Normal vibrational modes of phospholipid bilayers observed by low-frequency Raman scattering

Low-frequency Raman spectra of multilamellar vesicles made either of 1-palmitoyl-2-oleoyl-$sn$-glycero-3-phosphocholine (POPC) or 1,2-dipalmitoyl-$sn$-glycero-3-phosphocholine (DPPC) have been studied in a wide temperature range. Below $0{}^{\circ }\mathrm{C}$ two peaks are found at frequencies around 8–9 and $14-17\mathrm{c}{\mathrm{m}}^{-1}$ and attributed to the normal vibrational modes of the phospholipid bilayer, which are determined by the bilayer thickness and stiffness (elastic modulus). The spectral positions of the peaks depend on the temperature and the bilayer composition. It is suggested that the ratio of the intensities of the first and second peaks can serve as a measure of the interleaflet elastic coupling. The addition of cholesterol to the phospholipid bilayer leads to peak shift and broadening, which may be assigned to the composition heterogeneities commonly attributed to the lipid raft formation.

Figure 1

The representative low-frequency Raman spectra: the DPPC (a) and POPC (b) suspensions of phospholipid vesicles, for different temperatures. The arrows indicate the first two vibrational harmonics seen at low temperatures.

Figure 2

The low-frequency Raman spectra: the DPPC (a) and POPC (b) suspensions of phospholipid vesicles at $T=120\mathrm{K}$ (circles) and its five-Lorentzian fits (thick lines) constituted of the central peak (dashed lines), the first three harmonics (thin solid lines), and acousticlike states (dotted lines).

Figure 3

The Raman spectrum of suspensions of POPC-cholesterol (circles) vesicles at low-temperature $(T=120\mathrm{K})$ and its five-Lorentzian fit (thick line) constituted of the central peak (dashed lines), the first three harmonics (thin solid lines), and acousticlike states (dotted lines). For comparison the spectrum of the cholesterol-free POPC sample is shown by the solid line shifted upward for convenience.

Figure 4

The frequency of the second normal mode versus temperature: (a) DPPC suspensions, and (b) POPC (squares) and POPC-cholesterol (stars) suspensions.