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Structure and phase behavior of hydrated mixtures of L-dipalmitoylphosphatidylcholine and palmitic acid. Correlations between structural rearrangements, specific volume changes and endothermic events

1988; Elsevier BV; Volume: 48; Issue: 3-4 Linguagem: Inglês

10.1016/0009-3084(88)90091-6

1873-2941

Rumiana Koynova, B.G. Tenchovl, Peter J. Quinn, P. Laggner,

Thermodynamic properties of mixtures

Several new features of the phase diagram of L-dipalmitoylphosphatidylcholine (DPPC)/palmitic acid mixtures in excess water were established by means of static and time-resolved X-ray diffraction, densitometry and differential scanning calorimetry (DSC). At low temperatures, palmitic acid has a biphasic effect on the lamellar subgel phases: at concentrations below 5–6 mol%, it prevents formation of the DPPC subgel phase (Lc), while at higher contents (between about 40 and 90 mol%) another subgel phase (Lccom) is formed as a result of lipid co-crystallization at 1 DPPC : 2 palmitic acid stoichiometry. A crystalline palmitic acid phase separates from Lccom above 70–80 mol% of fatty acid. The Lccom phase transforms into a lamellar gel phase (Lβ) in an endothermic transition centered at 38°C. At high temperatures, the mixtures form hexagonal liquid-crystalline phase (HII) in the region of 60–70 mol% and an isotropic phase (I) at 90–100 mol% of palmitic acid. No coexistence of HII phase with the fluid lamellar phase of DPPC was observed at intermediate compositions (20 and 50 mol% of palmitic acid) but rather formation of a complex phase with non-periodic geometry characterized by molten chains and a broad, continuous small-angle scattering band. No evidence for fluid phase coexistence was found also at compositions between HII and I phases. The Lβ-HII transition at 60–70 mol% of palmitic acids is readily reversible and two-state in both heating and cooling modes. It is characterized by the coexistence of initial and final phases with no detectable intermediates by time-resolved and static X-ray diffraction. The crystalline-isotropic transition in palmitic acid is two-state only in heating direction. On cooling, it is characterized by strong undercooling and gradually relaxing lamellar crystalline structures. The slowly reversible Lccom-Lβ transition proceeds continuously through intermediate states. Although clearly discernible by both DSC and X-ray diffraction, it is not accompanied by specific volume changes.