Raman studies of interdigitation in bilayers of cerebroside sulphate

Stevenson, Christopher Carl (1988) Raman studies of interdigitation in bilayers of cerebroside sulphate. Masters thesis, Memorial University of Newfoundland.

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Abstract

Spontaneous Raman scattering studies of bilayer systems of the lipid molecule Galactosylceramide I³-sulphate, or cerebroside sulphate (CBS) in 0.5 and 2.0 M KCl and LiCl solutions in H₂O have been carried out with the samples at temperatures between 10 and 90°C and at thermal equilibrium. By monitoring the intensities of the 2845, 2885, and 2935 cm⁻¹ bands in the C-H stretching region of the lipid hydrocarbon chains, packing characteristics of these chains within the lamina of the bilayers are elucidated. For example, the 2885 cm⁻¹ band is attributed to the Raman-active asymmetrical stretching mode of CH₂ units along the chains superposed over a broader, intense band due to Fermi resonances with the CH₂ rocking modes at ≈1450 cm⁻¹. Drops in the relative intensity of this feature indicate a lessening of the degree of chain-chain interaction in the bilayer, and an abrupt change in this intensity with a change in temperature is an indication that the lipid bilayers have undergone a phase transition with its associated changes in hydrocarbon chain packing. This transition might be between two gel phases, perhaps interdigitated ones, or between a gel phase and the fluid phase. -- Unlike for the case or neutrally-charged lipids such as the phosphatidyl cholines, having either symmetric or asymmetric hydrocarbon chain lengths, the observed main phase transition temperatures to the liquid crystalline or fluid state Tm in CBS were only slightly affected by the effective length of the acyl chain attached to the sphingosine base of CBS. This was seen in the differential scanning calorimetry results of J. Boggs et.al., and in the results reported here. Tm depends more heavily on the species of cation in the aqueous environment of the CBS bilayers than on the acyl chain lengths. Potassium, having a smaller hydrated ionic radius, is found to shield the SO₃ charge in the CBS headgroup to a larger extent than lithium cations, and this is reflected in a higher gel-liquid crystalline phase transition temperature. Another effect studied is the further increase of Tm through the addition of a single OH group to the carboxyl end of the acyl chain in the saturated 18-carbon species of CBS. The additional inter-headgroup hydrogen bonding is thought to produce this increase in Tm. Finally, the myriad of smaller-enthalpy phase transitions observed by Boggs et.al occurring below Tm in their DSC data are not observed here in general; hints of other phase transitions occur only (or some CBS samples in K⁺, and not for Li⁺. -- Galactosylceramide I³-sulphate is a predominant charged lipid in the myelin sheath surrounding axons in the nervous system of all vertebrates. Knowledge of the equilibrium phase behavior of CBS in environments similar to those in nervous tissue would aid those involved in the medical study of nervous disorders, as well as contributing to the understanding of lipid dynamics in general.

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