STRUCTURAL-ANALYSES OF TRIACYLGLYCEROL POLYMORPHS WITH PT-IR TECHNIQUES .1. ASSIGNMENTS OF CH2 PROGRESSION BANDS OF SATURATED MONOACID TRIACYLGLYCEROLS

As the first step for full assignment of infrared (IR) active vibratio nal modes in crystalline triacylglycerols (TAGs), methylene wagging an d rocking regions of beta-forms of a series of monoacid TAGs having C1 0 through C22 acids were investigated. The wagging region was especial ly complicated, because dispersion curves of methylene wagging (nu(3)) and twisting-rocking (nu(7)) modes were overlapped and the three acyl chains of TAGs exhibited their own IR bands. In order to resolve the complicated spectra of TAGs, three polarized IR techniques (oblique tr ansmission, attenuated total reflection, and reflection-absorption spe ctroscopy) were employed in addition to the ordinary transmission meth od, and the obtained spectra were compared with those of a series of m onoacid 1,3-diacylglycerols. It was found that the two straight sn-l a nd sn-2 chains of monoacid TAGs showed different series of IR progress ion bands from those of the bend sn-3 chain having a gauche bond in th e neighborhood of the ester bond. The nu(7) modes of the bend chain ma de a significant contribution to the CH2 wagging region as well as the nu(3) modes of both the straight chains and the bent chain.