Colloidal gels from proteins and peptides - Physical Gels from Biological and Synthetic Polymers - page 275

0.08

Ovalbumin

0.06

0.04

BSA

0.02

β -lactoglobulin

0

3.5 4.5 5.5

d eff (nm)

6.5 7.5 8.5

Critical percolation volume fraction ( ɸ crit ) versus effective diameter for semi-flexible amyloid
fibrils of β -Lg, BSA and ovalbumin. The vertical lines indicate the diameter of the rods for
ovalbumin (4 nm) and BSA or β -Lg (5 nm). Adapted from Sagis et al.( 2004 ) © 2004 American
Chemical Society.

Figure 9.9

9.4.3 Phase behaviour

According to Sagis et al.( 2004 ), their isotropic solution becomes an isotropic gel at the
percolation transition, before reaching the biphasic region predicted by the Onsager
model. At larger concentrations, a two-phase gel is observed, consisting of nematic
droplets dispersed in an isotropic gel. At even higher concentrations (>10 wt%) a single
nematic phase develops. What this does suggest is that some solutions containing
amyloid fibrils have interesting phase behaviour, which resembles that expected for an

assembly of rod-like systems ( Chapter 3 ).
Incubation of proteins at relatively high concentrations (10

- 15% w/v) results in substan-

tial yields of amyloid fibrils, and according to some reports these solutions gel after several

days. Polarized optical microscopy images reveal that the gels contain large numbers of
small birefringent domains, indicating the presence of liquid crystal inclusions which do not
allow the

fibrils to align over long distances. Sagis et al.( 2004 ) have suggested the term

' liquid crystal glass to describe the appearance of many small liquid crystal domains.

Adamcik et al.( 2010 ) re-examined the morphology of

'

β -Lg fibrils. They performed

β

AFM measurements of -Lg fibrils produced by heat denaturation at pH 2: the images

show semi- fibrils several micrometres in length, in agreement with previous

literature reports such as those mentioned above. Maximum height measurements were
performed to characterize the various

exible

fibril cross-sections. Hierarchical helical structures

were observed. Periodic fluctuations in cross-section, persistence length and height along

the contour length of the lament, double-, triple-

and multi-stranded left-handed helical aggregates, the most frequently occurring popula-
tion being the double-stranded helix. In particular, the authors showed that a strong
tendency to inter-

fibrils allowed them to distinguish single-

bril aggregation exists, in particular for a positive large linear charge
density. They suggested that a strong energetic driving force must exist to promote