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SynopsisThe interaction of tilorone with DNA and five synthetic polydeoxyribonucleotides [(I): poly[d(G-C)]; (IV): poly(dG).poly(dC); and (V): poly(dA)-poly(dT)] has been investigated. Binding isotherms for the homopolymers were obtained by microdialysis equilibria using ''CC-labeled tilorone and interpreted with different models: exclusion effect, associated or not associated with cooperativity, or variable exclusion. Affinity appears to be related more to local structure than to base composition and decreases in the following order: (I) > (11) > (111) > (IV) > (V). Intercalation in circular DNA was demonstrated by electrophoresis migration and electron microscopy, which yielded an average unwinding angle of 7 O per bound dye. The behavior observed in CD and UV spectroscopy shows a sequence similar to the affinities. Tilorone seems to be less intercalated in (IV) and not a t all in (V). The experimental binding isotherm of tilorone to DNA was well fitted on the basis of a model where DNA acts as a heterogeneous lattice built with the six different possible couples of adjacent base pairs, each potential site behaving as if it were in the corresponding homopolymer. The results are discussed in terms of specificity of alternating Pyr-Pur sequences and related to theoretical calculations on intercalation energies of DNA.