Chemistry Reference
In-Depth Information
25
A
pH 5
pH 7
20
pH 9
15
10
5
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
A (m
2
/mg)
40
B
pH 5
pH 7
pH 9
30
20
10
0
0
5
10
15
20
25
π
(mN/m)
Figure 5 The effect of pH on (A) the surface pressure versus area isotherm and (B) the
elasticity modulus for
b
-casein monolayers spread at the air-water interface at
201C and ionic strength 0.05 M; O, pH 5;
D
,pH7;
r
,pH9
from the BAM images it is impossible to distinguish between the different
structures that b-casein adopts at the air-water interface. The images suggest
that the b-casein residues spread at the air-water interface have the same
isotropy in the plane vertical to the molecular chain, no matter what is the
structure adopted by b-casein residues at the interface. This isotropy is also
confirmed by the evolution during the monolayer compression of the reflectiv-
ity of the BAM images. Figure 6 shows that the reflectivity increases as the
monolayer is compressed and goes to a maximum at the collapse point, which
means that during the monolayer compression a denser film is formed. The sur-
face pressure dependence on reflectivity (Figure 6) and film thickness (Table 3)
for b-casein spread monolayers is essentially the same when viewed at higher
levels of magnification by AFM.
24
The differences in surface coverage and
packing are clearly seen. The monolayer thickness is 1.1 and 1.6 nm at
p
values
below and above
p
(t), respectively. A value of 1.1 nm is consistent with a flat
two-dimensional network composed of individual b-casein molecules. The
increase in film thickness at
p
4
p
(t) is consistent with a deformable protein
structure influenced by surface packing.
24
The arrows in Figure 6 indicate the
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