Biophysical studies of membrane proteins/peptides. Interaction with ...

the vesicle through bilayer deformation in the interface and to the insertion of
phospholipids in the interface region with intermediate conformations (between gel and
fluid like conformations) in order to substantially decrease the interface stress between
lipid domains (Jorgensen and Mouritsen, 1995). Another possibility is that the finite
size domains correspond to non-equilibrium structures, detected due to the very long
lifetime of the process of phase separation (de Almeida et al., 2002).
Even when at the same lamellar phase, lipids do not mix ideally. This is particularly
true for the gel phase, as the strict packing restrictions force dissimilar lipids to
immiscibility. As a result, clustering or domain formation is observed and the bilayer
gains lateral heterogeneity. This type of behaviour is observed e.g. for the highly
nonideal mixture DLPC-DSPC (Fig. I.7), as two different gel phases (each enriched in
one of the lipids) are present below the phase coexistence region.
Deviations from ideal lipid mixing and domain formation also occur in the fluid
phase, and PC lipids with a significantly different thickness were already shown to
segregate into domains due to hydrophobic mismatch stress. Also for this situation, the
discrepancy of hydrophobic thickness of the two PC lipids can create a packing stress in
the bilayer due to exposure of hydrocarbons to water, and stimulate lipid segregation
(Lehtonen et al., 1996). The lateral structures created by this type of phase separation
are expected to be on the nanometer scale in opposition to gel-fluid phase separation
which can be detected in the micrometer scale. Therefore, these domains are elusive to
most of the imaging techniques, and other spectroscopic techniques must be applied,
namely macroscopic fluorescence methodologies.
The ideality degree of a lipid mixture dictates not only the phase separation of the
mixture but also the size and shape of the phase-separated domains. Apart from
temperature, other factors influencing the lateral structuring of a multicomponent lipid
bilayer are dehydration and the presence of divalent ions. Dehydration can cause
lamellar to inverted hexagonal phase transition and induce phase separation (Webb et
al., 1993). Mixtures containing an anionic phospholipid can experience severe phase
separation in the presence of divalent ions like Ca 2+ . Calcium ions induce clustering and
phase separation of anionic phospholipids due to intermolecular cross bridges (Silvius,
1990).
A phase coexistence of particular biological relevance is that of L α and L o phases. In
1997 Simons and Ikonen proposed that small lipid domains in the L o phase (lipid rafts)
composed of saturated lipids and cholesterol coexisted with a matrix of lipids in the
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