STUDIES ON THE INTERACTION BETWEEN NUCLEIC-ACIDS AND CATIONIC PORPHYRINS .1. EFFECT OF THE BASICITY OF THE PORPHYRIN ON THE BINDING MODE TOTHE NUCLEIC-ACID

Interaction between nucleic acids (Calf Thymus, poly(dG-dC).poly (dG-d C), poly(dA-dT).poly(dA-dT)) and two cationic porphyrins (tetrakis-[4- {(3-trimethylaminopropyl)-oxy} phenyl]-porphyrin, TTHETAOPP, and tetra kis-[4N-{3-trimethyl-aminopropyl} pyridyl]porphyrin, TTHETA(py)P) has been studied by UV/Vis and CD spectroscopy as well as viscosimetry. Th e octapositively charged TTHETA(py)P was found to be an intercalator w ith the GC base-pairs though its size is much bigger than that of the classical intercalator TM(py)P(4). On the contrary, TTHETAOPP having n early the same size as TTHETA(py)P, is not able to intercalate even at low salt concentration: it is an outside binder with both GC and AT b ase-pairs and shows a great tendency to aggregate. Both binding modes were supported by the appropriate changes in optical spectral and by t he viscosimetric data. The results clearly show the important role of the basicity on the binding mode of porphyrin to DNA: the low pK(a) (a pproximately 1 for TTHETA(py)P) prefers intercalation whereas high pK( a) (4.6 for TTHETAOPP) makes the porphyrin an outside binder.