The only sphingolipid occurring naturally in mammalian cells, sphingomyelin (SM) is of
great importance. Having predominantly long, saturated acyl chains it has an unusually
high melting temperature (around physiological temperature) and has been strongly
associated with lipid microdomains often referred to as “lipid rafts”. These domains are
proposed to be enriched in SM and cholesterol (Chol) and have been linked to many cell
processes and disease states.
Despite the important role SM plays in the cell membrane, there is relatively little
published data available in the literature. The first chapter of this work investigates the
behaviour of SM from three natural sources, bovine brain (BBSM), egg yolk (EYSM)
and milk (MSM). This allows a comparison of the phase behaviour in terms of the
different chain compositions in each extract. The results show that the extracts differ in
their gel phase structures and we present the first x-ray diffraction data for a ripple phase
(Pβ’) in BBSM and EYSM.
At sufficiently high concentrations, Chol acts to disrupt the gel phase (Lβ) and order the
fluid phase (Lα) of SM, leading to the formation of the ‘intermediate’ liquid ordered (Lo)
phase. This phase retains characteristics of both the Lβ and Lα phases and has been
strongly associated with lipid rafts. Using a combination of x-ray diffraction and 31P
solid-state NMR, we have investigated the disruption of the gel phase of BBSM at low
Chol concentrations. The results show disruption of the regular packing of the gel phase
and also the removal of the ripple phase at 15mol% Chol.
Finally, time-resolved x-ray diffraction techniques were utilised to investigate the
kinetics of the different lamellar transitions observed. The Lβ to Lα and the Pβ’ to Lα
transitions are compared in MSM and BBSM.