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

INTRODUCTION: BIOMEMBRANES
and the only way to achieve effective packing is through a tilt of the hydrocarbon
chains.
Above the T m of the lipid, the acyl chain ordering characteristic of the gel phase is
lost (gauche conformations are favoured), and in this new phase, the lateral diffusion of
the lipid molecules is substantially higher (D is on the order of 10 -8 cm 2 s -1 ). This phase
is called fluid or liquid-crystalline phase (L α ). As a result of the smaller number of trans
conformations in the acyl-chains, bilayers in the fluid phase are considerably thinner
than in the gel phase (up to 6 Å in dipalmitoyl-sn-glycero-3-phosphatidylcholine-DPPC
bilayers), but the cross-sectional area increases dramatically (from 52 Å to 71 Å in
DPPC individual molecules) (Nagle and Tristram-Nagle, 2000). This increase in area is
a consequence of the weaker van der Waals attractive interactions between acyl-chains
in the fluid phase (Gennis, 1989). As a rule, a strong coupling exists between the lipid
phases in each monolayer (Bagatolli and Gratton, 2000), exceptions however have
already been observed (Devaux and Morris, 2004).
The acyl-chains of a phospholipid dictate to a large extent the stability of each of the
gel and fluid phases, and as a result they define the T m of the lipid. This is due to the
importance of the van der Waals interactions in stabilizing the gel phase. Long acylchains
permit stronger van der Waals attractive forces, stabilizing the gel phase and
increasing the lipid T m . Unsaturated chains prevent effective ordered packing into the
gel state, inducing a decrease in lipid T m . As most lipids in biomembranes present
unsaturations, lipid membranes are highly fluid. Headgroup structure can also influence
the transition temperature of the lipid. PE lipids present higher T m than PC due to
stabilizing hydrogen bonding in the gel phase. pH, ionic strength and pressure can also
influence T m .
PC lipids, as seen in Figure I.5, experience an additional phase transition. This
transition is called pretransition and occurs between two different gel states, L β’ , and a
ripple or periodic gel phase (P β’ ), at a characteristic temperature (T P ). Due to the
presence of the planar ring structure, cholesterol disrupts the packing of lipids when
mixed with lipids in the gel phase. For lipids in the fluid phase, cholesterol has an
ordering influence (Ipsen et al., 1987). The mixture of cholesterol and saturated
phospholipids can give rise to an additional phase called liquid ordered phase (L o ). In
the L o phase, the acyl-chains present a higher degree of ordering as compared to the one
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