The self-association equilibria of doxorubicin at high concentration and ionic strength characterized by fluorescence spectroscopy and molecular dynamics simulations

The self-association equilibria of doxorubicin hydrochloride (DX), at high drug and NaCl concentrations, are studied by temperature scan fluorescence spectroscopy, with the support of molecular dynamics (MD) calculations. Even though all anthracyclines show dimerization equilibria, DX only can further associate into long polymeric chains according to DXmon ⇄ DXdim ⇄ DXpol. This is reflected not only in the mechanical properties of DXpol solutions (behaving as thixotropic gels) but also in their spectroscopic behaviour. Fluorescence, in particular, is the technique of election to study this complex set of equilibria. Upon increasing the temperature, DXpol melts into DXdim, which in turn is in equilibrium with DXmon. Since DXdim is non fluorescent, with a fluorescence temperature scan experiment the DXpol⇄ DXmon equilibrium is probed. However, also information on the DX dimerization equilibrium can be derived together with the relevant thermodynamic parameters ruling the dimerization process (ΔHdim °= -56 kJ mol-1; ΔSdim °= -97 J mol-1 K-1). The residence time of DX molecules in the dimer (74.7 μs), as well as the monomers mutual orientation in the dimer, are characterized by means of theoretical and computational modelling. © 2019 Elsevier B.V.