Interaction of anthracycline antibiotics with biopolymers: 7. Equilibrium binding studies on the interaction of iremycin and DNA

We report spectrophotometric equilibrium studies of both the self-association of the new antibiotic iremycin and of its binding to calf thymus DNA in solution (ionic strength 0.2 M; pH 6.0). Iremycin forms dimers in this solution with a dimerization constant K₄=(1.19 ± 0.10) × 10³ M⁻¹. This equilibrium is taken into account in the evaluation of the interaction of iremycin with DNA. The binding behaviour can be completely described by a single binding mechanism of monomeric iremycin to DNA with allowance both for neighbour exclusion and for cooperativity of interaction. The three intrinsic binding parameters for the homogeneous model were determined simultaneously by a least squares fit of the original titration data: equilibrium constant of cooperative binding K = (2.72 ± 0.66) × 10⁵ M⁻¹ cooperativity parameter σ=0.38±3.27 ± 0.32. The binding parameters of iremycin and adriamycin and their microbial activities are compared.