Physical state of the bilayers

The difference spectra of the sample without cholesterol show that the interchain coupling (reflected in the two band minima) is removed between 40 and 43 C coinciding with Tc. Above T0, X-ray diffraction studies (33) have demonstrated the lack of interdigitation in DHPC bilayers. It is also at this temperature, that the parameter I f increases demonstrating a direct relationship between the physical state of the bilayer and the properties of the guest ketone. A similar observation is made for the sample containing 8 mol % cholesterol. 

Interestingly the shape of the curve follows the membrane transition, which is a direct indication of the physical state of the bilayer. If many such experiments are carried out using cells with different transition temperatures, the results are always the same the ratio parallels the membrane transition. In the extreme case, when the transition is at -20 C for cells grown in an oleate-enriched medium, the ratio above 0 C is a straight line with a gentle positive slope but no inflections. The total fatty acid analysis of laidlawii grown in oleate similarly shows almost no temperature dependence. To verify that the response is not necessarily enzyme dependent, the... 

Even this is not the entire story, however. The properties of the bilayer, and hence the diffusion rates, also depend on the composition and physical state of the bilayer. For example, in the studies of benzene diffusion in a dimyristolylphosphatidyl-choline (DMPC) bilayer mentioned above, temperature was varied in a controlled manner. As the temperature varied, so did the distribution of free volume. At low temperatures (still above the gel to liquid-crystal phase transition) the free volume was largely localized in large voids in the center of the... 

In an attempt to compare the physical state of the bilayers with the extent of hydrogen bonding to the keto group of 9HP, as a function of cholesterol concentration, we studied the temperature dependence of the CH2 symmetric stretching vibrational modes, (CH2). (24)... 

As might be expected, many membrane activities reflect the physical state of the membrane bilayer. An example of this is seen in Figure 4. Here the kinetics of protein mediated sugar transport through a lipid bilayer is seen to follow the melting of the bilayer. For these studies the human red blood cell hexose transport protein was removed from its native membrane and reconstituted into artificial membranes with a predetermined thermal profile. Sugar transport was monitored by a turbidometric method to obtain the parameters (maximum rate... 

Solid-state NMR studies of biomembranes have been carried out with deuterium or phosphorus-31. H-NMR spectroscopy has been of great help in the study of the physical state of the hydrocarbon chain region of phospholipid bilayers (Smith, 1979 Chan et al, 1981). This requires the synthesis of specifically deut-erated phospholipids. These lipids are then dispersed... 

Figure 1. Various physical states of phospholipids in aqueous solution. Note the following features (a) phospholipids residing at the air/water interface are arranged such that their polar head groups maximize contact with the aqueous environment, whereas apolar side chains extend outward toward the air (b) solitary phospholipid molecules remain in equilibrium with various monolayer and bilayer structures (c) bilayer vesicles and micelles remain in equilibrium with solitary phospholipid molecules, provided that the total lipid content exceeds the critical micelle concentration.

A problem that is rarely taken into account is related to the fact that the water concentration shows a conspicuous gradient within the stratum corneum thickness. These factors are likely to influence the physical state of lipids in bilayer formations, and therefore we expect lipid barrier structure to vary within the thickness of the stratum corneum.53... 

The earliest general model of adaptation to temperature in membrane lipids focused on the physical state ( static order or viscosity [= 1 / fluidity ]) of the bilayer. The finding that the physical state of membrane lipids from Escherichia coli cultured at different temperatures was similar at the different growth temperatures led to the homeoviscous adaptation hypothesis, which states that lipid composition is modified during thermal acclimation to facilitate retention of a relatively stable membrane physical state (Sinensky, 1974). At the outset of any discussion of homeoviscous adaptation, it is important to examine carefully what is meant by physical state (or the related terms static order, viscosity, and fluidity ). In such an analysis, one must also consider the physical methods that are used to make such measurements—and the limitations of these techniques. 

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