Location of Structural Transitions in an Isotopically Labeled Lung Surfactant SP-B Peptide by IRRAS

Pulmonary surfactant, a lipid/protein complex that lines the air/water interface in the mammalian lung, functions to reduce the work of breathing. Surfactant protein B (SP-B) is a small, hydrophobic protein that is an essential component of this mixture. Structure-function relationships of SP-B are currently under investigation as the protein and its peptide analogs are being incorporated into surfactant replacement therapies. Knowledge of the structure of SP-B and its related peptides in bulk and monolayer phases will facilitate the design of later generation therapeutic agents. Prior infrared reflection-absorption spectroscopic studies reported notable, reversible surface pressure-induced antiparallel β-sheet formation in a synthetic peptide derived from human SP-B, residues 9–36 (SP-B 9–36). In the current work, infrared reflection-absorption spectroscopy is applied in conjunction with isotopic labeling to detect the site and pressure dependence of the conformational change. SP-B 9–36, synthesized with 13C=O-labeled Ala residues in positions 26, 28, 30, and 32, shifted the β-sheet marker band to ∼1600 cm −1 and thus immediately identified this structural element within the labeled region. Surface pressure-induced alterations in the relative intensities of Amide I band constituents are interpreted using a semiempirical transition dipole coupling model. In addition, electron micrographs reveal the formation of tubular myelin structures from in vitro preparations using SP-B 9–36 in place of porcine SP-B indicating that the peptide has the potential to mimic this property of the native protein.