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

INTRODUCTION: BIOMEMBRANES
Lipid immiscibility gives rise to lateral structuring of the lipid bilayer and the
resulting lateral organizations are called domains. Lipid domains are not static
structures. They exhibit dynamics both in space and time, as they can exist as short
(fluctuations) or long-lived structures in the lipid bilayer. Large size domains (in the
order of micrometers) have already been detected through imaging techniques (Korlach
et al., 1999; Bagatolli and Gratton, 2000). These techniques make use of fluorescent
probes that show preferential partition in one of the lipid phases or differential
fluorescent properties in each phase (Klausner and Wolf, 1980; Bagatolli and Gratton,
2000). They are however restricted by the diffraction limit to detect domains larger than
~ 200 nm. An example is shown in Figure I.8.
Figure I.8 - Gel-Fluid coexistence in a giant unilamellar vesicle (GUV) (see 1.10) detected by
confocal microscopy. The system is a DLPC/DSPC (0.4/0.6 mol/mol) mixture. Red areas correspond to
the fluorescence arising from a probe with preferential partition to the gel phase (enriched in DSPC) and
green areas correspond to the fluorescence of a probe with preferential partition to the fluid phase
(enriched in DLPC) (taken from Korlach et al., 1999).
For a situation of thermodynamical equilibrium, one should expect complete phase
separation, i.e., only two very large domains inside the vesicle, one in the gel phase, and
the other in the fluid phase. This is due to the interfacial tension between lipid domains.
In the interface, the phases show differences in hydrophobic thickness, and the
consequent exposure of the hydrocarbon chains to water molecules create a packing
stress in the interface of domains. This tension is the driving force for the fusion of
small domains into larger lateral structures after phase separation, as larger domains
correspond to a smaller interface area. At infinite time, the complete macroscopic phase
separation should be achieved. However, several domains of limited size are observed
(Figure I.8). This is possibly due to the coupling of phase separation to the curvature of
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