Monolayer-multilayer transitions in a lung surfactant model: IR reflection-absorption spectroscopy and atomic force microscopy.
A hydrophobic pulmonary surfactant protein, SP-C, has been implicated in surface-associated activities thought to facilitate the work of breathing. Model surfactant films composed of lipids and SP-C display a reversible transition from a monolayer to surface-associated multilayers upon compression and expansion at the air/water (A/W) interface. The molecular-level mechanics of this process are not yet fully understood. The current work uses atomic force microscopy on Langmuir-Blodgett films to verify the formation of multilayers in a dipalmitoylphosphatidylcholine, dipalmitoylphosphatidylglycerol, cholesterol, and SP-C model system. Isotherms of SP-C-containing films are consistent with exclusion and essentially complete respreading during compression and expansion, respectively. Multilayer formation was not detected in the absence of SP-C. Most notable are the results from IR reflection-absorption spectroscopy (IRRAS) conducted at the A/W interface, where the position and intensity of the Amide I band of SP-C reveal that the predominantly helical structure changes its orientation in monolayers versus multilayers. IRRAS measurements indicate that the helix tilt angle changed from approximately 80 degrees in monolayers to a transmembrane orientation in multilayers. The results constitute the first quantitative measure of helix orientation in mixed monolayer/multilamellar domains at the A/W interface and provide insight into the molecular mechanism for SP-C-facilitated respreading of surfactant.
Wang
Lin
L
Department of Chemistry, Newark College of Arts and Sciences, Rutgers University, 73 Warren Street, Newark, NJ 07102, USA. flach@andromeda.rutgers.edu
Cai
Peng
P
Galla
Hans-Joachim
HJ
He
Huixin
H
Flach
Carol R
CR
Mendelsohn
Richard
R
D008051:Lipid Bilayers / Q000737:chemistry*; D008560:Membrane Fluidity; D018625:Microscopy, Atomic Force / Q000379:methods*; D008954:Models, Biological; D008956:Models, Chemical; D008968:Molecular Conformation; D010743:Phospholipids / Q000737:chemistry*; D011487:Protein Conformation; D037721:Pulmonary Surfactant-Associated Protein C / Q000737:chemistry*; D011663:Pulmonary Surfactants / Q000032:analysis / Q000737:chemistry*; D013499:Surface Properties
D008051:Lipid Bilayers; D010743:Phospholipids; D037721:Pulmonary Surfactant-Associated Protein C; D011663:Pulmonary Surfactants