Raman spectra and vibrational assignments for deuterated membrane lipids: 1,2-Dipalmitoyl phosphatidylcholine-d 9 and -d 62

Abstract

Vibrational Raman spectra of polycrystalline 1,2-dipalmitoyl phosphatidylcholine-d 9 (fully deuterated choline methyl groups) and 1,2-dipalmitoyl phosphatidylcholine-d 62 (fully deuterated acyl chains) were recorded in the 2800–3050, 2050–2250 and 700–1800 cm −1 regions. The fundamental vibrational modes were assigned primarily on the basis of isotopic frequency shift ratios, group frequency correlations and comparisons with specific model compounds. Since deuterium-substituted lipids provide well-isolated spectral probes, particularly in the carbon-deuterium stretching region, the dependence of the 2050–2250 cm −1 region on lipid phase was examined for the dipalmitoyl phosphatidylcholine-d 62 species. The methylene CD 2 deformation and twisting modes at 984 and 919 cm −1, respectively, also exhibit intense, isolated vibrational transitions which should prove useful for monitoring molecular order in mixed deuterated and undeuterated lipid systems. Except for the relatively weak choline methyl C-D and C-H stretching modes, the spectrum of 1,2-dipalmitoyl phosphatidylcholine-d 9 is not distinguishable from that of the undeuterated system. For both the d 9 and undeuterated species, the vibrational modes associated with the lipid head group region are sensitive to slight hydration.