科研之友
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摘要
We present the results of a comparative differential calorimetric and Fourier transform infrared spectroscopic study of the effect of cholesterol and five of its analogues on the thermotropic phase behaviour and organization of dipalmitoylphosphatidylcholine bilayer membranes. These sterols/steroids differ in both the nature and stereochemistry of the polar head group at C3 (beta OH, alpha OH or C=O) and in the position of the double bond (C4-C5 in ring A or C5-C6 in ring B). In the three Delta(5) sterols/steroid series, the concentration of these compounds required to abolish the DPPC pretransition, inversely related to their relative ability to disorder gel state DPPC bilayers, decreases in the order beta OH> OH > C=0 and these differences in concentration are significant. However, in the Delta(4) series, these concentrations are more similar, regardless of polar head group nature or stereochemistry. Similarly, the residual enthalpy of the main phase transition of DPPC at 50 mol.% sterol/steroid, which is inversely related to the miscibility of these compounds in the DPPC bilayer, also increases in the order beta OH> etOH > C=O, but this effect is attenuated in the Delta(4) as opposed to the Delta(5) series. Both of these results indicate that the presence of a double bond at C4-C5 in ring A, as compared to a C5-C6 double bond in ring B, reduces the effect of variations in the structure of the polar group at C3 on the properties of the host DPPC bilayer. The movement of the double bond from C5 to C4 in the two sterol pairs results in a greater decrease in the temperature and enthalpy of both the pretransition and the main phase transition, whereas the opposite result is observed in the ketosteroid pair. Similarly, the ability of these compounds to order the DPPC hydrocarbon chains decreases in the order beta OH> alpha OH > C=O in both series of compounds, but in the two sterol pairs, hydrocarbon chain ordering is greater for the Delta(5) than the Delta(4) sterols, whereas the opposite is the case for the steroid pair. All of these results indicate that the typical effects of sterols/steroids in increasing the packing density and thermal stability of fluid lipid bilayers are optimal when an OH group rather than C=0 group is present at C3, and that this OH group is more effective in the equatorial rather than the axial orientation. We can explain all of our sterol results by noting that the shift of the double bond from Delta(5) to Delta(4) introduces of a bend in ring A, which in turn destroys the coplanarity of the steroid fused ring system and reduces the goodness of sterol packing in the host DPPC bilayer. However, this conformational change should also occur in the ketosteroid pair, yet our experimental results indicate that the presence of the Delta(4) double bond is less disruptive than a double bond at As. We suggest that the presence of keto-enol tautomerism in the conjugated Delta(4) ketosteroid, but not in the nonconjugated As compound, may provide additional H-bonding opportunities to adjacent DPPC molecules in the bilayer, which can overcome the unfavourable conformational change in ring A induced by the Delta(4) double bond.
- 出版日期2014-1
