Conformational conversion of DNA G-quadruplex induced by a cationic porphyrin

The interactions between cationic meso-tetrakis(4-(N-methylpyridiumyl))porphyrin (TMPyP4) and the G-quadruplex (G4) of human telomeric single-strand oligonucleotide d(TTAGGG)₂ (S12) have been investigated by means of circular dichroism (CD), UV-visible absorption and fluorescence spectroscopies. It is found that TMPyP4 can preferentially induce the conformational conversion of the G4 structure from the parallel type to the parallel/antiparallel mixture in the presence of K⁺, and that it can directly induce the formation of antiparallel G4 structure from the single-strand oligonucleotide S12 in the absence of K⁺. Furthermore, the comparable experiments of TMPyP4 with two single-strand oligonucleotides S6 d(TTAGGG) and S24 d(TAGGG(TTAGGG)₃T) in the absence of K⁺ show that TMPyP4 can also induce the formation of antiparallel G4 from S24 but not from S6, indicating that the end-loops of the G4 structure are the key factors for the formation of G4 induced by TMPyP4.