Infrared spectroscopy of aqueous biophysical monolayers

We have used infrared external reflectance spectroscopy at the air/water interface to study: (1) polarized infrared (TR) monolayer spectroscopy at different incoming angles of incidence, and (2) ternary mixed model systems (i.e. two lipids and a peptide) that mimic the composition of pulmonary lung surfactant. Using polarized IR radiation, we have observed a splitting of the symmetric and antisymmetric methylene stretching vibrations in the spectra of long-chain hydrocarbon amphiphiles that has been previously unreported for unpolarized monolayer spectra. The splitting of the C-H bands results in sub-bands identifiable with ordered and disordered chain conformations. The splitting of these bands qualitatively tracks the fractional composition of ordered/disordered states upon monolayer compression. In a separate series of experiments, we have obtained the IR external reflectance spectra of monomolecular films of model mixtures relevant to pulmonary surfactant physiology. Monolayers were composed of 2:1 DPPC-d(62):DOPG containing 0, 1 or 2 wt.% of the hydrophobic surfactant proteins SP-B and SP-C (SP-B + C). The CH2 antisymmetric and symmetric stretching bands (similar to 2920 and 2852 cm(-1)) along with the analogous CD2 stretching bands (similar to 2194 and 2089 cm(-1)) were analyzed, and band heights, integrated intensities and peak frequency positions were plotted as a function of measured surface pressure. These data indicate that the presence of surfactant protein appears the disorder the acyl chains of the DPPC-d(62) component in the film, opposite of what is seen for the protiated component, DOPG. Data from these model mixtures indicate that the surfactant protein interacts differently with each of the lipid components. (C) 1998 Elsevier Science S.A. All rights reserved.