A conformation transition of lung surfactant lipids probably involved in respiration

X-ray scattering and freeze-fracture electron microscopy of a lung surfactant extract show the existence of a complex lamellar phase, L gamma, over a wide range of concentrations and temperatures. This lamellar phase, which consists of two bilayer motifs comprised of monolayers with stiff chains alternating with monolayers with disordered chains, allows us to propose a structural model of a collapse phase at the alveolar pulmonary interface. This model accounts for the increase in surface pressure during the compression as well as the easy respreading upon expansion of the interface during the respiratory cycle.