Structure and Polymorphism of l,2-Dioleoyl-3-acyl-sn-glycerols. Three- and Six-Layered Structures

Triacylglycerols, which usually contain at least one unsaturated fatty acid, are the most important forms of stored biological lipids in teleosts, mammals, and most plants. Since the physical properties of such mixed-chain triacylglycerols are poorly understood, a systematic study of such compounds has been initiated. Stereospecific l,2-dioleoyl-3-acyl-sn-glycerols were synthesized with even carbon saturated fatty acyl chains of 14-24 carbons in length. Their polymorphic behavior was examined by differential scanning calorimetry and X-ray powder diffraction. The thermal behavior revealed from one to four major polymorphic transitions depending upon saturated chain length. Plots of enthalpy of fusion and entropy vs. carbon number for melting of the most stable polymorph were linear throughout the series with slopes of 1.0 kcal/mol per carbon atom and 2.6 cal/(mol K) per carbon atom, respectively. These slopes indicate that the saturated chains are packed in a well-ordered tightly packed lattice. When the compounds were rapidly cooled to 5 °C, X-ray powder diffraction revealed strong β' (ca. 3.8 and 4.2 Å) reflections and weak β (ca. 4.6 Å) reflections. The β subcell reflections intensified when the compounds were heated to within 5 °C of the melting temperature of the highest melting polymorph. Evidence of an α phase was not seen on 30-min X-ray exposures for any of the compounds. In the proposed packing arrangement the saturated and unsaturated chains are segregated into layers. The stable form of all compounds exhibits a triple layer packing mode in which a bilayer of oleoyl chains is segregated from an interdigitated layer of saturated chains. The thickness of the glycerol backbone normal to the base plane is about 4.1 Å. The saturated chains are tilted 55.8° with respect to the base plane. This tilt is close to the value of 56.5° obtained from the single crystal study of oleic acid, which may indicate that the saturated chains project from the glycerol backbone like a linear extension of the first nine carbons of the oleoyl chain. In addition, a metastable six-layer packing mode was identified for l,2-dioleoyl-3-myristoyl-sn-glycerol. While some of the complex polymorphisms involve adjustments in chain packing, others may relate to transitions from unstable six-layer to stable three-layer structures.