Direct observation of phase and morphology changes induced by lung surfactant protein SP-B in lipid monolayers via fluorescence, polarized fluorescence, Brewster angle and atomic force microscopies

Both human lung surfactant protein SP-B and its amino terminus (SP-B_1-25) alter the phase behavior of palmitic acid (PA) monolayers by inhibiting the formation of condensed phases and creating a new fluid PA-protein phase. This fluid phase increases the compressibility of the monolayers by forming a network that separates condensed phase domains at coexistence and persists to high surface pressures. The network changes the monolayer collapse nucleation from a heterogeneous to a more homogeneous process through isolating individual condensed phase domains. This results in higher surface pressures at collapse, and monolayers easier to respread on expansion, factors essential to the in vivo function of lung surfactant. The network is stabilized by low line tension between the coexisting phases as confirmed by the formation of extended linear domains or "stripe" phases. Similar stripes are found in monolayers of fluorescein-labeled SP-B_1-25, suggesting that the reduction in line tension is due to the protein. Comparison of isotherm data and observed morphologies of monolayers containing SP-B_1-25 with those containing the full length SP-B protein shows that the peptide retains most of the native activity of the protein, which may lead to cheaper and more effective synthetic replacement formulations.