THE EFFECT OF SIDE-CHAIN ANALOGS 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 th e thermotropic phase behavior of bilayers formed from 1-stearoyl-2-ole oyl-sn-glycero-3-phosphocholine (SOPC), an important subclass of natur ally occurring phosphatidylcholines (PCs). The synthetic sterols we st udied contained either a terminally unbranched (n-series) or a single methyl-branched (iso-series) side chain of 3 to 10 carbon atoms. The p hase transition behavior was examined by high-sensitivity differential scanning calorimetry (DSC). The main phase transition endotherm of SO PC/sterol bilayers consists of superimposed sharp and broad components , which represent the hydrocarbon chain melting of sterol-poor and ste rol-rich phospholipid domains, respectively. The transition temperatur e and the cooperativity of the sharp component are moderately reduced upon sterol incorporation and the enthalpy decreases to zero when ster ol levels of 20-30 mol% an reached. The enthalpy of the broad componen t transition initially increases to a maximum around 25 or 25-30 mol% sterol and thereafter decreases with further increases in sterol conce ntration. However, the broad transition of SOPC bilayers containing bo th short (C-22, i-C5 and n-C3) and long (i-C9 and i-C10) side-chain st erols still persists at levels of 50 mol% sterol. Thus the effective s toichiometry of SOPC-sterol interactions varies with changes in sterol alkyl side-chain length. The incorporation of short linear or branche d side-chain sterols (C-22, n-C3, n-C4, i-C5) causes the broad compone nt transition temperature and cooperativity to decrease dramatically, whereas the incorporation of medium- and long-chain sterols in both th e n- and iso-series has less effect on the transition temperature and cooperativity of the broad component. Overall, no significant differen ces were found between the n- and iso-series sterols for a given side- chain length. A comparison of the phase behavior of dipalmitoylphospha tidylcholine (DPPC)/sterol (McMullen et al. (1995) Biophys. J. 69, 169 -176) and SOPC/sterol mixtures indicates that the primary factor respo nsible 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 th us the stoichiometry of lipid-sterol interactions, also appears to dep end on the degree of unsaturation of the host lipid bilayer.