Cationic porphyrins bearing diazolium rings: Synthesis and their interaction with calf thymus DNA

Two novel cationic porphyrins bearing five-membered rings at the meso- positions, meso-tetrakis(1,3-dimethylimidazolium-2-yl)porphyrin (H₂TDMImP) and meso-tetrakis(1,2-dimethylpyrazolium-4-yl)porphyrin (H₂TDMPzP), have been synthesized. These two compounds interact with calf thymus DNA (CTDNA) in different binding modes from that of meso-tetrakis(N-methylpyridinium-4- yl)porphyrin (H₂TMPyP). H₂TDMImP outside binds to the minor groove of CTDNA while H₂TDMPzP intercalates into CTDNA. These two novel cationic porphyrins strongly bind to CTDNA even at high ionic strength and the binding constant of H₂TDMPzP to CTDNA is comparable to that of H₂TMPyP. The binding of H₂TDMImP to CTDNA is enthalpically driven. The favorable free energy changes in binding of H₂TDMPzP to CTDNA come from the large negative enthalpy changes accompanied by small positive entropy changes.