A nuclear magnetic resonance study of the self-association of adriamycin and daunomycin in aqueous solution

Abstract

A study of the self-association of the anthracycline antibiotics adriamycin and daunomycin was undertaken in D2O and methanol using nmr at 400 MHz. From the concentration dependence of the 2H linewidths obtained on selectively deuterated daunomycin it was determined that daunomycin forms a dimer in aqueous solution. The concentration dependence of the chemical shift of the 4 methoxy group of adriamycin was fit to a dimer model in which only the non-protonated neutral species of adriamycin participated in the self-association process. The data were in agreement with the dimer model if a pK of 7.4 was used. The pH dependence of the self-association was fit to two ionizations, one with a pK of 8.2 and the other with a pK of 8.8. The latter pK was attributed to the ionization of the phenol based on the work of Eksborg. It was concluded that only the deprotonated amine of adriamycin took part in the dimerization process. The dimerization of adriamycin is characterized by a ΔS of approximately 3 cal deg−1 M−1 and a ΔH of −3.45 kcal M−1. At room temperature the association constant for the dimerization process was found to be 4.0 × 105 M−1. These results were discussed in terms of earlier studies by Barthelemy-Clavey etal., Martin, and Chaires etal.