Thermodynamic quantification of sodium dodecyl sulfate penetration in cholesterol and phospholipid monolayers

Sodium dodecyl sulfate (SDS) is one of the extensively used surfactants in bioprocesses. Present work analyzes the penetration of SDS in pure and mixed Langmuir monolayers consisting of 1,2-Dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and cholesterol. A series of isotherms are obtained by compressing the monolayers over subphase containing varying concentrations of SDS. The extent of penetration of SDS is quantified in terms of its mole percentage in the monolayer by applying various thermodynamic equations derived from Gibbs adsorption equation. Analysis of pure monolayers shows that SDS penetration is directly correlated with the monolayer fluidity and the lipid packing density. We find that POPC monolayers, which do not pack closely on compression due to a kink in one of the acyl chains, exhibit higher SDS penetration than DPPC monolayers which are more rigid due to two saturated acyl chains that pack very efficiently. The SDS penetration is also analyzed in mixed lipid monolayers in which the lipids are present in molar proportions similar to that in the plasma membranes of mammalian cells. Thermodynamic quantification shows that mixed monolayers of DPPC with POPC (in 2:1 molar ratio) exhibit penetration similar to pure POPC monolayers indicating dominating effect of unsaturated phospholipids on membrane packing. We find lower SDS penetration in mixed cholesterol-POPC monolayers than pure POPC monolayers at all surface pressures and for all SDS subphase concentrations used. This confirms with the well-known condensation effect of cholesterol on POPC monolayers. Cholesterol is known to have a rigidifying effect on DPPC monolayers at low surface pressures and a fluidizing effect at high surface pressures. Consistent with these experimental observations, our thermodynamic quantification predicts lower SDS penetration at low pressures and higher penetration at high pressures when compared to pure DPPC monolayers. Thus, thermodynamic theories are able to accurately predict the experimentally observed trends for penetration in mixed monolayers of cholesterol with POPC and DPPC. Our work demonstrates that thermodynamic quantification is a reliable technique to estimate extent of penetration of various additives in pure and mixed lipid monolayers.