Toward Intelligent Materials - Molecular Self-Assembly: Advances and Applications - page 7

Several examples of amyloid materials have been identified across
the different branches of life. Fungi exploit amyloid aggregates as
epigenetic, heritable elements that form the basis of distinct strains
[51]. For example, Het-s amyloid peptide co-assembly mediates
self/non-self recognition and prevents incompatible fusions of
fungal strains [20]. expresses amyloid as

a material component of their egg shell as a structural barrier for
the developing silk moth embryos [52], and in barnacles, amyloid
provides a durable adhesive for surface attachment [53]. Prokaryotes
produce amyloid as the structural foundation for biofilms [54,55],
and mammals may use amyloid fibers formed from Pmel17, an
integral membrane glycoprotein found in melansomes, for directing
the polymerization of the skin pigment polymer melanin [56,57].
As the diversity of cross-

Austrofundulus limnaeus

assembly structures continue to be
discovered [21,58], there is likely to be an ever increasing range of
new materials applications.

β

assembly is
derived from the nucleating core of the 42-residue A

Currently, the most thoroughly analyzed cross-

β

β

peptide of

Alzheimer's disease, KLVFFAE or A (16-22) [21,59-66]. When end

capped, acetylated at the N-terminus and amidated at the C-terminus,
the charge in the Aβ(16-22) peptide resides only in the side chains of
the terminal lysine (K) and glutamic acid (E) residues. A remarkable
feature of this simpler peptide is that these charges have a profound
effect on their assembled morphology, directing either twisted 5 nm-
diameter soluble fibers under neutral conditions or soluble hollow
52 nm-diameter nanotubes when the conditions are more acidic
(Fig. 1.3). Initial scattering analyses supported a bilayer architecture
for the nanotubes with the amphiphilic strands arranged in parallel
β

β

-sheets [21]. These constraints logically placed the charged lysines
on the outer surface of the tube walls; however, solid-state NMR
analyses established that both fiber [23,61] and tube [23]

β

-sheets

were anti-parallel. Further, the fibers contain in-register -sheets

while the nanotube strand registry is shifted to expose the terminal
lysine on the tube surface (Fig. 1.3B,E). This structure model is
remarkable in two ways. First, that a simple protonation of the E
side chain could so impact the morphology, and second, that fully
half of these charged side chains in both structures must be buried
within a bilayer interface.

β