Direct measurement of phase coexistence in DPPC/Cholesterol vesicles using Raman spectroscopy.

The phase behavior of bilayers of binary mixtures of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and cholesterol has been studied using Raman spectroscopy. It is observed that the shape of the cholesterol vibrational spectrum in lipid-cholesterol binary mixtures does not vary significantly with either the cholesterol concentration or the temperature. This permits determination of the lipid vibrational signatures of the liquid-disordered (ld), solid-ordered (so) and liquid-ordered (lo) phases. Within the phase coexistence region, the measured spectra are described very well by a linear combination of the different spectral components, which permits a quantitative analysis of the phase diagram. In contrast to earlier findings, our experiments provide no indication of a phase boundary at low cholesterol concentration. The upper boundary of the phase coexistence region is found at not, vert, similar27 and not, vert, similar22 mol% for ld-lo and so-lo coexistence region, respectively. Within these phase coexistence regions, the partitioning of cholesterol between the cholesterol-poor and the cholesterol-rich phases is in close agreement with the lever rule.