Calorimetric and UV spectroscopic study of the interaction of paraquat with phospholipids

Paraquat (PQ) is a popular herbicide and its main effect is the modification of the membrane integrity. The PQ-dipalmitoylphosphatidic acid (DPPA) and PQ-dipalmitoylphosphatydilcholine (DPPC) water suspension systems at pH 7 have been investigated in order to determine the kind of the molecular interaction that exists between PQ and phospholipids (PL) with different charges, and the influence of the presence of Ca++ ions on this interaction. The experimental results demonstrate that the interaction between PQ and PL is mainly electrostatic; PQ strongly binds to DPPA, which bears one negative charge at pH 7, but only weakly to DPPC, which, unlike DPPA, is uncharged. The presence of Ca++, in an amount sufficient to completely bind the lipid (1 M), 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.