Identifying Macromolecules The authors review a surface enhanced laser Raman spectroscopy study of lung surfactant protein interaction with bacterial lipopolysaccharide

The molecular architecture of the surfactant lipoprotein system is important in understanding its interaction with the microbial world. It consists of 90% lipids — mainly phospholipids — and 5–10% specific lipoproteins consisting of four major apoproteins (SP-A, SP-B, SP-C, and SP-D). The importance of SP-A and SP-D for innate immunity of the lung is recognized now (1,2). The surfactant complex is secreted into the alveolar lumen (hypophase) in the form of multilamellar structures and transformed into tubular myelin lattices (TMLs), which form the precursor of the lipid monolayer to form a surface film for reducing the surface tension at the air–liquid interface during the expansion of the alveoli, and thereby limit their collapse during inspiration and expiration (3,4). Based upon the above structure, lung lipoproteins play a dual role in reducing the surface tension and, most importantly, provide host defense at the air–liquid interface (2,5). The focus of our studies is to use the biophysical techniques of Raman spectroscopy to examine in detail the mode of interaction of these lipoproteins with defined phospholipids to form the TML and its role in innate immunity, especially the interaction of SP-A and SP-D with the bacterial and viral membranes. The biological application of biophysical techniques such as Raman spectroscopy has been well documented (6). Raman spectroscopy is well suited for the study of molecular interactions because the vibrational spectra provide a very sensitive and noninvasive means of probing the intermolecular and intramolecular interactions in surfactant lipoprotein matrix and the microbial pathogen membranes. In this study we report data on the molecular spectra of SP-D, lipopolysaccharide (LPS), and Lipid A and molecular interactions of SP-D with bacterial LPS using the surface enhanced Raman spectroscopy technique (SERS). The results indicate specific vibrational shifts in the Raman spectra that underlie changes in the conformational structure of the protein–LPS complexes.