Biophysical studies of membrane proteins/peptides. Interaction with ...
Biophysical studies of membrane proteins/peptides. Interaction with ...
Biophysical studies of membrane proteins/peptides. Interaction with ...
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functional role in process such as endo- and exocytosis, <strong>membrane</strong> recycling, protein<br />
trafficking, fat digestion, <strong>membrane</strong> budding and fusion. PE lipids show propensity to<br />
arrange into this structures (Mouritsen, 2005). The cubic phase, which consists <strong>of</strong> short<br />
tubes connected in a hexagonal array, is sometimes found on mixtures <strong>of</strong> lipids that are<br />
in transition from a lamellar phase to an inverted hexagonal phase. The inverted<br />
hexagonal phase consists <strong>of</strong> hexagonally packed water cylinders <strong>with</strong> an outer lining <strong>of</strong><br />
lipid molecules oriented <strong>with</strong> their acyl-chains away from the aqueous cylinders<br />
(Yeagle, 1993).<br />
1.5. Lamellar phase transitions in lipid bilayers<br />
Apart from transitions between different lipid morphologies, lipids also experience<br />
phase transitions <strong>with</strong>out drastic changes in morphology. In a lamellar symmetry, lipids<br />
can experience several packing conditions that correspond to different lipid phases.<br />
These different structures are classified as distinct phases because upon crossing the<br />
transition temperature (or other thermodynamic variable), several physical properties <strong>of</strong><br />
the lipid bilayer change abruptly, including the heat capacity, lateral diffusion,<br />
permeability, thickness, area, vesicle shape, etc.<br />
In Figure I.5, differential scanning calorimetry (DSC) pr<strong>of</strong>iles <strong>of</strong> three<br />
phospholipids are presented. In these, the specific heat <strong>of</strong> the phospholipids is shown as<br />
a function <strong>of</strong> temperature. For dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC)<br />
two peaks are clearly visible, and each peak corresponds to a phase transition. From<br />
this, it is clear that DMPC can exist as three different phases between 10 and 30 ºC. All<br />
other PC lipids present identical behaviour. For the PE and PA lipids the first transition<br />
is absent, the main transition (T m ) is however generally common to all phospholipids.<br />
This transition is considered to have first order characteristics (Gennis, 1989).<br />
Below the main transition, lipids are tightly packed, the acyl chains are ordered and<br />
extended in an all-trans chain configuration, while the molecules are arranged in a<br />
regular lattice as in a crystalline solid (Figure I.5). This phase is for that reason called<br />
solid-ordered or gel phase (s 0 ). In the gel phase, individual molecules diffuse slowly in<br />
the plane <strong>of</strong> the <strong>membrane</strong>, and the lateral diffusion coefficient (D) <strong>of</strong> a phospholipid<br />
molecule is on the order <strong>of</strong> 10 -10 cm 2 s -1 . Lipids <strong>with</strong> large headgroups as PC must also<br />
tilt in the plane <strong>of</strong> the bilayer while in the gel state (Figure I.5). The cross-sectional area<br />
<strong>of</strong> the headgroup <strong>of</strong> these lipids is larger than the cross-sectional area <strong>of</strong> the acyl-chains,<br />
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