Ether linkages in glycerolipids. Their effect on long chain packing

Abstract

Although very little difference in overall molecular volume results in the substitution of ester linkages of long acyl chains to glycerol by ether linkages, a profound change in molecular conformation occurs nevertheless. This is evident in electron diffraction measurements of crystal packing for ether glycerolipids. Ether analogs of triacylglycerols, 1,2-diacylglycerols, and phosphatidylethanolamines have crystal structures similar to those of the corresponding ester lipids only when the chains are packed in the rotationally disordered hexagonal methylene subcell. The more crystalline polymorphic forms for respective ether and ester lipids are quite different. Trialkylglycerols favor a β′-packing rather than the β; the β′-form of the 1,2-diether represents a chain tilt about the 5.0 A O ⊥ subcell axis and not the 7.5 A axis; the β packing of the 1,3-diether is unlike that of the diester and furthermore a stable β′-polymorph is also found. The favored chain packing of the diether phospholipid is the hexagonal methylene subcell and not the orthorhombic subcell found for the natural lipid. Diffraction and melting point data, in general, indicate that the greater rotational freedom around ether bonds tends to disorder the acyl chain packing.