Multilayers of Lung Surfactant at the Air/Water Interface Observed by Neutron Reflectometry under Compression-Expansion Cycles

Abstract

The alveolar interface is covered by film composed of lipids and proteins, generally referred to as “lung surfactant”. It has several important functional roles, including stabilizing the interface, allowing for area expansions and controlling diffusional transport. Prematurely born babies can develop neonatal respiratory distress syndrome (NRDS) due to deficiencies of lung surfactant. This syndrome, if left untreated, is lethal. We aim to further understand the structure and formation of the interfacial film.The majority of biophysical studies of model systems for the alveolar interface focus on single monolayers. However, several studies of lung surfactant extracts have demonstrated formation of multilayer interfacial structures [1-3] suggesting that this might also be the situation in the endogenous system.In this study we use Curosurf, an extract used in clinics to treat infants suffering from NRDS. We have characterized a similar extract in the bulk phase using ssNMR and SAXS/WAXS showing co-existing lamellar phases (liquid disordered/ordered and gel phase). Furthermore we have been able to resolve order parameters for the acyl chains of the lipids in the biological extract of lung surfactant.We use neutron reflectometry to study the lung surfactant at the air-water interface under controlled high humidity and under repeated compression-expansion cycles in order to mimic the work of breathing. We demonstrate multilayer formation at the interface and how the film responds to compression-expansion cycles. Moreover, we show how the structure of the film evolves under the cycles, the effect of ion composition and how the film thickness reaches a steady state.1. Schurch et al. 1995, Biol. Neonate, 67, 61-76.2. Perez-Gil et al. 2008, Biochim. Biophys. Acta-Biomembr. 1778, 1676-1695.3. Follows et al. 2007, Biochim. Biophys. Acta 1768, 228-235.