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 <SCP>l</SCP>-α-1,2-dipalmitoyl-sn-glycerol-3-phosphocholine (<SCP>l</SCP>-DPPC) monolayers are studied using surface pressure versus area isotherms measured in the temperature range 10−35 °C in the presence and absence of 5 mM odorants in the subphase. The data indicate that the molecular area occupied by the <SCP>l</SCP>-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. <SCP>l</SCP>-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.