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An Investigation of the Functionality of Synthetic Peptides in Pulmonary Surfactant Formulations from Molecular Dynamics Simulations

Parag S. Adhangale and Donald Gaver III. Dept. of Biomedical Engineering, Tulane University, Lindy Boggs Center Suite 500, New Orleans, LA 70118

Pulmonary surfactant, a mixture of ~90 wt % lipids and ~10 wt % proteins secreted by the type II epithelial cells, is essential for the normal functioning of the lungs during the respiration cycle. It helps lower the surface tension by forming a surface active film at the air/liquid interface thus reducing the effort required for breathing. It also prevents alveolar collapse after expiration. Low amounts of pulmonary surfactant in the alveolar space of neonates results in Respiratory Distress Syndrome (RDS). Deactivation of pulmonary surfactant is also known to exacerbate Acute Respiratory Distress Syndrome (ARDS). Endogenous pulmonary surfactant is a complex mixture of phospholipids and proteins. Dipalmitoylphosphatidylcholine (DPPC), a major component of the mixture (~40 wt %), can form a tightly packed gel phase at physiological temperatures and reduce the surface tension to near-zero values. The commonly held view with respect to surfactant function is that the surface film is able to withstand high surface pressures after expiration through the enrichment of the film with DPPC. Two possible mechanisms have been proposed for DPPC enrichment viz. selective DPPC adsorption from the underlying subphase that acts as a lipid reservoir to the monolayer and the selective removal or “squeezing out” of other lipid components from the monolayer to the subphase during compression. Both mechanisms are thought to occur. The hydrophobic surfactant protein SP-B has been known to play a critical role in lung function. This was evidenced by studies in which the removal of SP-B via genetic mutation in mice resulted in RDS-like symptoms in neonates. The surfactant formulations in use at present for surfactant replacement therapy (SRT) can be broadly classified into two types: one involving phospholipids but devoid of proteins and another including extracted and purified bovine surfactant proteins. The extraction and purification of bovine surfactant proteins involves considerable cost and a risk of bacterial contamination. A promising class of surfactant formulations being proposed for SRT regimens involves the use of synthetic peptide mimics of SP-B. We use fully atomistic molecular dynamics simulations to investigate the functionality of one such synthetic peptide viz. KL4 in DPPC monolayers at the air-water interface. We study the structure of the monolayer in the presence of the peptide at different levels of compression and at different initial peptide orientations. We use these results to compare the efficacy of KL4 to the peptide SP-B1-25 as a component of synthetic surfactant mixtures.