Lyso-phosphatidylcholines in Langmuir monolayers : influence of chain length on physicochemical characteristics of single-chained lipids

Single-chained phospholipids constitute a class of membrane components found in normal cells in relatively low concentration; however, these group of compounds are known owing to their broad phys- iological activities. Despite that the knowledge concerning fundamental physicochemical properties of lyso-lipids is very limited and in contrast to double-chained phospholipids there is an obvious deficiency of studies focused on correlation between their amphipathic character and film-forming properties with biological activities. In the present paper we have attempted to explain the main issues regarding the characteristics of lyso-PCs in monolayers at the air/aqueous interface. Our results show that all the investigated phospholipids differing in the length of hydrophobic chain: C 18 lyso-PC, C 22 lyso-PC and C 24 lyso-PC form stable Langmuir monolayers of a relatively low degree of con- densation. It was found that during compression the investigated monolayers significantly change their organization at the interface which is strongly connected with temperature of the subphase. The appli- cation of X-ray reflectivity confirmed that the bulky choline head-groups in molecules of lyso-PCs are strongly penetrated by water molecules, while the hydrophobic chains are significantly tilted from the surface normal. The obtained results show that the phase transitions observed in the course of the reg- istered isotherms result from decrease in immersion of molecules in the subphase as well as from the decrease in hydrating water molecules. On the basis of GIXD experiments it turned out that in the mon- olayers of C 22 lyso-PC and C 24 lyso-PC at higher surface pressures (>20 mN/m) small fractions of periodi- cally ordered phase appear at the interface. For the monolayer of C 24 lyso-PC in the periodically ordered fraction the untilted (U) to tilted (t) phase transition was found.