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

Minato A., Yano J., Ueno S., Smith K., Sato K., FT-IR study on microscopic structures and conformations of POP-PPO and POP-OPO molecular compounds, Chemistry &  Physics of  Lipids, 1997, 88, 63-71

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.