Atomic resolution structure analysis of β′ polymorph crystal of a triacylglycerol: 1,2-dipalmitoyl-3-myristoyl-sn-glycerol

The crystal structure of the β′-2 form of a mixed chain triacylglycerol (TAG), 1,2-dipalmitoyl-3-myristoyl-sn-glycerol (PPM), was determined to a final reliability factor of 0.074. This work is the first to resolve the atomic-level structure of the β′ polymorph, which is of the highest functionality among multiple polymorphs in asymmetric TAG. In particular, fat crystals present in food emulsions are in β′, whose transformation into β causes deterioration in their physical properties. β′-2, one of the two β′ forms of PPM, forms a monoclinic unit cell with a space group of C2; Z = 8, a = 16.534(5) Å, b = 7.537(2) Å, c = 81.626(9) Å; β = 90.28(2)°, V = 10171(3) ų, density = 1.018 g/cm³, and μ = 4.96 cm^-1. The following characteristics were obtained: 1) two asymmetric units, named A and B, form a hybrid-type orthorhombic perpendicular subcell; 2) the two asymmetric units reveal different glycerol conformations: trans for sn-1 palmitic acid and sn-2 palmitic acid, but gauche for sn-3 myristic acid in A; and trans for sn-2 palmitic acid and sn-3 myristic acid, but gauche for sn-1 palmitic acid in B; 3) a unit lamellae reveals a four-chain-length structure consisting of two double-layer leaflets; 4) the two double-layer leaflets are combined end-by-end in a unit lamellae; and 5) the chain axes are alternatively inclined against the lamellar interface.