Self-Assembly of Synthetic and Biological Polymeric Systems of ...

In vivo, the self-assembly of proteins into highly ordered, β-sheet enriched, fibrillar, insoluble,
supramolecular structures known as amyloid fibrils (or amyloid-like fibrils, term referring to
protein fibrils that not associated with any disease) is linked to the onset of a growing number
of human disorders, including Alzheirmer’s, Parkinson’s, Huntington´s diseases, spongiform
encephalopathies, and type II diabetes mellitus. On the other hand, the controlled self-
assembly of proteins and peptides into amyloid-like structures may constitute an attractive
alternative to develop nanomaterials (106)(107).
Amyloid fibrils can be obtained in vitro through different ways by adjusting different external
parameters as, for example, pH, ionic strength, and temperature, and cosolute or cosolvent
additions. This means that the formation of amyloid-like conformations is not restricted to
peptide chains of specific length or having a particular primary or secondary structure; this
suggests that the conformational shift into β-sheet rich is energetically preferred. The
characterization of the fibrillation process of proteins in vitro has largely been focused on
biophysical characterization in order to determine the structure of the fibril, and on
biochemical studies to get into the mechanism and kinetics of the process. Also, through
techniques such X-ray diffraction, electron microscopy, and solid state NMR spectroscopy we
now have a good understanding of the core architecture of individual fibrils too.
In general, amyloid fibrils possess distinct physical properties as: i) they are characterized by a
cross-β sheet structure, where individual β-strands are perpendicular and each β-sheet is
parallel to the fibril axis (Figure 5.4). The X-ray diffraction pattern shows two characteristic
reflections at 4.7 Å and 10-11 Å, corresponding to the inter-strand and stacking distances
between individual β-sheets, respectively; ii) amyloids bind to histological dyes such as Congo
red (CR) and Thioflavin T (ThT). After binding to CR, an apple-green birefringence under cross-
polarized light is observed. CR and ThT dyes, however, do not bind to monomeric proteins or
peptides. These dyes fluoresce when they bind to β-sheet rich fibrils and are, therefore, useful
for spectroscopic monitoring of fibril´s growth and kinetics; iii) under electron microscope
(EM), amyloid fibrils appear to be few micrometers long, non-branched filaments, with 7-12
nm diameter. In a majority of cases, amyloids are composed of 2-4 protofilaments that are
either helically twisted or laterally associated with each other forming higher order fibrils; iv)
amyloids are resistant to heat, wide ranges of pH and proteases (107)-(113).
Over the past several years, soluble proteins and protein fragments self-assembled into
amyloid aggregates have attracted the interest of researchers from diverse fields. From a
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