Similarities in Self-Assembly of Proteins and Surfactants: An Attempt to Bridge the Gap - Food Colloids: Self-Assembly and Material Science - page 66

will have a pronounced influence on the fibril length distribution. In the early
stages of the self-assembly, when the typical time that the protein remains
attached to the fibril's end is much longer than the time necessary to form a
permanent bond, the fibril will grow and will be able to form irreversible bonds
(b-sheet structures). The proteins in the fibril are in a 'trapped state' as a result
of the high bond energy of the b-sheet. This, together with the high charge
density at low pH, and the strong hydrophobic interactions at elevated tem-
perature, may explain that, under these circumstances, one obtains fibrils as
opposed to aggregates of fractal dimension much higher than 1.
The irreversibility does, in general, not apply to protein molecules attached
to both ends of the fibril. If this were to be the case, all the proteins would
eventually be incorporated in the fibrils - something contrary to our observa-
tions. At concentrations low enough, the typical time the average protein
molecule remains attached to the fibril's end is much shorter than the time
necessary to form a permanent bond; it is a two-step mechanism, where the
protein first attaches to the fibril and subsequently becomes incorporated in the
b-sheet of the fibril. Due to the partially irreversible assembly of most of the
proteins in the fibril, only limited rearrangement of protein monomers can
occur. Therefore we expect the final length distribution to be somewhat wider
than the Poisson distribution, but narrower than predicted by Equation (21).
After a mild denaturation step, at an elevated temperature and pH ¼ 2, the
protein partially unfolds leading to an effective attraction between the proteins.
As the protein molecules are in a head-to-tail arrangement within the fibril, this
suggests that the attraction is of a dipolar nature. This attraction could very
well be the sum of the different forces involved, i.e., electrostatic, hydrophobic,
etc. In order to initiate fibril growth, a nucleation event is necessary. Initially
the assembly is reversible, but after some time (typically hours) the fibril is
stabilized by a crossed b-sheet structure, running perpendicular to the long axis
of the fibril. As a result of this irreversibility, thermodynamic equilibrium
theories can only be applied with caution. The system might be kinetically
trapped in the later stages of the assembly, and as such cannot reach its
equilibrium by rearranging the proteins between different fibrils or even
spherulites. It is unknown if the assembly is entropy-driven or enthalpy-driven.
It remains further to be tested whether at short times the above suggested
attempt for an equilibrium approach to protein assembly into fibrils is appli-
cable, and whether the equilibrium approach can be extended to regions of
higher salt concentration and different pH.
References
1. J. Israelachvili, Intermolecular and Surface Forces, 2nd edn, Academic
Press, London, 1992.
2. J.N. Israelachvili, D.J. Mitchell and B.W. Ninham, J. Chem. Soc. Faraday
Trans. II, 1976, 72, 1525.
3. C. Tanford, J. Phys. Chem., 1974, 78, 2469.