Deciphering the Fluorescence Signature of Daunomycin and Doxorubicin

The fluorescence characteristics of daunomycin (DNM), doxorubicin (DXR), and other anthracycline drugs are often used to monitor localization of the drug within lipid bilayers and liposomal delivery systems and to assess interaction of the drug with DNA and other macromolecules. However, the binding of DNM and DXR to proteins and membrane systems has been observed to exhibit variable effects on the anthracycline's fluorescence. We have delineated the spectroscopic response of DXR and DNM to their surroundings in several systems, including solvents of differing dielectric constant, aqueous solutions of varying pH or fluorophore concentration, and the reverse micellar system of AOT/heptane/water with a range of doxorubicin concentrations. We have observed that the ratio of fluorescence intestinal at the two principal lambda max values shows a parabolic dependence on solvent dielectric constant, i.e. inverted solvatochromism. This behavior has been overlooked by previous investigators and, together with the appearance of a long-wavelength band near 630 nm in solvents of low dielectric strength (also previously not reported), is key to understanding the partitioning of anthracyclines in membrane systems as well as resolving the conflicting interpretation of data in the literature.