Single planar bilayers of DMPC free-standing in air (black films) have been studied by confocal micro Raman spectroscopy in the 600-4000 cm -1 spectral range. The DMPC vibrational bands are well-defined. This allows the derivation of two parameters associated with the packing and mobility and with the fluidity of the alkyl chains and to quantify the rigidity enhancement accompanying the L R liquid crystalline to gel phase transition (T m ) 24 °C). The intensity of the water band allows to evaluate the thickness of the interstitial aqueous core which is 22 ( 4 Å in the L R phase. This water band exhibits a spectral change relative to bulk liquid water which is significant of the confinement of water molecules in the aqueous core.
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 peptidephospholipid interaction in ultrathin films.
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