Free-Standing Black Films:  An Alternative to Langmuir Monolayers for the Study by Raman Spectroscopy of Peptide−Phospholipid Interaction in Ultrathin Films

This paper reports on a study by Raman spectroscopy of free-standing black films of dimyristoylphosphatidylcholine (DMPC) in interaction with melittin, a hemolytic peptide. In a first step, the Raman spectra of these films are compared with those of Langmuir monolayers, using the same acquisition conditions. Black films give much better spectra than Langmuir monolayers and, hence, are used to follow the effects induced by melittin at different melittin/DMPC molar ratios ranging from 1/1000 to 1/55. Two parameters deduced from the intensity of some specific bands in the Raman spectra show that melittin induces an increase of the lateral and conformational order in DMPC alkyl chains. Moreover, increasing amounts of melittin induces an increase of the water core thickness of black films from 22 to 400 Å. This swelling is explained in terms of the DLVO theory. For the thinnest films, the shape of the multicomponent water band changes which reveals a confinement effect in the aqueous core. Finally, discussion of these results is oriented toward the suitability of the black film configuration for Raman studies of peptide−phospholipid interaction in ultrathin films.