Conformational transitions gauche-trans, spectroscopy

Modern methods of vibrational analysis have shown themselves to be unexpectedly powerful tools to study two-dimensional monomolecular films at gas/liquid interfaces. In particular, current work with external reflection-absorbance infrared spectroscopy has been able to derive detailed conformational and orientational information concerning the nature of the monolayer film. The LE-LC first order phase transition as seen by IR involves a conformational gauche-trans isomerization of the hydrocarbon chains a second transition in the acyl chains is seen at low molecular areas that may be related to a solid-solid type hydrocarbon phase change. Orientations and tilt angles of the hydrocarbon chains are able to be calculated from the polarized external reflectance spectra. These calculations find that the lipid acyl chains are relatively unoriented (or possibly randomly oriented) at low-to-intermediate surface pressures, while the orientation at high surface pressures is similar to that of the solid (gel phase) bulk lipid. 

The present analysis by C-D stretching infrared spectroscopy has clarified several conformational properties of the alkyl chain in the Ci2E3-water system. The conformational change at the phase transition from L2 to L is not significant and only a small increase in the trans fraction is observed for the C-C bonds close to the alkyl/oxyethylene interface. This implies that the conformational states of the alkyl chain in the L2 and L phases in the vicinity of their boundary are substantially not different. In the L phase, when the composition or the temperature approaches the region of the phase separation or transition, the trans fractions for the C-C bonds closer to the alkyl/oxyethylene interface and those closer to the chain terminal decrease. These observations indicate that the conformational transformation from trans to gauche at these chain positions makes the lamellar structure less stable and leads eventually to the structural destruction. 

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