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

changes abruptly in the interfacial region, varying from the value in water (ε = 78) to
the value in the hydrocarbon core (ε = 2) (McIntosh, 2002).
5) After adsorption of the peptide to the bilayer surface, conformational changes
generally take place that entail changes in the thermodynamic properties of the binding
process. One common scenario is that peptides are unstructured (random coil) while in
the aqueous environment and after binding to the bilayer take on an amphipatic
structure. The bound structure can also be dependent on the L/P in the membrane
(transition from an alpha-helical to beta-sheet structure). The change to an amphipatic
structure leads to multiple intermolecular hydrogen bonds and results in a largely
favourable contribution to partition. Isothermal titration calorimetry studies with some
amphipatic peptides revealed that the favourable enthalpic contributions for peptide
binding arising from alpha-helix formation amounted to half of the free energy of
binding (McIntosh, 2002).
The thermodynamic parameter that relates directly to the partition of the peptide
between the lipid and water phases is the partition coefficient (K p ):
K =
p
n
n
S,L
S,W
/ V
/ V
L
W
2.1
where V i are the volumes of the phases, and n S,i are the moles of solute in each phase (i
=W, aqueous phase; i=L, lipid phase) (Mateo et al., 2006). The partition coefficient is
converted in free energy of binding to the membrane (∆G 0 ) by:
o
∆G = -RT ln
( Kp)
2.2
After binding, amphipatic peptides are expect to reside in the lipid/water interface of
the membrane. The exact position is nevertheless dependent on both the membrane and
the peptide sequence. From X-ray diffraction measurements on DOPC bilayers, the axis
of an ideally amphipatic peptide was located between the mean positions of the glycerol
and carbonyl groups (Fig. I.6), exactly at the interface between the polar headgroups
and the hydrocarbon core (Mishra et al., 1994).
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