Structure of Self-Assembled Globular Proteins - Food Colloids: Self-Assembly and Material Science - page 37

If the binding between the particles is reversible, the aggregates and gels that
are formed are only transient. Because the assembly is reversible, it will yield
configurations that minimize the free energy of the system. This can lead to phase
separation between a dense phase of close-packed particles and a dilute phase of
small transient aggregates. It is possible that during the phase separation a
transient gel is formed by the aggregating particles. 6,7 Although phase separation
will finally lead to two macroscopic phases, this may be a very slow process, and
so the transient gel may be long-lived. When both reversible and irreversible
binding occur together, transient structures formed by reversible binding may
become permanently frozen-in by the formation of irreversible bonds.
Many other factors can modify the assembly process. Particles may feel both
attractive and repulsive interactions or may contain only a limited number of
specific binding sites. Furthermore, different kinds of particles may be involved,
interacting differently with their own kind and with other kinds. The interac-
tion itself may be time-dependent; e.g., reversible bonds may become perma-
nent as the assembly process progresses. Computer simulations are probably
the only way to understand in detail how each of these factors influences the
large-scale structure. 8 In real systems, various factors often play a role simul-
taneously and it is not always evident which one is predominant.
Self-assembly is frequently used to prepare food systems. For example,
cheese and yoghurt are the result of self-assembly of casein; and sauces and
puddings may be thickened by the self-assembly of polysaccharides or gelatin
into a system spanning network. In all these cases the self-assembly is induced
by a change in the medium which changes the interaction between the particles/
polymers. This paper considers the structures formed by self-assembled glob-
ular proteins. Gelled egg-white is an obvious example of such a food system.
Self-assembly of globular proteins can be induced in different ways, e.g.,by
adding solvents or denaturants, heating, or applying pressure. The aggregation
leads to gel formation if the concentration is suciently high. 9 Aggregation and
gelation can also be induced by changing the pH or adding salt. In the latter
case one often uses small aggregates formed by a pre-treatment of the native
globular protein. This process is generally called as 'cold gelation' in the
literature. 10 The structure of globular protein aggregates and gels has been
investigated using the complementary techniques of microscopy and scattering.
Globular proteins have different sizes and shapes, and different secondary
structures. Depending on the pH they have different charge densities and
charge distributions. The types of interaction involved in the assembling
include covalent bonding through the formation of disulfide bridges, charge
interactions, hydrophobic interactions, and hydrogen bonding. 10 It would
therefore be naı¨ ve to imagine that one can use the simple model of attractive
hard spheres to explain the self-assembly of all globular proteins. Nevertheless,
remarkably similar structures are formed by different globular proteins. The
differences in the structure for a given protein under different conditions of pH
and ionic strength are often larger than between different proteins under the
same conditions. This observation justifies discussing the self-assembly of
globular proteins in general terms.