CONICET | Buscador de Institutos y Recursos Humanos

Grafted lipids with poly(ethylene glycol) (PEG) with different degrees of polymerization are widely used in mixed liposomal formulation for drug delivery. These polymers bound to the phospholipid head group region of the membrane form a hydrophilic layer which stabilize vesicles and extend their circulation lifetime, preventing the self-aggregation and reducing or preventing protein adsorption from the blood sera. PEGs ability to fulfil this role has been attributed mostly to its physical properties such as unlimited water solubility, large excluded volume and high degree of conformational entropy. Mixtures of dipalmitoyl phosphatidylcholine (dpPC) with dipalmitoyl phosphatidylethanolamine grafted poly(ethyleneglycol) (PE-PEG) with mean PEG molecular masses of 350, 1000 and 5000 Da were prepared in chloroform-methanol 2:1 with molar fractions of PE-PEG varying between 0 and 1. The isotherms of all PE-PEGs at low pressures were gaseous with lift-off areas increasing with the molecular mass of the PEG moiety (660, 1000 and 6000 A/molec, respectively). At 15 mN/m they all acquired more packed arrangements with molecular areas 140, 180 and 500 A/molec. and PE-PEG1000 and PE-PEG5000 completed a smooth phase transition. PE-PEG5000 collapsed at 42 mN/m while PE-PEG350 and PE-PEG1000, after a phase transition at 25 and 30 mN/m, collapsed at 60 and 55 mN/m respectively. The transition from the highly expanded monolayer to more condensed phase with relative small minimal molecular areas (40, 50 and 140 A/molec) suggested the transition form the already described mushroom to the linear brush conformations. All the mixtures with dpPC exhibited marked negative deviations from ideallity suggesting a strong condensation due to molecular interdigitation at the polymer interfacial region. Isothermal phase diagrams at 22ºC were characterized by two compositional regions. At the limit of miscibility with lower mol fractions, the surface pressure was composition-independent, but above a mixture-specific stoichiometry the collapse pressure remained constant suggesting a constant composition with lost of PE-PEG. All the mixtures with dpPC exhibited marked negative deviations from ideallity suggesting a strong condensation due to molecular interdigitation at the polymer interfacial region. Isothermal phase diagrams at 22ºC were characterized by two compositional regions. At the limit of miscibility with lower mol fractions, the surface pressure was composition-independent, but above a mixture-specific stoichiometry the collapse pressure remained constant suggesting a constant composition with lost of PE-PEG. The isotherms of all PE-PEGs at low pressures were gaseous with lift-off areas increasing with the molecular mass of the PEG moiety (660, 1000 and 6000 A/molec, respectively). At 15 mN/m they all acquired more packed arrangements with molecular areas 140, 180 and 500 A/molec. and PE-PEG1000 and PE-PEG5000 completed a smooth phase transition. PE-PEG5000 collapsed at 42 mN/m while PE-PEG350 and PE-PEG1000, after a phase transition at 25 and 30 mN/m, collapsed at 60 and 55 mN/m respectively. The transition from the highly expanded monolayer to more condensed phase with relative small minimal molecular areas (40, 50 and 140 A/molec) suggested the transition form the already described mushroom to the linear brush conformations. All the mixtures with dpPC exhibited marked negative deviations from ideallity suggesting a strong condensation due to molecular interdigitation at the polymer interfacial region. Isothermal phase diagrams at 22ºC were characterized by two compositional regions. At the limit of miscibility with lower mol fractions, the surface pressure was composition-independent, but above a mixture-specific stoichiometry the collapse pressure remained constant suggesting a constant composition with lost of PE-PEG.

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