INTERACTION OF BIPYRIDILIUM HERBICIDES WITH MODEL MEMBRANES

Divalent cationic bipyridilium derivatives (Paraquat) have been widely used in agriculture; the primary target of these chemicals has been considered to be the phospholipids, and their effects could be modulated by the presence of bivalent cations. A study of paraquat (PQ)-phospholipid (PL), with different charge at pH 7, has been carried out by means of some different chemico-physical techniques. The systems PQ-dipalmitoylphosphatidic acid (DPPA) and PQ-dipalmitoylphosphatidylcholine (DPPC) water suspensions at pH 7 have been investigated. The experimental results demonstrate that the interaction between PQ and PL is mainly electrostatic; PQ strongly binds DPPA, which bears one negative charge at pH 7, but only weakly DPPC. The small changes in the DSC curves of DPPC can be explained by a small penetration into PL bilayer of the hydrophobic tail of PQ molecules. The presence of calcium ions, in an amount sufficient to bind completely the lipid, strongly alters the DPPA-PQ interactions, thus inhibiting the binding of PQ to lipid. On the contrary, the DPPC-PQ interactions are not modified by the presence of calcium ions. The behaviour of PQ is different from that of other polycations as polyamines, due to the steric effects present in the PO molecule. Our results confirm that the negatively charged PL could be the possible molecular target of PQ. The electrostatic interactions are strongly modulated by the presence of cations with a great affinity for the phosphate group, such as calcium ions.