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 c onsidered to be the phospholipids, and their effects could be modulate d by the presence of bivalent cations. A study of paraquat (PQ)-phosph olipid (PL), with different charge at pH 7, has been carried out by me ans of some different chemico-physical techniques. The systems PQ-dipa lmitoylphosphatidic acid (DPPA) and PQ-dipalmitoylphosphatidylcholine (DPPC) water suspensions at pH 7 have been investigated. The experimen tal results demonstrate that the interaction between PQ and PL is main ly electrostatic; PQ strongly binds DPPA, which bears one negative cha rge 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 th e hydrophobic tail of PQ molecules. The presence of calcium ions, in a n amount sufficient to bind completely the lipid, strongly alters the DPPA-PQ interactions, thus inhibiting the binding of PQ to lipid. On t he 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 a re strongly modulated by the presence of cations with a great affinity for the phosphate group, such as calcium ions.