Calorimetric and spin-label ESR studies of PEG:2000-DPPE containing DPPC/lyso-PPC mixtures

摘要

The thermotropic behavior of dipalmitoylphos-phatidylcholine (DPPC) multibilayers containing up to 10 mol% of lyso-palmitoylphosphatidylcholine (lyso-PPC) with and without low content of poly(ethylene gly-col:2000)-grafted dipalmitoylphosphatidylethanolamine (PEG:2000-DPPE) has been studied by high sensitivity differential scanning calorimetry (DSC) and electron spin resonance (ESR) using the spin probe di-tert-butyl-nitro-xide (DTBN).The three lipids,dispersed in buffer at appropriate concentrations,form thermosensitive liposomes used as site-specific drug-delivery systems.Without poly-mer-lipids,the DPPC main transition temperature is downshifted of 1.2-1.3 °C at the highest lyso-PPC content.The molar enthalpy and the cooperative unit of the DPPC main transition first decrease rapidly,then more slowly and finally slightly increase with lyso-PPC content.Moreover,in the mixed dispersions,the membrane fluidity increases at any temperature.The addition up to 5 mol% of PEG:2000-DPPE to DPPC/10 mol% lyso-PPC mixtures does not affect neither the thermotropic phase behavior nor the transition cooperativity and the fluidity of the dispersions.