INFRARED-SPECTROSCOPY OF AQUEOUS BIOPHYSICAL MONOLAYERS

We have used infrared external reflectance spectroscopy at the air/wat er interface to study: (1) polarized infrared (TR) monolayer spectrosc opy at different incoming angles of incidence, and (2) ternary mixed m odel systems (i.e. two lipids and a peptide) that mimic the compositio n of pulmonary lung surfactant. Using polarized IR radiation, we have observed a splitting of the symmetric and antisymmetric methylene stre tching 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 wit h ordered and disordered chain conformations. The splitting of these b ands qualitatively tracks the fractional composition of ordered/disord ered states upon monolayer compression. In a separate series of experi ments, we have obtained the IR external reflectance spectra of monomol ecular films of model mixtures relevant to pulmonary surfactant physio logy. 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 (s imilar to 2194 and 2089 cm(-1)) were analyzed, and band heights, integ rated intensities and peak frequency positions were plotted as a funct ion of measured surface pressure. These data indicate that the presenc e of surfactant protein appears the disorder the acyl chains of the DP PC-d(62) component in the film, opposite of what is seen for the proti ated component, DOPG. Data from these model mixtures indicate that the surfactant protein interacts differently with each of the lipid compo nents. (C) 1998 Elsevier Science S.A. All rights reserved.