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

functional role in process such as endo- and exocytosis, membrane recycling, protein
trafficking, fat digestion, membrane budding and fusion. PE lipids show propensity to
arrange into this structures (Mouritsen, 2005). The cubic phase, which consists of short
tubes connected in a hexagonal array, is sometimes found on mixtures of lipids that are
in transition from a lamellar phase to an inverted hexagonal phase. The inverted
hexagonal phase consists of hexagonally packed water cylinders with an outer lining of
lipid molecules oriented with their acyl-chains away from the aqueous cylinders
(Yeagle, 1993).
1.5. Lamellar phase transitions in lipid bilayers
Apart from transitions between different lipid morphologies, lipids also experience
phase transitions without drastic changes in morphology. In a lamellar symmetry, lipids
can experience several packing conditions that correspond to different lipid phases.
These different structures are classified as distinct phases because upon crossing the
transition temperature (or other thermodynamic variable), several physical properties of
the lipid bilayer change abruptly, including the heat capacity, lateral diffusion,
permeability, thickness, area, vesicle shape, etc.
In Figure I.5, differential scanning calorimetry (DSC) profiles of three
phospholipids are presented. In these, the specific heat of the phospholipids is shown as
a function of temperature. For dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC)
two peaks are clearly visible, and each peak corresponds to a phase transition. From
this, it is clear that DMPC can exist as three different phases between 10 and 30 ºC. All
other PC lipids present identical behaviour. For the PE and PA lipids the first transition
is absent, the main transition (T m ) is however generally common to all phospholipids.
This transition is considered to have first order characteristics (Gennis, 1989).
Below the main transition, lipids are tightly packed, the acyl chains are ordered and
extended in an all-trans chain configuration, while the molecules are arranged in a
regular lattice as in a crystalline solid (Figure I.5). This phase is for that reason called
solid-ordered or gel phase (s 0 ). In the gel phase, individual molecules diffuse slowly in
the plane of the membrane, and the lateral diffusion coefficient (D) of a phospholipid
molecule is on the order of 10 -10 cm 2 s -1 . Lipids with large headgroups as PC must also
tilt in the plane of the bilayer while in the gel state (Figure I.5). The cross-sectional area
of the headgroup of these lipids is larger than the cross-sectional area of the acyl-chains,
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