A calorimetric and spectroscopic comparison of the effects of cholesterol and its immediate biosynthetic precursors 7-dehydrocholesterol and desmosterol on the thermotropic phase behavior and organization of dipalmitoylphosphatidylcholine bilayer membranes.

We performed differential scanning calorimetric (DSC) and Fourier transform infrared (FTIR) spectroscopic studies of the effects of cholesterol (CHOL), 7-dehydrocholeterol (7DHC) and desmosterol (DES) on the thermotropic phase behavior and organization of dipalmitoylphosphatidylcholine (DPPC) bilayer membranes. 7DHC and DES are the immediate biosynthetic precursors of CHOL in the Kandutch-Russell and Bloch pathways and 7DHC and DES differ in structure from CHOL only by the presence of an additional double bond at C7 of ring B or C24 of the alkyl side chain, respectively. Our DSC results indicate that the incorporation of all three sterols produces comparable decreases in the temperature of the pretransition of DPPC, but CHOL decreases its cooperativity and enthalpy more strongly than 7DHC and especially DES. These findings indicate that all three sterols decrease the thermal stability of gel phase DPPC bilayers but that 7DHC and especially DES are less miscible in them. However, the incorporation of CHOL and DES produce comparable increases in the temperature of the broad component of the main phase transition of DPPC while 7DHC decreases it, but again CHOL produces greater decreases in its cooperativity and enthalpy then 7DHC and especially DES. These results indicate that CHOL and DES stabilize the sterol-rich domains of fluid DPPC bilayers, but that 7DHC and especially DES are less miscible in them. Our FTIR spectroscopic results indicate that CHOL increases the rotational conformational order of fluid DPPC bilayers to a somewhat and markedly greater degree than DES and 7DHC, respectively, consistent with our DSC findings. Our spectroscopic results also indicate that although all three sterols produce comparable degrees of H-bonding (hydration) of the DPPC ester carbonyl groups in fluid bilayers, CHOL is again found to be fully soluble in gel state DPPC bilayers at low temperatures, whereas 7DHC and especially DES are not. In general, we find that 7DHC and DES incorporation produce considerably different effects on DPPC bilayer membranes. In particular, the presence of an additional double bond at C7 or C24 produces a marked reduction in the ability of 7DHC to order fluid DPPC bilayers and in the miscibility of DES in such bilayers, respectively. These different effects may be the biophysical basis for the reduction of these double bonds in the last steps of CHOL biosynthesis, and for the deleterious biological effects of the accumulation of these sterols in vivo.