DIFFERENTIAL SCANNING CALORIMETRIC STUDY OF THE EFFECT OF STEROL SIDE-CHAIN LENGTH AND STRUCTURE ON DIPALMITOYLPHOSPHATIDYLCHOLINE THERMOTROPIC PHASE-BEHAVIOR

We have investigated the thermotropic phase behavior of dipalmitoylpho sphatidylcholine (DPPC) bilayers containing a series of cholesterol an alogues varying in the length and structure of their alkyl side chains . We find that upon the incorporation of up to similar to 25 mol % of any of the side chain analogues, the DPPC main transition endotherm co nsists of superimposed sharp and broad components representing the hyd rocarbon chain melting of sterol-poor and sterol-rich phospholipid dom ains, respectively. Moreover, the behavior of these components is depe ndent on sterol side chain length. Specifically, for all sterol/DPPC m ixtures, the sharp component enthalpy decreases linearly to zero by 25 mol % sterol while the cooperativity is only moderately reduced from that observed in the pure phospholipid. In addition, the sharp compone nt transition temperature decreases for all sterol/DPPC mixtures; howe ver, the magnitude of the decrease is dependent on the sterol side cha in length. With respect to the broad component, the enthalpy initially increases to a maximum around 25 mol % sterol, thereafter decreasing toward zero by 50 mol % sterol with the exception of the sterols with very short alkyl side chains, Both the transition temperature acid coo perativity of the broad component clearly exhibit alkyl chain length-d ependent effects, with both the transition temperature and cooperativi ty decreasing more dramatically for sterols with progressively shorter side chains. We ascribe the chain length-dependent effects on transit ion temperature and cooperativity to the hydrophobic mismatch between the sterol and the host DPPC bilayer (see McMullen, T. P. W., Lewis, R . N. A. H., and McElhaney, R. N. (1993) Biochemistry 32:516-522). More over, the effective stoichiometry of sterol/DPPC interactions is alter ed by a significantly large degree of hydrophobic mismatch between the sterol and the DPPC bilayer. Thus the short chain sterols appear to e xhibit considerable immiscibility in gel state DPPC bilayers, effectiv ely limiting their interaction with adjacent phospholipid molecules.