Spectrophotometric and nmr analysis of the interaction of fluorinated mono‐ and bifunctional interacalators with DNA, poly(dA‐dT), and poly(dG‐dC)

We have synthesized and investigated the DNA binding properties of three fluorinated acridine derivatives—a monomer (I), a short dimer (II) and a long dimer (III). Only III has a sufficiently long chain bridging the two acridine nuclei to permit binding by bisintercalation. Analysis of the equilibrium and kinetic binding properties of these compounds to poly(dA‐dT) demonstrates that they behave very similarly to their unfluorinated parent compounds. Helix extension, as determined by viscosity measurements, shows that both compounds I and II bind by monointercalation while III binds by bisintercalation. These results are confirmed by 19 F‐nmr analysis, which indicates, in particular, that the two chromophores of III share the same molecular environment as that of I in the presence of either calf thymus DNA or poly(dA‐dT). Negative nuclear Overhauser effects in the presence of DNA indicate tight binding such that the motion of the ligands is governed by the polynucleotide dynamics. Optical titrations establish that in 4 M NaCl, both I and III bind to calf thymus DNA, but no binding was observed with poly(dG‐dC). This result is in contrast to those for dimers of ethidium, which show substantial binding to polynucleotides under high salt conditions. Nuclear magnetic resonance experiments, however, carried out at considerably higher concentrations, show that compound I does indeed bind to poly(dG‐dC) under these high salt conditions, albeit weakly, and leads to a conversion of the polynucleotide from a left‐handed to a right‐handed conformation.