Amyloid-Based Natural Adhesives and Cements - The Functional Fold: Amyloid Structures in Nature - page 141

should not be confused with the persistence length of the fibril itself).
The contour length corresponds to the “hidden length” exposed on
release of the sacrificial bonds within each structural unit, and is
directly related to the number of amino acids within this hidden
length.
Quantitative variations have been found between functional
amyloid from different biological adhesive systems (Fig. 8.5).

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These
are thought to be primarily associated with the differing number
of amino acids in the constituent monomers. Where details of the
monomer are known, as in the case of curli (see Chapter 7), there
is a good correspondence between the quantitative characteristics
of the force-extension curve and the anticipated hidden length
within the folded monomer.

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The relative toughness in each case
could be calculated from the area under the force-extension curve
as this corresponds to the energy required to pull the material apart.
The relative toughness of functional amyloid measured was

found to be consistently higher than that measured for amyloid
fibrils formed

ex vivo

, which may indicate an underlying structural
difference between the two.

in vitro

However, in all cases, the molecular
level origin of the cohesive strength was associated with the generic
amyloid intermolecular β-sheet structure, whereas adhesive residues
appeared to lie outside the amyloid core, as deduced by comparing
the mechanical responses of amyloid fibrils with and without a
specific adhesive function.

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An additional material characteristic of amyloid fibrils of benefit
to the mechanical properties of the adhesive is the high degree
of rotational symmetry provided by the fibril structure itself.
Surprisingly, some force traces taken for algal adhesives while
approaching the surface have also shown sawtooth structures,
indicating unravelling of

β

-sheets as the tip is pushed into the

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adhesive matrix (Fig. 8.6). The smaller persistence length on

approach, compared to sawtooth structures in the retraction
curves, corresponds to the additional stiffness of the matrix
material acting in parallel with the fibril. Sawtooth mechanical
responses such as these indicate the multi-directional toughness
provided by the fibrillar structure. The ability of functional amyloid
in natural adhesives to rapidly self-assemble was also apparent in
the mechanical data from algae adhesives where sawtooth structures
re-formed between successive approach-retraction scans. This

rapid reassembly indicates the ability of the material to self-heal.

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