A DSC and FTIR spectroscopic study of the effects of the epimeric cholestan-3-ols and cholestan-3-one on the thermotropic phase behavior and organization of dipalmitoylphosphatidylcholine bilayer membranes: Comparison with their 5-cholesten analogs

Abstract

We present the results of a comparative differential calorimetric and Fourier transform infrared spectroscopic study of the effect of cholesterol and five analogs on the thermotropic phase behavior and organization of dipalmitoylphosphatidylcholine bilayer membranes. These sterols/steroids differ in both the nature and stereochemistry of the polar head group at C3 (β-OH, α-OH or C=O) and in the presence or absence of a double bond in ring B. In both the Δ(5) and saturated 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 β-OH > α-OH > C=O. However, in the saturated series, these concentrations are much more similar, regardless of polar head group chemical structure. Similarly, the residual enthalpy of the DPPC main phase transition at 50 mol% sterol/steroid, inversely related to the miscibility of these compounds in fluid DPPC bilayers, also increases in the order β-OH > α-OH > C=O, but this effect is again attenuated in the saturated series. Moreover, replacement of the double bond at C5 with a saturated linkage also reduces sterol/steroid solubility in all cases. Interestingly, the C5 double bond has no effect on DPPC hydrocarbon chain ordering in the βOH sterol pair, considerably increases ordering in the αOH pair, and considerably reduces ordering in the C=O pair. Moreover, the ability of these compounds to order the DPPC hydrocarbon chains decreases in the order β-OH > α-OH > C=O in the Δ(5) series of compounds, but in the order β-OH > C=O > α-OH in the saturated series. Our results indicate that the effects of the presence or absence of a double bond at C5 of ring B on the thermotropic phase behavior and organization of DPPC bilayers are influenced by the nature and stereochemistry of the polar group present at C3 and vice versa. Nevertheless, the characteristic effects of sterols/steroids on fluid lipid bilayers are optimal when an OH group rather than C=O group is present at C3, and when this OH group is in the equatorial (β) orientation. Moreover, the presence of a single double bond specifically at C5 is required to maximize sterol solubility in fluid DPPC bilayers, which is probably its primary function in natural sterols.

Keywords: Cholestanol; Cholesterol; Dipalmitoylphosphatidylcholine; Lipid bilayer membranes; Sterol/phospholipid interactions; Thermotropic phase behavior.