Sterol-phospholipid interactions in model membranes Effect of polar group substitutions in the cholesterol side-chain at C 20 and C 22

Abstract

The interactions of phospholipids with four different cholesterol derivatives substituted with one OH or one keto group at position C 20 or C 22 of the side-chain were studied. The derivatives were the 22, R-hydroxy; 22, S-hydroxy; 22-keto- and 20, S-hydroxycholesterol. Two aspects of the interactions were investigated: (1) the effect of the cholesterol derivatives on the gel → liquid crystalline phase transition of dipalmitoylphosphatidylcholine (DPPC) and of dielaidoylphosphatidylethanolamine (DEPE) monitored by differential scanning calorimetry and (2) The effect on the lamellar → hexagonal H II phase transition of DEPE monitored by DSC and by 31P-NMR to determine structural changes. The gel → liquid crystalline phase transition was affected by the cholesterol derivatives to a much larger extent in the case of DPPC than of DEPE. In both cases, there was a differential effect of the four derivatives, the 22, R-hydroxycholesterol being the less effective. In DPPC-sterol 1:1 systems, 22, R-hydroxycholesterol does not suppress the melting transition, the ΔH values becomes 7.1 kcal · mol −1 as compared to 8.2 kcal · mol −1 for the pure lipid. 22, S-OH cholesterol has a much stronger effect ( ΔH = 3.1 kcal · mol −1) and 22-ketocholesterol suppresses the transition completely. In DEPE mixtures of all these compounds, the melting transition of the phospholipid is still observable. The transition temperature was shifted to lower values (−13.5°C in the presence of 20, S-OH cholesterol). The ΔH of the transition was lowered by these compounds except in DEPE-22, R-OH cholesterol mixtures and the cooperativity of the transition (reflected by the width at half peak height) was reduced. The lamellar → hexagonal H II phase transition was also affected by the presence of these cholesterol derivatives. The transition temperature value was depressed with all these compounds. 20, S-OH cholesterol was the most effective followed by 22, R-OH cholesterol. The ΔH of the transition was not strongly affected. The molecular interfacial properties of these derivatives were studied by the monomolecular film technique. It is most likely that 22, R-OH cholesterol due to the hydroxyl groups at the 3β- and 22, R-positions orients with the sterol nucleus lying flat at the air/water interface, since the compression isotherm of either the pure sterol or the DOPC-sterol mixture (molar ration, 1:1) monomolecular film exhibits a transition at approx. 103 Å 2, corresponding to the area of revolution of the sterol nucleus. This remarkable property, due probably to the existence of a kink between the side-chain and the long axis of the steroid nucleus, might explain the smaller effect of this sterol on the melting transition of either PC or PE systems.