Phase behaviour of fats and their mixtures.

Structure and functional properties of diacylglycerols in membranes.

Investigations on the physical state of lipid nanoparticles by synchrotron radiation X-ray diffraction

The size-dependent behavior of different crystalline, colloidal monoacid and complex triglycerides in aqueous dispersion is investigated, mainly by differential scanning calorimetry and X-ray diffraction. With decreasing particle size the melting range of the triglycerides broadens and shifts to lower temperatures. The melting of particles in small-size dispersions (e.g., with mean particle sizes around 100 nm) of even, saturated monoacid triglycerides such as tripalmitin and trimyristin is characterized by a sequence of discrete sharp transitions which are not due to polymorphism. This complex melting process is observed with different types of stabilizers. The sequence of melting events originates from the shape and layered structure of the triglyceride nanocrystals leading to successive discrete transitions corresponding to the melting of particle fractions with different thickness. In dispersions of complex triglycerides, the individual transitions are less sharp or even undetectable. The heat of fusion and the crystallization temperature of the nanoparticles decrease slightly with decreasing particle size whereas the rate of polymorphic transitions increases. Moreover, dispersions containing extremely small nanoparticles display an X-ray diffraction pattern indicating the presence of a crystal form that seems not to have been described previously for these triglycerides.

Effect of Particle Size on Colloidal Solid Triglycerides

Calorimetric, X-ray diffraction, and 31P nuclear magnetic resonance (NMR) studies of aqueous dispersions of 1,2-dihexadecyl-sn-glycero-3-phosphocholine (DHPC) gel phases at low temperatures (-60 to 22 degrees C) show thermal, structural, and dynamic differences when compared to aqueous dispersions of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) gel phases at corresponding temperatures. Differential scanning calorimetry of DHPC dispersions demonstrates a reversible, low-enthalpy "subtransition" at 4 degrees C in contrast to the conditionally reversible, high-enthalpy subtransition observed at 17 degrees C for annealed DPPC bilayers. X-ray diffraction studies indicate that DHPC dispersions form a lamellar gel phase with dav congruent to 46 A both above and below the "subtransition". It is suggested that the reduced dav observed for DHPC (46 A as compared to 64 A in DPPC) is due to an interdigitated lamellar gel phase which exists at all temperatures below the pretransition at 35 degrees C. 31P NMR spectra of DHPC gel-phase bilayers show an axially symmetric chemical shift anisotropy powder pattern which remains sharp down to -20 degrees C, suggesting the presence of fast axial diffusion. In contrast, 31P spectra of DPPC bilayers indicate this type of motion is frozen out at approximately 0 degrees C.

Comparative study of the gel phases of ether- and ester-linked phosphatidylcholines

The contribution that x-ray diffraction has made to the understanding of triglyceride polymorphism is reviewed. The crystal structure of these compounds is explained in terms of molecular orientation in the crystal lattices. At the present time only the crystal structure of the monoacid saturated triglycerides has been reasonably well defined. Mixed triglycerides and mixtures thereof have not yet been fully investigated. This review stresses the need for systematic correlation of the crystal structure of fats with the molecular composition and configuration.

C. W. Hoerr

Papers

The role of X-ray diffraction in studies of the crystallography of monoacid saturated triglycerides