Lipids phase changes

Presumably stereospecific recognition of peptide hormone neurotransmitters would be primarily of importance in your conceptual framework for recognition. However, could it be possible to extend its significance for transduction in the context of efficacy, potency enhancement or loss, or even perhaps for antagonist activity I ask this because there has been much speculation that peptide receptor interaction might involve lipid phase changes which may in some cases be related to transduction or other related biological processes. 

The sharpness of the transition in pure lipid preparations shows that the phase change is a cooperative behavior. This is to say that the behavior of one or a few molecules affects the behavior of many other molecules in the vicinity. The sharpness of the transition then reflects the number of molecules that are acting in concert. Sharp transitions involve large numbers of molecules all melting together. 

An emergent field dealing with the physical/chemical processes that underlie the changes of lipid phase state during such cellular events as membrane fusion, vesicle trafficking, and cell disjunction. 

Because the area under the peaks in the scanning calorimeter is proportional to the heat of transition, the instrument can be calibrated by running a known amount of membrane lipid suspended in water. It is necessary to assume, of course, that all of the lipid is in the bilayer conformation in water. If the lipid content of the membranes is known, the fraction of the lipids contributing to the peak observed for the membranes can then be calculated by comparing peak areas for the membranes and the lipids in water. Our preliminary results using this approach indicate that at least 60% of the lipids in the membranes participate in the phase change. Work is in progress to obtain more precision. 

Interpretation of the Calorimetric Results. There is little doubt that the transition observed in M. laidlawii membranes arises from the lipids since it occurs at the same temperature in both intact membranes and in water dispersions of membrane lipids. It is reasonable to conclude that in both membranes and membrane lipids the lipid hydrocarbon chains have the same conformation. The lamellar bilayer is well established for phospholipids in water (I, 20, 29) at the concentration of lipids used in these experiments. In the phase change the hydrocarbon core of the bilayer undergoes melting from a crystalline to a liquid-like state. Such a transition, like the melting of bulk paraffins, involves association between hydrocarbon chains and would vanish or be greatly perturbed if the lipids were apolarly bound to protein. We can reasonably conclude that most of the lipids in M. laidlawii membranes are not apolarly bound to protein. 

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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