Trans-gauche isomerization, vibrational

The vibrational spectroscopy time scale (10-300 x 10 sec) is appropriate for the direct sampling of the fastest motion expected to occur in phospholipid acyl chains, namely trans-gauche isomerization. To date, most Raman ai)d FT-IR studies of phospholipid phase behavior and lipid/protein interaction have focused mainly on qualitative measures of acyl chain organization. For example, the... 

Fig. 3.3 Trans/gauche isomerism (a) a trans bond viewed normally to the plane of the three bonds required to define it (b) a view of (a) from the right-hand side, looking almost end-on to the trans bond (c) and (d) nearly end-on views of left- and right-handed gauche bonds, respectively. (Reproduced from The Vibrational Spectroscopy of Polymers by D. I. Bower and W. F. Maddams. Cambridge University Press 1989.)...

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. 

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