Structure of Self-Assembled Globular Proteins - Food Colloids: Self-Assembly and Material Science - page 35

Chapter 3
Structure of Self-Assembled Globular
Proteins
Taco Nicolai
POLYME ` RES, COLLOI ¨ DES, INTERFACES, UMR CNRS, UNIVERSITE ´
DU MAINE, 72085 LE MANS CEDEX 9, FRANCE
3.1 Introduction
Large structures can be created by assembling many smaller elements. The way
we do this is determined by the properties that are sought after in the structure.
So, for example, we build a bridge in such a way that it has the property of
spanning a river. There is feedback from the overall structure to the assembling
process, as the latter is adapted to obtain the desired structure. Similarly,
biomolecular structures formed by an evolutionary process, such as globular
proteins, also result from this feedback mechanism, because assembling proc-
esses that do not lead to successful structures are less frequently reproduced.
The elementary units - polymers, colloids, proteins, and droplets - may
themselves spontaneously assemble to form larger structures that can be very
complex. This self-assembly is caused by the interaction between the particles
without feedback from the large-scale structure, which therefore may be
considered epiphenomenal. The large-scale structure is entirely determined by
interactions on small length-scales. Sometimes, the structures formed by self-
assembly have useful properties that can be exploited to create new materials. If
we are able to predict the large-scale structure on the basis of the local
interaction, we can try to modify the latter to obtain desired properties for
the former.
Unfortunately, it is generally far from straightforward to predict the large-
scale structure even if we have detailed knowledge of the particles and their
interaction. A system of hard Brownian spheres that stick irreversibly when
they collide (diffusion-controlled aggregation) represents one of the simplest
self-assembly processes; nevertheless, it leads to very complex structures, 1 as
illustrated in Figure 1. These structures can only be understood by invoking the
concepts of self-similarity and fractal scaling. The number m of particles in the
aggregates increases with the radius of gyration R g of the aggregates following a
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