Complexation and organization of doxorubicin on polystyrene sulfonate chains: impacts on doxorubicin dimerization and quenching

Anthracycline doxorubicin hydrochloride (DX) is a positively charged fluorescent drug, which in water self-associates into non-fluorescent antiparallel dimers upon increasing concentration and/or ionic strength. The positive charge of DX allows for complexation with negatively charged polymers and drug carriers. The fluorescence of DX following complexation with polyanion polystyrene sulfonate (PSS) is studied here. The fluorescence emission of DX decreases in the presence of PSS, being almost completely quenched when the ratio (R) of PSS monomers-to-DX molecules is larger than 10. Increasing R values over 30 results in a progressive recovery of fluorescence. The circular dichroism of PSS-DX complexes shows inverted characteristic bands of DX dimers suggesting the presence of parallel dimers at a concentration of DX below dimerization in water. Molecular dynamics studies corroborate a preferential orientation of DX into parallel dimers when interacting with PSS and show that DX molecules interact with a binding pocket of PSS monomers rather than with one single monomer. Increasing the ionic strength results in a recovery of fluorescence without an apparent release of DX from the PSS-DX complex as shown by DOSY NMR. PSS acts as a template for concentrating DX, triggering dimerisation and orienting DX molecules with their charged groups facing the negatively charged PSS monomers.