Synthesis and polymorphism of 1,2‐dipalmitoyl‐3‐acyl‐<i>sn</i>‐glycerols

Abstract

Abstract Stereospecific 1,2‐dipalmitoyl‐ sn ‐glycerol and 1,2‐dipalmitoyl‐3‐acyl‐ sn ‐glycerols with even‐carbon saturated fatty acyl chains of 2–16 carbons in length were synthesized. The polymorphic behavior and packing arrangements of the most stable crystal form obtained, from the solvent of crystallization, were studied by differential scanning calorimetry and powder X‐ray diffraction. Three different layered packing modes were identified: (a) double‐layer diglyceride‐type; (b) triple‐layer triglyceride‐type; (c) double‐layer triglyceride‐type. The first type of packing was represented by 1,2‐dipalmitoyl‐3‐unsubstituted, 3‐acetyl and 3‐butyryl‐ sn ‐glycerols packed in a bilayer with their long hydrocarbon chains in a parallel arrangement. In the second type of packing, shown by 1,2‐dipalmitoyl 3‐hexanoyl and 3‐octanoyl‐ sn ‐glycerols, the shorter acyl chains formed a middle layer interposed between 2 layers of the 1,2‐palmitoyl chains of sn ‐glycerol. The third type of crystal packing was exhibited by 1,2‐dipalmitoyl‐3‐dodecanoyl and 3‐tetradecanoyl‐ sn ‐glycerols and tripalmitin, was analogous to trilaurin in which the acyl chains at the 1 and 2 positions of glycerol formed an extended, nearly straight line and the 3‐acyl chain was folded to lie parallel and alongside the acyl chain at the 1 position. The intermediate member of the series, 1,2‐dipalmitoyl‐3‐decanoyl‐ sn ‐glycerol, exhibited both the second and the third type of chain packings when obtained from different solvents of crystallization.