Condensing and Expanding Effects of the Odorants (+)- and (-)-Carvone on Phospholipid Monolayers

Interactions of the odorants (+) and (-)-carvone (5-isopropenyl-2-methyl-2-cyclohexenone) with L-R-1,2- dipalmitoyl-sn-glycerol-3-phosphocholine (L-DPPC) monolayers are studied using surface pressure versus area isotherms measured in the temperature range 10-35 °C in the presence and absence of 5mM odorants in the subphase. The data indicate that the molecular area occupied by the L-DPPC monolayer becomes larger when the monolayer is spread on the subphase containing (-)-carvone and that, in contrast, it becomes smaller when the monolayer is exposed to (+)-carvone. Both expanding and condensing effects are dependent on temperature. L-DPPC monolayers with (-)-carvone absorb twice as much heat as those with (+)-carvone when compressed at 30 °C. Under the same conditions, monolayers with (-)-carvone undergo a larger entropy change than monolayers with (+)-carvone. Variations in molecular areas and thermodynamic properties may contribute to membrane recognition of optical isomers.