Colloidal gels from proteins and peptides - Physical Gels from Biological and Synthetic Polymers - page 276

aggregation under these conditions. Once aggregation has occurred, the electrostatic
repulsive forces induce a twisting of the ribbons. The attractive force leading to aggregation
might arise from the amphoteric nature of the protein

fibrils, which can promote strong

attractive interactions. The ribbon-like cross-section may arise from the

balance of hydrophobic short-range attraction and electrostatic long-range repulsion.
Jung and Mezzenga ( 2010 ) determined a phase diagram for

' hydrophobic '

β -Lg by optical observa-

tions of flask contained a magnetic bar to stir the solution during the heating

process, and it is reported that stirring enhanced the conversion rate and the birefrin-
gence, and reduced the formation of spherulites. This method was also used by
Adamcik et al.( 2010 ) for sample preparation before their AFM observations. The
resulting systems are fundamentally different from the ones previously investigated in
quiescent conditions, because they contain virtually no spherulites. Consequently, the
structure of the

asks. Each

fibres is highly dependent on the exact heat denaturation process.
Sagis et al.( 2004 ) reported on the coexistence of nematic droplets (spherulites) in the
isotropic phase around 0.5 wt% and pH 2, whereas in the work of Jung and Mezzenga
( 2010 ) the transition was purely an isotropic

nal

nematic one. For Jung and Mezzenga,
solutions were liquid and transparent up to 5 wt%, where a gel was obtained. Under
polarized light, the sample at 0.3 wt% was not birefringent, whereas at 0.4 wt% the
sample started to be very slightly birefringent. The isotropic

-

-

nematic phase transition
occurred at 0.4 wt% at pH 2. The sample became progressively more birefringent as the
concentration was increased. At 2 wt% and above, the sample was birefringent with
bright colours, suggesting the possible presence of cholesteric liquid crystalline phases
(chiral helical conformations of the

fibres). The gel revealed a microscopic phase
separation, with coexisting isotropic (non-birefringent) and nematic (birefringent)
phases, visible under polarized light. According to this work, the entire region of the
phase diagram below the gelation threshold is presented as reversible to pH and concen-
tration changes, in agreement with the thermodynamic character of the phase diagram.
For the gel, however, dissolution of the nematic phase did not occur upon dilution, and
the system retained birefringent spots both between cross-polarizers and under polarized
light microscopy, indicating that the aggregation in the gel region does not follow
thermodynamic behaviour, but rather a concentration-induced irreversible aggregation
of the fibres, similar to the formation of spherulites. A gel was obtained by concentrating

the pure nematic region, which induced irreversible aggregation within the nematic
phase, and phase separation into a

fibre-enriched region (the aggregates) and a

fibre-impoverished region (the isotropic continuous phase). This scheme was different
from the one suggested by the work of Sagis et al.( 2004 ), who based their analysis on a
percolation transition in the isotropic phase. As a consequence of this complexity, direct
application of Onsager theory based on excluded-volume interactions appears to be
inappropriate to describe these systems. Flory s theory, accounting for hydrophobic

interactions of rod suspensions in water, seems to produce theoretical predictions
which are more consistent with experimental observations.
A challenging issue in the self-assembly of complex supramolecular structures is
therefore understanding how kinetically ef

'

cient pathways emerge from the multitude
of possible transition states and routes (Pouget et al., 2010 ). In general, in vitro