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 235

Table 2 The effect of pH on characteristic BAM geometrical and topographical
parameters (from image morphology and reflectivity) for LE and LC
domains of DPPC monolayers spread at the air-water interface, at the
transition between LC and LE phases (at p ¼ 6.7 mN m 1 )at201C
and ionic strength 0.05 M

pH 5 7 9

Domain size
Radius (mm) 6.7 10.2 14.8

Area (mm 2 )

143.5 397.0 816.6

Domain shape
Roundness 1.1 1.5 1.3

Border Circular Star shape Ellipsoidal

Thickness (a.u. 10 7 )
LE 2.9 2.9 5.5

LC 6.3 6.3 8.5

LE-LC transition 3.4 3.4 3

sub-phase pH exerts a significant influence on the self-assembly of b-casein
monolayers at the air-water interface. The p -A isotherm is displaced towards
the surface pressure axis, and the monolayer structure is more condensed, in the
acidic subphase. From the p A isotherm itself (Figure 5A) and the elasticity
modulus deduced from its slope (Figure 5B), it appears that there is a critical
surface pressure p (t,b-cas) (see Table 1) at which the monolayer properties
change significantly. With b-casein films we distinguish two different structures
(Structures 1 and 2) around the monolayer collapse at the highest surface
pressure, close to its equilibrium spreading pressure value, p (e,b-cas) (see Table 1).
According to Graham and Phillips, 23 a tail-train structure for b-casein occurs at
p o p (t,b-cas) (Structure 1). At higher surface pressures, and up to p ¼ p (e,b-cas),
amino acid segments are extended into the underlying aqueous solution and
adopt the form of loops and tails (Structure 2). The p (e,b-cas) value decreases
monotonically as the pH increases, a phenomenon which is consistent with a
reduction in the interactions between amino acid residues at the air-water
interface with the basic sub-phase. The acidic sub-phase caused an increase in
the p (t) value. This transition was observed at p (t,b-cas) ¼ 15.6 and 10.6 mN
m 1 at pH 5 and 7, respectively (Table 1). However, at pH 9 this transition
was not observed. The presence of a minimum E plateau value, which is
characteristic of a second-order phase transition, corroborates the existence of
two structures for b-casein monolayers at pH 5 and 7. However, such a
minimum E plateau was not observed at pH 9. Clearly, the electrostatic
repulsions between amino acid residues at pH 9 do not favour Structure 2 with
the formation of loops and tails. At p o p (t,b-cas), the E values are higher at pH
5 and 7 than at pH 9, but the opposite was observed at p 4 p (t,b-cas).
The BAM images confirm that only a homogeneous phase is present during
the compression of b-casein up to the monolayer collapse (Figure 6A). That is,