Configuration trans-gauche

The phase transition of bilayer lipids is related to the highly ordered arrangement of the lipids inside the vesicle. In the ordered gel state below a characteristic temperature, the lipid hydrocarbon chains are in an all-trans configuration. When the temperature is increased, an endothermic phase transition occurs, during which there is a trans-gauche rotational isomerization along the chains which results in a lateral expansion and decrease in thickness of the bilayer. This so-called gel to liquid-crystalline transition has been demonstrated in many different lipid systems and the relationship of the transition to molecular structure and environmental conditions has been studied extensively. 

At low temperatures, in lipid bilayers prepared from a single lipid species, the acyl/alkyl chains of the lipids in the bilayer are characterized by a high trans/gauche configurational ratio. 

Based on the values of hyperfine coupling constants due to fl-protons, the steric configuration of the atoms in the stable radical must be as illustrated in Fig. 5.5. It is well known that the conformation of the polyethyleneoxide chain in its normal crystalline phase is of trans-trans-gauche type i.e., a 7 -helical structure as illustrated by the left structure in Fig. 5.5. Since this conformation is in good agreement with that of the stable free radical represented by the right structure in Fig. 5.5., it can be assumed that the 7 -helical structure of the polyethyleneoxide chain still exists in irradiated UPEOC. The values of the line width AH j, and hyperfine coupling constants due... 

Subsequently, an x-ray crystallographic study showed that the molecular configuration was gauche with a perpendicular distance between the planes of the rings of 3.43 A. This gauche configuration is possibly stabilized with respect to the trans by weak van der Waals forces between atoms of the two six-membered rings. 

The decalin (bicyclo[4.4.0]decane) ring system provides another important system for study of conformational effects in cyclohexane rings. Equilibration of the cis and trans isomers reveals that the trans isomer is favored by about 2.8 kcal/mol. Note that this represents a change in configuration, not conformation. The energy difference can be analyzed by noting that the cis isomer has three more gauche butane interactions that are... 

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