Cationic 5,10,15,20-tetrakis(N-methylpyridinium-4-yl)porphyrin fully intercalates at 5 '-CG-3 ' steps of duplex DNA in solution

The interaction of 5,10,15,20-tetrakis(N-methylpyridinium-4-yl)p (T4MPyP(4)) with the oligonucleotide DNA duplex [d(GCACGTGC)](2) was studied by two- dimensional H-1 NMR spectroscopy, optical absorbance, circular dichroism, a nd molecular dynamics simulation employing particle mesh Ewald methods. T4M PyP(4+) is one of the largest aromatic molecules for which intercalative bi nding to DNA has been proposed, although this has been called into question by recent X-ray crystallographic work [Lipscomb et al. (1996) Biochemistry 35, 2818-2823]. T4MPyP(4+) binding to [d(GCACGTGC)](2) produced a single s et of (mostly) upfield-shifted DNA resonances in slow exchange with the res onances of the free DNA. Intra- and intermolecular NOEs observed in the com plex showed that the porphyrin intercalates at the central 5'-CG-3' step of the DNA duplex without disrupting the flanking base pairs. Absorption and circular dichroism spectra of the complex also support intercalative bindin g. Molecular dynamics simulations (using explicit solvent and PME methods), carried out for fully and partially intercalated complexes, yielded stable trajectories and plausible structures, but only the symmetrical, fully int ercalated model agreed with NOESY data. Stable hydrogen bonding was observe d during 600 ps of MD simulation for the base pairs flanking the binding si te.