.
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