The effect of side-chain analogues of cholesterol on the thermotropic phase behavior of 1-stearoyl-2-oleoylphosphatidylcholine bilayers: A differential scanning calorimetric study

In this study we have examined the effects of analogues of cholesterol differing with respect to alkyl side-chain length and structure on the thermotropic phase behavior of bilayers formed from 1-stearoyl-2-oleoyl-sn-olycero-3-phosphocholine (SOPC), an important subclass of naturally occurring phosphatidylcholines (PCs). The synthetic sterols we studied contained either a terminally unbranched (n-series) or a single methyl-branched (iso-series) side chain of 3 to 10 carbon atoms. The phase transition behavior was examined by high-sensitivity differential scanning calorimetry (DSC). The main phase transition endotherm of SOPC/sterol bilayers consists of superimposed sharp and broad components, which represent the hydrocarbon chain melting of sterol-poor and sterol-rich phospholipid domains, respectively. The transition temperature and the cooperativity of the sharp component are moderately reduced upon sterol incorporation and the enthalpy decreases to zero when sterol levels of 20-30 mol% are reached. The enthalpy of the broad component transition initially increases to a maximum around 25 or 25-30 mol% sterol and thereafter decreases with further increases in sterol concentration. However, the broad transition of SOPC bilayers containing both short (C-22, i-C5 and n-C3) and long (i-C9 and i-C10) side-chain sterols still persists at levels of 50 mol% sterol. Thus the effective stoichiometry of SOPC-sterol interactions varies with changes in sterol alkyl side-chain length. The incorporation of short linear or branched side-chain sterols (C-22, n-C3, n-C4, i-C5) causes the broad component transition temperature and cooperativity to decrease dramatically, whereas the incorporation of medium- and long-chain sterols in both the n- and iso-series has less effect on the transition temperature and cooperativity of the broad component. Overall, no significant differences were found between the n- and iso-series sterols for a given side-chain length. A comparison of the phase behavior of dipalmitoylphosphatidylcholine (DPPC)/sterol (McMullen et al. (1995) Biophys. J. 69, 169-176) and SOPC/sterol mixtures indicates that the primary factor responsible for changes in the thermotropic phase behavior of these systems is the extent of the hydrophobic mismatch between the sterol and the host lipid bilayer. However, sterol miscibility in PC bilayers, and thus the stoichiometry of lipid-sterol interactions, also appears to depend on the degree of unsaturation of the host lipid bilayer.