An infrared reflection-absorption spectroscopy study of the secondary structure in (KL4)4K, a therapeutic agent for respiratory distress syndrome, in aqueous monolayers with phospholipids.

KLLLLKLLLLKLLLLKLLLLK (KL(4)) has been suggested to mimic some aspects of the pulmonary surfactant protein SP-B and has been tested clinically as a therapeutic agent for respiratory distress syndrome in premature infants [Cochrane, C. G., and Revak, S. D. (1991) Science 254, 566-568]. It is of obvious interest to understand the mechanism of KL(4) function as a guide for design of improved therapeutic agents. Attenuated total reflection (ATR) IR measurements have indicated that KL(4) is predominantly alpha-helical with a transmembrane orientation in lipid multilayers (1), a geometry quite different from the originally proposed peripheral membrane lipid interaction. However, the lipid multilayer model required for ATR may not be the best experimental paradigm to mimic the in vivo function of KL(4). In the current experiments, IR reflection-absorption spectroscopy (IRRAS) was used to evaluate peptide secondary structure in monolayers at the air/water interface, the physical state that best approximates the alveolar lining. In contrast to the ATR-IR results, KL(4) (2.5-5 mol %) films with either DPPC or DPPC/DPPG (7/3 mol ratio) adopted an antiparallel beta-sheet structure at all surface pressures studied > or =5 mN/m, including pressures physiologically relevant for lung function (40-72 mN/m). In contrast, in DPPG/KL(4) films, the dominant conformation was the alpha-helix over the entire pressure range, a possible consequence of enhanced electrostatic interactions. IRRAS has thus provided unique molecular structure information and insight into KL(4)/lipid interaction in a physiologically relevant state. A structural model is proposed for the response of the peptide to surface pressure changes.