Limited miscibility in hydrated DPPC – Lyso-PPC systems

Abstract

Hydrated dipalmitoylphosphatidylcholine (DPPC) and lyso-palmitoylphosphatidylcholine (Lyso-PPC, abbreviated as LPC) mixtures form vesicles composed from periodically or randomly arranged layers and micelle-like nanoparticles, as revealed by a wide range of thermodynamical, structural, and morphological characterization methods. Based on five different experimental methods, we have reconstructed the phase diagram of the system. The micelle-like objects are strongly anisotropic and are identified as bicelles. Even a small amount of LPC induces drastic changes in the thermotropic phase behaviour of DPPC. Here, an interdigitated gel (LβI) structure, coexisting with the other gel phases, forms at moderate LPC concentration. Bicelles and LβI structured vesicles appear in the equimolar LPC ratio range (0.4 ≤ XLPC ≤ 0.6). Two complex phase transition regimes are detectable. Between 36–40 °C, bicelles are pre-melted while domains of LβI structure are “crystallized” through local endothermic and exothermic transitions, respectively. Then, around 42 °C the chains of both the LβI and bicelles melt completely. Above the equimolar LPC ratio range (XLPC > 0.6) the chain melting in the LβI structure vanishes, while this transition still exists in the chain region of bicelles. This wealth of structural forms, due to the weak first order transition characteristics, is highly sensitive for perturbations and interactions induced by e.g. the presence of membrane proteins.