FT–IR study on microscopic structures and conformations of POP–PPO and POP–OPO molecular compounds

Abstract

The microscopic structures and conformations of the molecular compounds of POP–PPO and POP–OPO were examined using microprobe polarized Fourier transform infrared (FT–IR) spectroscopy. The two molecular compounds are formed in a double chain length structure due to specific acyl–acyl interactions through oleic acid moieties, yet peculiar differences were observed between the two molecular compounds. The subcell structure of β form of the POP–PPO compound was T ‖, whose b s-axis was approximately parallel to the unit cell b-axis. The olefinic conformation were neither of skew- cis-skew′ type nor of skew- cis-skew type. On the other hand, two subcell structures were assumed for β form of the POP–OPO compound: (a) the palmitoyl leaflet forms the T ‖ subcell and the oleoyl leaflet O′ ‖, or (b) the two leaflets are both of T ‖ subcell in which the directions of subcell axes of the palmitoyl and oleoyl chains are different. The olefinic conformation of the β form of the POP–OPO compound revealed the skew- cis-skew′ type. It was assumed that steric hindrance between the palmitoyl and oleoyl chains resulted in the formation of the molecular compounds of two types of double chain length structure: (a) complete separation of the palmitoyl and oleoyl chain leaflets as revealed in the POP–OPO compound, and (b) one leaflet of the palmitoyl chains and other leaflet of the palmitoyl-oleoyl mixed acid chains in the POP–PPO compound. The olefinic conformation of the POP–PPO compound was largely deformed compared to that in the PPO–OPO compound.