On the binding of actinomycin and of daunomycin to DNA: a calorimetric and spectroscopic investigation

The “strong” binding of two antibiotics, actinomycin D and daunomycin, to native DNA (calf-thymus) in dilute aqueous solution has been studied by means of calorimetric and spectroscopic measurements. In essence our results show: (1) Daunomycin interaction with DNA is an exothermic process, all features of which depend in a discontinuous way on the fraction of DNA binding sites engaged by the drug. Fluorescence data indicate that such a discontinuous trend should be independent of the GC content of DNA. (2) Actinomycin binding to DNA is, on the contrary, characterized by a positive enthalpy. For such binding, no discontinuity appears discernible with increasing the molar ratio of drug to DNA (phosphorous) on the basis of calorimetric and fluorescence data. (3) Both antibiotics can be bound simultaneously to DNA: our results would suggest that their binding sites on the biopolymer are independent.Discussion is focussed on the possible information derivable from our data on whether or not intercalation may indeed be the main process through which each antibiotic considered “strongly” interacts with DNA.