Biomedical Engineering Reference
In-Depth Information
Amino-acid analysis of the spider silk used
for orb-web building showed that these silks are
characterized by the presence, in very high pro-
portion, of glycine and alanine residues for drag-
line silks and glycine and proline residues for
flagelliform silks [34] . This finding has strong
implications for silk properties in that glycine
and alanine are very simple molecules, compris-
ing hydrogen and a methyl group, respectively,
leading to linear polymers.
X-ray diffraction and nuclear magnetic reso-
nance spectroscopy studies demonstrated that
the dragline silk fiber protein contains glycine-
rich repeats alternating with alanine rich-repeats;
crystalline regions made of linked polyalanine
regions are organized in anti-parallel β -sheets
alternating with amorphous regions [35] . The
polyalanine domains can form noncovalent
cross-links or hydrogen bonds between the indi-
vidual proteins and alternate either with a gly-
cine-rich proline containing pentapeptide,
supposed to be responsible for the fiber's elastic-
ity [36] , or with a glycine-rich tripeptide, which
forms a small 3 10 helix (3 turns in 10 amino-acid
units), supposed to link highly crystalline and
more amorphous regions [37] . The structure of
the pentapeptide motif is modeled after that of
the elastin pentapeptide and is thus expected to
form β-turns, which in tandem would take the
shape of an elastic β-spiral and act as a spring
[37] . Spider dragline silks are very tough fibers
due to a combination of high tensile strength
(400 kpsi) and high extensibility (35%). Flagelli-
form silks are also very tough fibers, far more
elastic than dragline silks (up to 200%) as a result
of the presence of these long regions containing
repeats of several elastic motifs [31] .
FIGURE 10.6 Female Nephila clavipes on a female Homo
sapiens digit. (Photo: Janci Despain.)
are all orb-web weaver spiders whose survival
relies on catching insects in an elaborate web
structure. These types of spiders possess seven
types of glands and are able to produce seven
types silks of very different mechanical properties
and compositions that each have a determined
function and use (i.e., web-building silks, cocoon
silk, dragline, and swathing silk). More primitive
spiders (i.e., Mygalomorphae ) only have one type
of silk gland producing a silk used for cocoon
swathing or, in some cases, lining the borrow of
certain underground trapdoor spiders such as
Antroaediatus unicolor .
The best-characterized silks are the dragline
silk (major ampullate silk) used for construction
of the web frame or as a safety line when the orb
weaver spider drops from elevations, and the
viscid silk (or flagelliform silk) forming the cap-
ture spiral of the web. These silks differ in their
repeat structures, resulting in different tensile
strengths and elasticities.
HAGFISH SLIME THREADS
In recent years, spider silks have been presumed
to be a model material for the design of protein
fibers that possess substantial physical proper-
ties and are ultimate sustainability; unfortu-
nately, attempts at making artificial spider silks
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