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

The crystal structure of the beta'-2 form of a mixed chain triacylglycerol (TAG), 1,2-dipalmitoyl-3-myistoyl-sn-glycerol (PPM), was determined to a fi nal reliability factor of 0.074. This work is the first to resolve the atom ic-level structure of the beta' polymorph, which is of the highest function ality among multiple polymorphs in asymmetric TAG. In particular, fat cryst als present in food emulsions are in beta', whose transformation into beta causes deterioration in their physical properties. beta'-2, one of the two beta' forms of PPM, forms a monoclinic unit cell with a space group of C2; Z = 8, a = 16.534(5) Angstrom, b = 7.537(2) Angstrom, c = 81.626(9) Angstro m; beta = 90.28(2)degrees, V = 10171(3) Angstrom (3), density 1.018 g/cm(3) , and mu = 4.96 cm(-1). The following characteristics were obtained: 1) two asymmetric units, named A and B, form a hybrid-type orthorhombic perpendic ular subcell; 2) the two asymmetric units reveal different glycerol conform ations: 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 myristi c 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) t he two double-layer leaflets are combined end-by-end in a unit lamellae; an d 5) the chain axes are alternatively inclined against the lamellar interfa ce.