Interaction between antitumor antibiotic chromomycin A₃ and Mg²⁺. I. Evidence for the formation of two types of chromomycin A₃-Mg²⁺ complexes

Abstract

Chromomycin A₃ (CHRA₃) is an antitumor antibiotic which binds to Mg²⁺. In the present communication, we show, by means of equilibrium spectroscopic studies (such as absorption, fluorescence and circular dichroism), that two types of CHRA₃- Mg²⁺ complexes (of 1:1 and 1.9:1 stoichiometries in terms of CHRA₃: Mg²⁺ respectively) are formed depending on the concentrations of CHRA₃ and Mg²⁺. The rate constant and activation energy for the formation of two complexes are different, thereby reinforcing the proposition that they are different molecular species. This observation is novel and significant in order to understand the anticancer property of the drug. It also provides explanation for earlier observations that site, affinity parameters and mode of interaction of the drug with DNA in the presence of Mg²⁺ depend on the relative concentration of Mg²⁺.