Biomedical Engineering Reference
In-Depth Information
fibrils can be stained with the dye Congo Red (Hughes and Dunstan,
2009
), and
show a distinct green birefringence when viewed under polarized light. An alternative stain
is Thio
Amyloid
avin T (Krebs et al.,
2005
). Under appropriate microscopy all show uniform,
smooth and persistent
fibrils with a strand thickness of the order of 8 nm, and all show a
distinct form of
-sheet structure. Similar properties have already been noted above for the
low-pH gels, although amyloid structures in vivo often involve very speci
β
c proteins, which
can convert spontaneously from soluble protein to insoluble plaques (Dobson,
2001
).
Gosal and co-workers (Gosal et al.,
2002
) studied the gelation and amyloid formation
of
-Lg by dissolving the protein in aqueous alcohols. Images of the systems (made at
sub-gelling concentrations) show the expected
β
fibrils, and rheological measurements of
the gels showed some features to be very similar to those of the heat-set pH 2 systems,
although, compared to these, G
00
sometimes showed a minimum, perhaps suggesting that
these were more
flexible networks.
9.5.6
Gels from peptide self-assembly
Over the last decade or so there has been a great deal of interest in developing
fibrillar
self-assembled, so-called biomimetic, systems from specially synthesized peptide
sequences. Among the pioneering work, that by Boden and co-workers is particularly
noteworthy. They were able to design anti-parallel
β
'
'
(Aggeli et al.,
1997a
,
1997b
) which formed highly entangled gel-like systems, and their rheological properties
could be dictated by either shear history or chemistry, for example by altering solvent
polarity or hydrogen bonding ability. Altering the response to such in
-sheet
tapes
uences could be
achieved by modifying the properties of the
β
-sheet. A vast array of
fibrillar structures
(proto
fibrils and ribbons) were obtained by changing the pH or ionic
conditions. Yet another useful peptide
laments,
'
motif
'
is the coiled coil (Petka et al.,
1998
;
Wang et al.,
1999
).
Both pH and temperature control assembly, making the resultant structures thermo-
reversible, and the response can be encoded into the peptide sequence; even conforma-
tional switching from a coiled-coil to
fibril has been achieved by introducing acyl
chains (Takahashi et al.,
1999
). This introduces a hydrophobic defect, which controls
assembly. Recent reviews which cover this topic in more detail are those by Hamley
(
2007
), Jung et al.(
2010
) and Ho et al.(
2011
).
Of particular note here is the recent themed journal issue on peptide- and protein-based
materials, which summarizes both systems and applications (Ulijn and Woolfson,
2010
).
Many of the articles in this issue are of relevance, but we concentrate on the review
of rheological measurements on peptide gels (Yan and Pochan,
2010
). Pochan and
co-workers have established a very sound reputation in this area, and their work on the
MAX1 and MAX8 peptides, described there, is particularly signi
β
-sheet
cant.
β
-turn sequence, with adjacent residues of
the hairpin containing alternating hydrophobic (valine) and hydrophilic (lysine) residues.
In aqueous solution this forms a short piece of
MAX1 is a 20 amino acid containing a central
coil because the positively charged
lysines prevent folding. However, in electrolyte solution the positive charges are screened
out, and then the peptides fold into hairpins, which in turn self-assemble to give a
'
random
'
'
rigid
'
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