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

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 beta form of the POP-PPO compound was T-parallel to, whose b(s)-axis was approximately parallel to the unit cell b-axis. The olefinic conformation were neither of skew-cia-skew' type nor of skew-cia-skew type. On the other hand, two subcell structures were assumed for beta form of the POP-OPO compound: (a) the palmitoyl leaflet forms the T-parallel to subcell and the leaflet O'(parallel to), or (b) the two leaflets are both T-parallel to subcell in which the directions of subcell axes of the palmitoyl and oleoyl chains are different. The olefinic conformation of the beta form of the POP-OPO compound revealed the skew-cia-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. (C) 1997 Elsevier Science Ireland Ltd.