The effect of cholesterol on the interfacial elastic packing interactions of various molecular species of phosphatidylcholines (PCs) has been investigated by using a Langmuir-type film balance and analyzing the elastic area compressibility moduli (C(s)-1) as a function of average cross-sectional molecular area. Emphasis was on the high surface pressure regions (Π ≤ 30 mN/m) which are thought to mimic biomembrane conditions. Increasing levels of cholesterol generally caused the in-plane elasticity of the mixed monolayers to decrease. Yet, the magnitude of the cholesterol- induced changes was markedly dependent upon PC hydrocarbon structure. Among PC species with a saturated sn-1 chain but different an-2 chain cis unsaturation levels [e.g., myristate (14:0), oleate (18:1(Δ9(c)) linoleate (18:2(Δ9,12(c)), arachidonate (20:4(Δ5,8,11,14(c)), or docosahexenoate (22:6(Δ4,7,10,13,16,19(c))], the in-plane elasticity moduli of PC species with higher sn-2 unsaturation levels were less affected by high cholesterol tool fractions (e.g., >30 mol %) than were the more saturated PC species. The largest cholesterol-induced decreases in the in-plane elasticity were observed when both chains of PC were saturated (e.g., di-14:0 PC). When both acyl chains were identically unsaturated, the resulting PCs were 20-25% more elastic in the presence of cholesterol than when their sn-1 chains were long and saturated (e.g., palmitate). The mixing of cholesterol with PC was found to diminish the in-plane elasticity of the films beyond what was predicted from the additive behavior of the individual lipid components apportioned by mole and area fraction. Deviations from additivity were greatest for di-14:0 PC and were least for diarachidonoyl PC and didocosahexenoyl PC. In contrast to C(s)-1 analyses, sterol-induced area condensations were relatively unresponsive to subtle structural differences in the PCs at high surface pressures. C(s)-1 versus average area plots also indicated the presence of cholesterol concentration-dependent, low-pressure (<14 mN/m) phase boundaries that became more prominent as PC acyl chain unsaturation increased. Hence, area condensations measured at low surface pressures often do not accurately portray which lipid structural features are important in the lipid-sterol interactions that occur at high membrane-like surface pressures.
Bibliographical noteFunding Information:
The authors thank Dr. Burt Litman for helpful discussions during portions of this work. This work was supported by United States Public Health Service Grant GM45928 (to R.E.B.) and the Hormel Foundation. The automated Langmuir film balance used in this study receives major support from United States Public Health Service Grants HL49180 and HL17371 (to H.L.B.).