Porphyrin Bound to i-Motifs: Intercalation versus External Groove Binding.

G-rich and C-rich DNA can fold into the tetrastranded helical structures G quadruplex or C quadruplex (i-motif), which are considered to be specific drug targets for cancer therapy. A large number of small molecules (so-called ligands), which can bind and modulate the stability of G quadruplex structures, have been widely examined. Much less is known, however, about the ligand binding interactions with the C quadruplex (i-motif). By combining steady-state measurements (UV/Vis, fluorescence, and induced circular dichroism (ICD)) with time-resolved laser flash photolysis spectroscopy, we have studied the binding interactions of cationic porphyrin (5,10,15,20-tetrakis(N-methylpyridinium-4-yl)-21 H,23 H-porphyrin, abbreviated as TMPyP4) with i-motifs (C