Influence of a single ether bond on assembly, orientation, and miscibility of phosphocholine lipids at the air–water interface

Abstract

How does a small change in the structure of a phospholipid affect its supramolecular assembly? In aqueous suspensions, the substitution of one ester linkage in DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) by an ether linkage alters its phase behaviour completely. To unravel the effect of replacing a phospholipid's ester linkage by an ether linkage in lipid monolayers, we characterized pure monolayers of the model lipid DPPC and its sn-2 ether analogue PHPC (1-palmitoyl-2-O-hexadecyl-sn-glycero-3-phosphocholine) as well as mixtures of both by measurements of surface pressure–molecular area (π–A_mol) isotherms. In addition, we used infrared reflection absorption spectroscopy (IRRAS) to study lipid condensation, lipid chain orientation, headgroup hydration, and lipid miscibility in all samples. Mixed monolayers consisting of DPPC and PHPC were studied further using epifluorescence microscopy. Our results indicate a strong influence of the sn-2 ether linkage on headgroup hydration and ordering effects in the regions of the apolar chains and the headgroups. Both effects could originate from changes in glycerol conformation. Furthermore, we observed a second plateau in the π–A_mol isotherms of DPPC/PHPC mixtures and analysis of the mixed π–A_mol isotherms reveals a non-ideal mixing behaviour of both lipids which may be caused by conformational differences in their headgroups.