Interfacial behaviour and mechanical properties of spread lung surfactant protein/lipid layers

The surface behaviour of spread dipalmitoyl phosphatidyl choline (DPPC), lung surfactant protein C (SP-C), and their mixtures were characterised using a captive bubble surfactometer. The surface tension was determined by using axisymmetric bubble shape analysis. Surface dilatational rheological behaviour was characterised by sinusoidal oscillation of the bubble volume and at frequencies 0.006-0.025 Hz. The pi/A isotherms of DPPC, SP-C, and their mixtures were described with a generalised equation of state. Monolayer cycling of mixed DPPC/SP-C layers yields isotherms with a plateau in the range of 50-53 mN/m. When the surface pressure becomes higher SP-C is squeezed out of the film, but it re-enters the film upon expansion. Surface dilatational elasticities of DPPC films had a maximum at about 30 mN/m. At higher surface pressures, the films became brittle and the elasticity decreased. A slightly pronounced maximum was found at a surface pressure exceeding 55 mN/m. The dilatational viscosity had two distinct maxima, corresponding with those in the elasticity curves, i.e. one before the minimum area demand, and one in the range of over-compression. This was explained by the formation of a second ordered complex structure in the range of film over-compression. SP-C films show continuously increasing dilatational elasticities and viscosities with a maximum at f approximately 0.02 Hz. Mixed monolayers, DPPC+2 mol% SP-C, had dilatational elasticities increasing with surface pressure. In contrast to DPPC alone, an elasticity maximum appeared in the range of the squeeze out plateau. The dilatational viscosity had two distinct maxima as observed for DPPC, whereas the maximum before the squeeze out plateau is very broad like that of SP-C. The viscosity decreased for frequencies higher 0.02 Hz favouring elastic properties of the film. Our data provide experimental evidence that SP-C mixed with DPPC yield higher elasticities and viscosities as compared with films formed by the single components. This behaviour is likely to support breathing cycles, especially for the turn from inspiration to expiration and vice versa.