| Lung surfactant is a lipid rich fluid that coats the inner surface of the alveoli, the primary units of respiration where oxygen diffuses into the bloodstream. Due to the small radii and high curvature of the alveoli, significant pressure is required to overcome surface tension and to inflate the lung. Lung surfactant is a biological coating that lowers surface tension and minimizes the work required for breathing. It is primarily composed of two phospholipids, the zwitterionic 1,2-dipalmitoyl-sn-Glycero-3-Phosphocholine (DPPC) and the anionic 1-Palmitoyl-2-Oleoyl-sn-Glycero-3-[Phospho- rac-(1-glycerol)] (POPG). The 79-amino acid surfactant protein B (SP-B), however, is an absolutely essential component of lung surfactant. Although present at low levels (approximately 1% by weight), mutation or loss of SP-B results in respiratory distress syndrome (RDS). The design of simple peptides to mimic the charge distribution and the hydrophobic characteristics of SP-B was investigated in the early 1990s. A peptide composed of only the basic residue lysine and hydrophobic leucine systematically repeated (KLLLLKLLLLKLLLLKLLLLK) has demonstrated clinical efficacy in the treatment of RDS when administered with DPPC, POPG and palmitic acid.;In this dissertation, solid-state NMR spectroscopy (ssNMR) was used to examine in detail the phase properties of these lipids and the effects of KL4 on lipid organization and dynamics. Structural measurements were also performed to determine the secondary structure of KL4. These studies were carried out using two model lipid systems: (1) 3:1 POPC: POPG, a bilayer system used in many studies probing peptide:lipid interactions, including amphipathic antibiotics of similar size and hydrophobicity to KL 4, and (2) 4:1 DPPC:POPG, which is similar to the lipid composition in the lung. Complementary studies using differential scanning calorimetry (DSC) and circular dichroism (CD) were also carried out. Comparative experiments were also performed on residues 59-80 of SP-B (SP-B59-80), the C-terminal region upon which the sequence of KL4 was derived. Based on these studies, a molecular model incorporating the structure of KL 4 and its interactions with 3:1 POPC:POPG bilayers and 4:1 DPPC:POPG bilayers was developed. This model can serve as a guide for understanding how proteins modulate surface tension and for developing more effective peptide mimetics for clinical use. |
