The dissolution and formation of egg phosphatidylcholine (PC) vesicles by the detergent octyl glucoside were examined systematically by using resonance energy transfer between fluorescent lipid probes, turbidity, and gel filtration chromatography. Resonance energy transfer was exquisitely sensitive to the intermolecular distance when the lipids were in the lamellar phase and to the transitions leading to mixed micelles. Turbidity measurements provided information about the aggregation of lipid and detergent. Several reversible discrete transitions between states of the PC-octyl glucoside system were observed by both methods during dissolution and vesicle formation. These states could be described as a series of equilibrium structures that took the forms of vesicles, open lamellar sheets, and mixed micelles. As detergent was added to an aqueous suspension of vesicles, the octyl glucoside partitioned into the vesicles with a partition coefficient of 63. This was accompanied by leakage of small molecules and vesicle swelling until the mole fraction of detergent in the vesicles was just under 50% (detergent:lipid ratio of 1:1). Near this point, a transition was observed by an increase in turbidity and release of large molecules like inulin, consistent with the opening of vesicles. Both a turbidity maximum and a sharp increase in fluorescence were observed at a detergent to lipid mole ratio of 2.1:1. This was interpreted as the lower boundary of a region where both lamellar sheets and micelles are at equilibrium. At a detergent:lipid ratio of 3.0:1, another sharp change in resonance energy transfer and clarification of the suspension were observed, demarcating the upper boundary of this two-phase region. This latter transition is commonly referred to as solubilization.(ABSTRACT TRUNCATED AT 250 WORDS)

The polymorphism and phase transitions of cocoa butter (CB) have been reexamined separately by differential scanning calorimetry (DSC) and X‐ray diffraction as a function of temperature (XRDT) at scanning rates between 0.1 to 5°C/min and 0.1 to 2°C/min, respectively. A new instrument, which allowed simultaneous DSC and XRDT recordings from the same sample by taking advantage of the high‐energy flux of a synchrotron, was employed for characterization of the intermediate phase transitions. These techniques allowed us to confirm the existence of the six polymorphic forms of CB (called I to VI) by in situ characterization of their formation in the DSC + XRDT sample holder. A detailed study of Form I structure led us to propose a liquid‐crystal organization in which some of the chains displayed sharp long‐spacing lines (d001=52.6±0.5 Å) and a β′ organization (4.19 and 3.77 Å), while the others remained unordered with broad scattering (maxima at about 112 and 36.5 Å). The organization of this liquid crystalline phase is compared to that of fat and oil liquids. This liquid crystalline phase progressively transformed on heating into a more stable phase (Form II, α type, d001=48.5±0.5 Å and short‐spacing at 4.22 Å). Form III was only observed in a sharp temperature domain through its specific short‐spacings. The existence of the six species has been essentially related to the crystallization of monounsaturated triacylglycerols (TAG), while trisaturated species were found partly solid‐soluble in these six polymorphic forms. An insoluble fraction crystallized independently of the polymorphism of the monounsaturated TAG in a separate phase with long‐spacings that were either of the α (49.6±0.5 Å) or β (44.2±0.5 Å) form. In mixture with Form V, this fraction melts and solubilizes in the liquid phase at 37.5°C. Isolation of these high‐melting crystals shows a melting point of about 50°C. High‐performance liquid chromatography analysis of this fraction confirmed an increase from 3.0 to 11.3% of saturated TAG and their association with part of the 1,3‐stearoyl‐2‐oleoylglycerol preferentially to 1‐palmitoyl‐2‐oleoyl‐3‐stearoylglycerol and (1,3‐palmitoyl‐2‐oleoylglycerol).

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