Structural analyses and triacylglycerol polymorphs with FT-IR techniques. 2. β′₁-form of 1,2-dipalmitoyl-3-myristoyl-sn-glycerol

On the basis of the assignments of the methylene wagging bands described in the preceding paper (Part 1), FT-IR studies have been applied to the β′₁-form of 1,2-dipalmitoyl-3-myristoyl-sn-glycrerol (PPM). PPM has the most stable form of β′-type, in contrast to ordinary triacylglycerols, which have the most stable form of β-type. The degrees of inclination of the acyl chains with respect to the lamellar interface were determined by two FT-IR techniques of attenuated total reflection and oblique transmission. The acyl chains were packed in orthorhombic perpendicular (O_⊥) subcell as revealed in the absorption spectra of polymethylene scissoring and rocking modes. As for the acyl chain conformation, it was found that a palmitoyl chain and a myristoyl chain extend to form an all-trans conformation, while the second palmitoyl chain takes a bent conformation in the neighborhood of the ester bond. The acyl chain axis tilts against both the a_s- and the b_s-axis of the O_⊥ subcell, yet the degree of inclination against the b_s-axis is slightly larger than that against the a_s-axis. The a_s-axis is inclined unidirectionally, while the b_s-axis is alternately inclined along the successive layer.