Relationships between Equilibrium Spreading Pressure and Phase Equilibria of Phospholipid Bilayers and Monolayers at the Air−Water Interface

Abstract

The intricate interplay between the bilayer and monolayer properties of phosphatidylcholine (PC), phosphatidylglycerol (PG), and phosphatidylethanolamine (PE) phospholipids, in relation to their polar headgroup properties, and the effects of chain permutations on those polar headgroup properties have been demonstrated for the first time with a set of time-independent bilayer-monolayer equilibria studies. Bilayer and monolayer phase behavior for PE is quite different than that observed for PC and PG. This difference is attributed to the characteristic biophysical PE polar headgroup property of favorable intermolecular hydrogen-bonding and electrostatic interactions in both the bilayer and monolayer states. This characteristic hydrogen-bonding ability of the PE polar headgroup is reflected in the condensed nature of PE monolayers and a decrease in equilibrium monolayer collapse pressure at temperatures below the monolayer critical temperature, T(c) (whether above or below the monolayer triple point temperature, T(t)). This interesting phenomena is compared to equilibrated PC and PG monolayers which collapse to form bilayers at 45 mN/m at temperatures both above and below monolayer T(c). Additionally, it has been demonstrated by measurements of the equilibrium spreading pressure, pie, that at temperatures above the bilayer main gel-to-liquid-crystalline phase-transition temperature, T(m), all liquid-crystalline phospholipid bilayers spread to form monolayers with pie around 45 mN/m, and spread liquid-expanded equilibrated monolayers collapse at 45 mN/m to form their respective thermodynamically stable liquid-crystalline bilayers. At temperatures below bilayer T(m), PC and PG gel bilayers exhibit a drop in bilayer pi(e) values < or =0.2 mN/m forming gaseous monolayers, whereas the value of pic of spread monolayers remains around 45 mN/m. This suggests that spread equilibrated PC and PG monolayers collapse to a metastable liquid-crystalline bilayer structure at temperatures below bilayer T(m) (where the thermodynamically stable bilayer liquid-crystalline phase does not exist) and with a surface pressure of 45 mN/m, a surface chemical property characteristically observed at temperatures above bilayer T(m) (monolayer T(c)). In contrast, PE gel bilayers, which exist at temperatures below bilayer T(m) but above bilayer T(s) (bilayer crystal-to-gel phase-transition temperature), exhibit gel bilayer spreading to form equilibrated monolayers with intermediate pie values in the range of 30-40 mN/m; however, bilayer pie and monolayer pic values remain equal in value to one another. Contrastingly, at temperatures below bilayer T(s), PE crystalline bilayers exhibit bilayer pie values < or =0.2 mN/m forming equilibrated gaseous monolayers, whereas spread monolayers collapse at a value of pic remaining around 30 mN/m, indicative of metastable gel bilayer formation.