Laser time-resolved fluorescence study of the interaction between anthracyclines and cardiolipin. 
The molecular interaction between cardiolipin vesicles and two representative anthracyclines, daunomycin and 5-iminodaunomycin, has been studied at pH 7.1 by laser time-resolved fluorescence, for a cardiolipin-to-anthracycline ratio r ranging from 0.02 to 5.  The fluorescence lifetime of daunomycin is 1.03 ns.  For r = 0.3 - 5 a longer-lived transient (1.91 - 1.49 ns) is present and originates from the excitation of daunomycin bound on a single phosphate group of cardiolipin.  At r = 0.3 two lifetimes are observed, the second one being due, partially, to free daunomycin and bound drug molecules embedded in the lipid bilayer.  The fastest-decaying species is present for r = 0.5 - 2.0 and identified as two adjacent, stacked-up daunomycin molecules bound onto the two phosphate groups of the cardiolipin.  In the case of 5-iminodaunomycin, a less cardiotoxic analogue, three-exponential decay is never observed and a fast-decaying component, pi approximately 0.2 ns, is already present at low r and vanishes for r greater than 0.5.  The constancy of the lifetimes of the longer-lived species may originate from the reorientation of the bound drug from the hydrophilic to the lipid domain.