Influence of acyl chain mobility on the properties of monolayers of some phospholipids

Abstract

Surface properties of various phospholipids and their simple mixtures have been studied to investigate the requirements of lipid phase in lung surfactant. A series of saturated and unsaturated lecithins have been examined using a modified Wilhelmy surface balance for their ability to reach low surface tension (γ). Both monolayers of unsaturated and saturated lecithins were capable of achieving near zero γ as long as they were compressed below their respective bulk phase transition temperatures (tc). When monolayers were formed at 37°C using mixtures of dipalmitoyl phosphatidylcholine (DPPC) and other lower melting lecithins the ability to reach low surface tension was dependent on: (1) the relative amounts of the two components (2) the tc of the lower melting lecithin and (3) the rate at which the monolayer was compressed. It has been suggested that compression of binary monolayers may result in a “squeeze-out” of the more fluid lecithin producing a DPPC-enriched monolayer capable of reaching low γ. -- Surface tension-area isotherms of monolayers of dipalmitoyl phosphatidyl ethanolamine (DPPE) and it methylated analogues, N-methyl dipalmitoyl phosphatidylehanolamine (N-me DPPE) and N,N-dimethyl dipalmitoyl phosphatidyl-ethanolamine (N,N-dime DPPE) have been examined. The minimum surface tensions achieved were γDPPC < γN,N-dimeDPPE < γN-MeDPPE < γDPPE. Monolayer mixtures of DPPE and DPPC showed that lower tensions were achieved with increasing proportions of DPPC. -- Results from monolayer studies suggested that the ability of lung surfactant to reach low γ may be dependent on lipid phase and hence its composition. Furthermore, infants born with Respiratory Distress Syndrome (RDS) may contain the “wrong mixture” of lipids in their surfactant so that low γ can not be achieved to stabilize the lung. The results presented here, using simple systems, may offer some insight in the consideration of potential mixtures to be used in artificial replacement surfactant.