Contact angle measurements suggest that PLUNC possesses surface activity.

(A) Advancing contact angles were measured by the sessile drop technique for various solutions dispensed onto siliconized glass, a hydrophobic surface. Bars depict the mean values for contact angles (in degrees) measured one minute after drops were dispensed onto the solid surface. Error bars represent the standard error about the mean (n = 6). Contact angles (θ) of less than 90° (dotted line in panel A) indicate wetting of the surface by the drop, whereas contact angles greater than 90° (to the right of the vertical line) indicate that a sample is “non-wetting”. The solid vertical line separates the solutions that have greater wetting ability than buffer alone (bars pointing left) from the ones that have lesser wetting ability (bars pointing right). Asterisks denote measurements that are significantly different from buffer, as determined by Student's two-tailed t-test (P-value <0.01). On a hydrophobic surface, PLUNC solutions transitioned from “non-wetting” to “wetting” at concentrations greater than 10 µg/mL. Inset: spreading behavior over time was compared for Tris buffer and a PLUNC-containing solution dispensed onto a hydrophobic surface. On each coverslip, the drop on the left is buffer, while drops of PLUNC (140 µg/mL) are shown on the right. Drop spreading was photographed after 5 minutes, 25 minutes, and 45 minutes, revealing that the presence of PLUNC conferred an increased tendency for an aqueous solution to spread on a hydrophobic surface. In panel (B), drops of test solutions were formed on unmodified glass, a hydrophilic surface, and advancing contact angles were measured as described above. On a hydrophilic surface, PLUNC enhanced wetting when at lower concentrations (1–2 µg/mL), while higher PLUNC concentrations (5–150 µg/mL) appeared to reduce wetting ability. Infasurf, a commercial lung surfactant, displayed significant wetting ability on both hydrophilic and hydrophobic surfaces.