The role of palmitic acid in pulmonary surfactant systems by Langmuir monolayer study: Lipid–peptide interactions

Abstract

Palmitic acid (PA) has been widely used as an useful and effective additive to pulmonary surfactants (PS) such as Survanta and Surfaxin. Moreover, PA has well-known biological and physicochemical properties, and its simple chemical structure allows its manufacture at low cost. However, the role and significance of PA in PS formulations are still controversial. In the present study, the PS model preparation containing PA has been investigated to clarify its additional effect (molecular recognition and interaction in particular) employing the Langmuir monolayer technique. The elucidation of the PA effect is performed by comparison with the data from its analogue (or homologue) lipids; i.e. three fatty acids and four fatty alcohols. The surface pressure (π)-molecular area (A) and surface potential (ΔV)−A isotherms for the eight DPPC/fatty lipid systems containing a PS model peptide (Hel 13-5) have been measured. In addition, the morphology and phase behavior are examined with in situ fluorescence and ex situatomic force microscopy. As a result, it is found that a balance between the hydrophilicity of the headgroup and the hydrophobicity of the tail group in the lipids is an important factor in pulmonary functions. In particular, the dissociation degree of the fatty acids, which depends on the pH or the surface pH, strongly affects their electrostatic interaction with positively charged Hel 13-5. The chain length is also significant and the carbon number 16 is found to be most appropriate. The present molecular-level information on the interaction between the lipids and Hel 13-5 in the presence of DPPC indicates that PA takes on a hydrophile–lipophile balance suitable for the PS preparation.