The interaction of DAPI with phospholipid vesicles and micelles

The interactions of the dye 4',6-diamidino-2-phenylindole (DAPI) with phospholipids ordered in single bilayer vesicles of dioleylphosphatidylserine (DOPS) or dimyristoylphosphatidylcholine (DMPC) or micelles of monomyristoylphosphatidylcholine (MPC) have been investigated. Somewhat unexpectedly, the binding of this dye to such ordered structures is not affected by the ionic strength of the external medium, which suggests an embedding of DAPI into the hydrocarbon phase. The fluorescence enhancement of DAPI bound can be accomodated within a model previously proposed for the behaviour of DAPI bound to proteins (Mazzini et al., Biophys. Chem. 42 (1992) 101). From both static and dynamic anisotropy measurements, bound DAPI results severely restricted in its rotational freedom but insensitive to the temperature dependent phase transition of the saturated DMPC vesicles. The considerable tightness and specificity of the interactions between DAPI and ordered phospholipids are also deduced from preliminary fluorescence quenching studies (reduced accessibility of iodide ions towards DAPI bound and quenching effects by the chaotrop Nonidet P-40).