Imaging of monolayers composed of palmitic acid and lung surfactant protein B

Near-field scanning optical microscopy and atomic force microscopy are used to probe the sub-micrometre phase structure in palmitic acid monolayers containing the 25 peptide amino terminus of lung surfactant protein B (SP-B_1?25). Monolayers deposited onto mica substrates at a surface pressure of 15 mN m^?1 exhibit a two-phase coexistence across a broad range of SP-B_1?25 concentrations. Monolayers containing 5 wt.% SP-B_1?25 or less exhibit an expanse of liquid condensed phase in which elliptical liquid expanded (LE) domains with areas of approximately 25 µm² coexist. By contrast, monolayers containing 20 wt.% SP-B_1?25 exhibit an expanse of liquid expanded phase in which circular liquid condensed domains coexist. The phase distribution dependence on SP-B_1?25 concentration suggests that the peptide induces disorder in the monolayer.