Pulmonary surfactant self-assembles into a functional film of defined molecular architecture irrespective of concentration and solvent of the spreading solution: A fluorescence and atomic force microscopy study

Pulmonary surfactant forms a mixed protein-lipid film at the air-water interface in the lung epithelia that reduces the surface tension of the interface to a very small value. Lack or inhibition of the lung surfactant leads to severe respiratory dysfunction. The mechanisms of surfactant inhibition may be studied in vitro, but relevance to the lung depends on the experimental protocol. To develop effective surfactant formulations for treatment of related diseases it is important to investigate the molecular mechanism of surfactant function, and understand what factors impact the effectiveness of surfactant performance. It has been shown that the ability of surfactant to reduce surface tension is concentration dependent. On the other hand, we have previously shown that the function of pulmonary surfactant is related to the film specific molecular arrangement. Here, we investigated how concentration and solvent of the spreading solution affect the molecular arrangement of surfactant at the air-water interface of a Langmuir film balance. Clinically used bovine lipid extract surfactant (BLES) was spread from either a high concentration bolus (27 mg/ml) such as used in clinical treatment or at lower concentration (5 mg/ml). Surfactant was either spread from an aqueous or a chloroform suspension; all treatments resulted in functional films. The film structure was similar for all conditions and consisted of a lipid monolayer with patches of lipid bilayer stacks attached to it. We have previously identified such molecular arrangement to be a hallmark of functional surfactant. In this paper, we discuss why film formation was unaffected by the concentration of the spreading solution and the solvent whereas this parameter effected film function in other in vivo- and in vitro studies.