Role of Electrostatic Interactions on Molecular Self-Assembly of Protein + Phospholipid Films at the Air–Water Interface - Food Colloids: Self-Assembly and Material Science - page 241

Figure 10 BAM images for DPPC + b -casein mixed monolayers spread at the air-water
interface (201C, ionic strength 0.05 M, shutter time ¼ 1/125 s) at p 4 p (e, b -
cas): A, pH 5; B, pH 7; C, pH 9. Mass fraction of DPPC in the mixture is
X DPPC ¼ 60%. Image sizes are 470 600 m m
15.4 Conclusions
The structure, morphology, and film thickness of DPPC, b-casein, and their
mixtures at the air-water interface has been determined using a combination of
p -A isotherms, BAM, and AFM. Pure DPPC monolayers show a structural
polymorphism dependent on p and pH, displaying film anisotropy, and heter-
ogeneous domain structures. In contrast, pure b-casein monolayers display a
homogeneous morphology, with a flat two-dimensional network composed of
individual b-casein molecules at low p value and protein structure influenced by
surface packing at high p value. This behaviour was observed at all surface
pressures and at pH 5 and 7.
For DPPC + b-casein mixed films, the role of attractive interactions
(hydrophobic and/or electrostatic) or repulsive interactions (electrostatic) is
more complex, being dependent on the surface pressure and the DPPC/b-casein
ratio. At microscopic and nanoscopic levels, respectively, BAM and AFM have
corroborated the structural polymorphism deduced from the p -A isotherms
and the role of electrostatic interactions on mixed film structure. Increasing
the repulsive electrostatic interactions between DPPC and b-casein by raising
the pH results in a greater degree of phase separation, and an earlier onset of
structural transitions of the b-casein component. Nonetheless, in addition to
the purely electrostatic forces, other types of interactions - hydrophobic forces,
the restructuring of solvent and ionic distribution around different parts of
the molecules, and the loss of mobility of the hydrophobic chains - must also
be considered in the self-assembly of mixed protein + lipid films at fluid
interfaces.
Acknowledgements
The authors acknowledge the support of CICYT through grants AGL2001-
3843-C02-01 and AGL2004-01306/ALI. Research at IFR is funded through the
core BBSRC grant to the Institute.