Extended cholesterol molecule

This effect of cholesterol on the location of guest molecules has been cited for chlorophyll a (36) and tetracaine (5) both molecules with specific biological function. In the present work we have been able to observe this effect over the physiological ranges of temperature and cholesterol concentration. Future experiments with other molecules would clarify if this property of cholesterol is generally applicable to other systems and, furthermore, if it extends to interactions between two molecules solubilized in a membrane. 

Cholesterol substitutes and evolution of biosynthetic pathways leading to cholesterol. In their review of cholesterol effects on membranes, Robertson and Hazel (1997) outline a broad evolutionary picture in which the origins and the functional roles of cholesterol substitutes and bilayer-spanning membrane stabilizers in bacteria, Archaea, and plants are presented. Figure 7.28 portrays the types of molecules that serve as membrane stabilizers in these different taxa rigid hemilayer inserts, which have a cholesterol-like chemistry and membrane localization rigid bilayer inserts, which extend... 

This chapter examines the biosynthesis of three important components of biological membranes—phospholipids, sphingolipids, and cholesterol (Chapter 12). Triacylglycerols also are considered here because the pathway for their synthesis overlaps that of phospholipids. Cholesterol is of interest both as a membrane component and as a precursor of many signal molecules, including the steroid hormones progesterone, testosterone, estrogen, and cortisol. The biosynthesis of cholesterol exemplifies a fundamental mechanism for the assembly of extended carbon skeletons from five-carbon units. 

The nanosized detection area Ar or volume created by STED also extends the power of fluorescence correlation spectroscopy (FCS) and the detection of molecular diffusion [74,95]. For example, STED microscopy has probed the diffusion and interaction of single lipid molecules on the nanoscale in the membrane of a living cell (Fig. 19.6). The up to 70 times smaller detection areas created by STED (as compared to confocal microscopy) revealed marked differences between the diffusion of sphingo- and phospholipids [74]. While phospholipids exhibited a comparatively free diffusion, sphingolipids showed a transient ( 10 ms) cholesterol-mediated trapping taking place in a < 20-nm diameter area, which disappeared after cholesterol depletion. Hence, in an unperturbed cell putative cholesterol-mediated lipid membrane rafts should be similarly short-lived and smaller. 

Pseudo-rotaxane complex formation was used to reversibly trigger the conformational preference of the bis-cholesterol derivative 58 (Scheme 22). Compound 58 was observed to gel cycloalkane and diphenyl ether at 19 mM but failed to gelate aromatic solvents under the same conditions [82]. However, addition of a diammonium guest into a benzene sol of 58 induced gelation to occur. This effect was attributed to a change in host conformation. Indeed pseudo-rotaxane formation forced the molecule to adopt the extended geometry, which resulted in a more favorable situation for one-dimensional aggregation and, in turn, gel formation [82],... 

Although we have limited our discussion to liquid crystals made from small molecules, the field of investigation of systems producing this type of state is very wide. It extends from the lyotropic systems described by G. Porte in Chap. 5 to mineral colloids, liquid crystal polymers, and biologically interesting systems such as DNA, myelin sheaths or cholesterol deposits in the arteries. It is not easy to predict which features will lead to new discoveries or applications, and it is clear that many mysteries remain unsolved. 

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