Binding of Dipyridamole to DPPG and DPPC Phospholipid Vesicles:  Steady-State Fluorescence and Fluorescence Anisotropy Decay Studies

Interaction of the coronary vasodilator dipyridamole (DIP) with vesicles of dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylglycerol (DPPG), and mixtures of them has been studied by fluorescence spectroscopy of the drug. Association constants were determined both below and above the phase transition temperature for the lipids, at different pHs (4.0 and 7.0). These constants at pH 7.0 were 633 M-1 (30 °C) and 1.15 × 103 M-1 (50 °C) for pure DPPC and 727 M-1 (30 °C) and 1.48 × 103 M-1 (50 °C) for pure DPPG. At pH 4.0 the association constants showed different behavior; K b values decreased by a factor of 3 for DPPC but increased for DPPG due to the electrostatic interaction of the protonated drug with the phospholipid headgroup. In the mixed system formed by DPPC and DPPG (11% and 20%) the resulting variations in the mixture compositions have a marked effect on drug−vesicle interaction; a decrease of the association constants was observed consistent with a more tightly packed bilayer. Steady-state anisotropy binding data demonstrated the validity of the two-state binding model. Fluorescence quenching experiments with 5-doxylstearic acid (5-DSA) were also performed at pH 4 and pH 7 and with different compositions of lipids. The results support the interfacial location of the drug (close to the fifth carbon of the alkyl chain), suggesting also a strong dependence of binding on lipid packing and the presence of charges at the membrane interface. Fluorescence anisotropy decay experiments were also performed for dipyridamole (DIP) in pure DPPC and DPPG at different pHs and temperatures. The results show clearly that the initial anisotropy r 0 of DIP in model systems (glycerol, sucrose) is quite high, above 0.3, attaining values of 0.20−0.24 in DPPG and being smaller in DPPC. These values are consistent with steady-state anisotropy values at saturating lipid concentrations. The anisotropy decays are best described as a single rotational correlation time which varies in the range 1.5−5.0 ns together with a limiting anisotropy value which also varies between 0.03 and 0.08. Data for anisotropy are in agreement with the binding data, also suggesting the location of the drug at the membrane interface.