Journal of Physical Chemistry B, Vol.101, No.41, 8120-8128, 1997
Structural-Analyses and Triacylglycerol Polymorphs with FT-Ir Techniques .2. Beta’(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 beta’(1)-form of 1,2-dipalmitoyl-3-myristoyl-sn-glycrerol (PPM). PPM has the most stable form of beta’-type, in contrast to ordinary triacylglycerols, which have the most stable form of beta-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-perpendicular to) 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-perpendicular to 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.