Secondary structure in lung surfactant SP-B peptides: IR and CD studies of bulk and monolayer phases

Pulmonary surfactant protein SP-B is known to facilitate adsorption and spreading of surfactant components across the air/water interface. This property appears essential for in vivo function in the alveolar subphase and at the air/alveolar surface. Three peptides with amino acid sequences based on SP-B containing predicted α-helical regions (SP-B_1-20, SP-B_9-36A, SP-B_40-60A) have been synthesized to probe structure-function relationships and protein-lipid interaction in bulk phase and monolayer environments. IR and CD studies are reported along with traditional surface pressure-molecular area (π-A) isotherms and IR reflection-absorption spectroscopy (IRRAS) investigations conducted at the air/water interface. In bulk phase, helix-promoting environments (methanol and aqueous dispersions of lipid vesicles), SP-B_1-20 and SP-B_9-36A contained significant amounts of α-helical structure, whereas varying degrees of α-helix, random coil, and β-sheet were observed in aqueous solutions and monolayers. The most striking behavior was observed for SP-B_9-36A, which displayed reversible surface pressure-induced β-sheet formation. Bulk phase lipid melting curves and monolayer experiments with peptide-lipid mixtures showed subtle differences in the degree of bulk phase interaction and substantial differences in peptide surface activity. The uniqueness of IRRAS is emphasized as the importance of evaluating secondary structure in both bulk phase and monolayer environments for lung surfactant peptide mimics is demonstrated.