Interaction of benzoic acid, aspirin and para-hydroxy benzoic acid with dipalmitoyl phosphatidic acid vesicles

The effect of benzoic acid (BA), aspirin (ASA) and para-hydroxy benzoic acid (p-HOBA) on the thermotropic behaviour and molecular mobility of dipalmitoyl phosphatidic acid (DPPA) vesicles has been investigated using DSC and 1H NMR. These investigations were carried out with DPPA dispersion in both multilamellar vesicular (MLV) and unilamellar vesicular (ULV) forms. The DSC data indicated that the mechanism by which BA, ASA and p-HOBA interact with the DPPA is similar in MLV and ULV. In the presence of BA, ASA and p-HOBA, the gel to liquid crystal phase transition temperature (Tm) of the DPPA vesicles increases. The DPPA vesicles thus becomes more rigid in the presence of BA, ASA and p-HOBA molecules, due to increased interaction between the lipid headgroups. The increase in rigidity of the DPPA membrane was found to be maximum when doped with ASA and least when doped with salicylic acid (SA). There seems to be good correlation between the acidity of –COOH group present in the drugs and the increase in Tm value (representing rigidity). The transition width (Δm) decreases with increasing concentration of BA, ASA and p-HOBA, implying an enhanced co-operativity of the acyl chain. DSC and NMR data indicate that BA, ASA and p-HOBA molecules, at all concentrations are located close to the interfacial region of the DPPA bilayer but not in the acyl chain region.