Influence of sebacate plasticizers on the thermal behaviour of dipalmitoylphosphatidylcholine liposomes

Sebacic acid esters are a promising class of low toxicity plasticizers for PVC. They are lipophylic substances and thus, their interaction with biomembranes must be considered. DPPC liposomes in the presence of increasing amounts of dimethylsebacate (DMS), diethylsebacate (DES), dibutylsebacate (DBS) and di-(2-ethylhexyl)sebacate (DEHS) have been studied by DSC, both on heating and cooling. As a general feature, the melting point decreases and the enthalpy ΔH increases along with the half-width and asymmetry of the transition in most of the systems. The observed effects are strongly dependent on the length and the shape of the ester-chains suggesting that both polar interactions between carboxylic groups of sebacates and the polar heads of the lipids as well the hydrophobic interactions involving penetration into the bilayer are important. The effects are noticeable also in the presence of small amounts of plasticizers. In the presence of large amounts of DES and DBS, a complex structure of the transition arising from the coexistence of aggregates of different types and also the appearance of new phases were observed. The behaviour of sebacates to model biomembranes is quite different for other classes of lipophylic PVC plasticizers, such as phthalates and we explain the differences on the basis of the greater flexibility of the sebacic acid chain.