Direct determination by Raman scattering of the conformation of the choline group in phospholipid bilayers

For clarification of the assignment of the vibrational modes of the choline group, Raman spectra of choline iodides selectively deuterated at three different positions were investigated. The isotope shifts of the C-N stretching vibrations suggested that they are conformation sensitive. When the Raman spectra of choline chloride, carbamoylcholine iodide, carbamoylcholine chloride, and methoxycarbonylcholine iodide are compared with the crystal structures of these compounds, a correlation between the vibrational frequency and the conformation of the O-C-C-N+ backbone could be established. The Raman bands attributed to the "totally" symmetric stretching (v1) and symmetric stretching vibrations (v2) of the C-N bonds of the quaternary ammonium group appeared at about 720 cm-1 and about 870 cm-1, respectively, for the gauche conformation of the O-C-C-N+ backbone, and in the trans conformation, they shifted to about 770 cm-1 (v1) and about 910 cm-1 (v2), respectively. On the basis of this correlation and from measurements of phosphatidylcholine and sphingomyelin bilayers, it was concluded that most of the choline groups in both bilayers take the gauche conformation not only in the solid state but also in the gel and liquid-crystalline states. These data represent the first direct evidence that a gauche conformation for the O-C-C-N+ bond is preferred in the gel and liquid-crystalline states. These key bands, especially the v1 band, are a powerful tool to study the conformation of the choline group in situ not only in the membrane field but also in the neuroscience in connection with acetylcholine.