Biomedical Engineering Reference
In-Depth Information
9.5.4
Non-ligated fibrin gels
The precursor of the blood clotting
fibrin gels is
fibrinogen, a small rod-like
protein. The usual
fibrin gels are produced by enzymatic treatment of
fibrin with
the peptide
fibrinoligase
-
known to experts in the
field as clotting factor XIIIa
(Berg et al.,
2007b
)
cross-linked structure, the so-called
ligated gels. Nevertheless, gels can be produced without this treatment and so they
arediscussedhere.TheycanbepreparedbyalteringpHandionicstrength,and
show typical
-
to produce a
'
chemical
'
fibrillar strands under microscopic examination. Another reason for
including them here is that the rheological work carried out in the mid-1970s is a
model study from what was then the world
'
s leading laboratory in this area, led by
the late J. D. Ferry.
Roberts and co-workers ( Gerth et al.,
1974
; Roberts et al.,
1974
) described a series of
measurements of such
fibrin clots, both ligated and unligated, using the specialized
Birnboim apparatus. Various preparation regimes gave rise to gels described as
'
ne
'
. Unligated clots showed little G
0
frequency dependence, and a
concentration dependence around c
1.5
. Creep measurements showed some
and
'
coarse clots
'
ow, perhaps
slip induced in the coarse clots, but little for the
finer materials. A later study found a
slightly higher concentration exponent, around 1.9. It was suggested that the elasticity
did not arise simply from steric effects, which would give higher exponents still, but that
there was some branching, as seen in later EM pictures (Muller et al., 1984), and recent
work has examined strain stiffening. Both the similarities and the differences between the
rheology of these materials and gels formed from tubulin and actin have been discussed
(Janmey et al., 1991; Rammensee et al., 2007).
9.5.5
'
Amyloid
'
protein gels formed in non-aqueous media
The self-assembly of proteins and peptides can be induced by a variety of treatments
other than heating, most of which initiate partial unfolding of the native monomer, so that
such methods as solvent or chemically induced unfolding of the protein are possible. The
denaturation of proteins in alcohol
water mixtures, in particular, is a well-documented
feature of protein chemistry, leading to the formation (in most cases, and at appropriate
molar concentrations of solvent) of an expanded helical conformation often referred to as
the
-
'
H-state
'
(Tanford et al.,
1962
; Buck,
1998
).
-Lg, numerous studies (for example, using ORD and CD spectroscopy) have
shown that the largely
For
β
β
-sheeted secondary structure of this protein can be converted to
amainly
-helical form upon addition of various alcohols, and at the same time the
protein expands (Tanford et al.,
1960
; Townend et al.,
1967
; Kamatari et al.,
1996
). In
addition, the partially folded intermediate
α
is formed prior to such
transformation to the H-state. The H-state itself, however, seems to be unstable and
prone to aggregation. Hence, in some cases,
'
molten globule
'
fibrillar aggregates have been shown to
occur in alcohol
for example, lysozyme in high molar concentrations
of ethanol (Goda et al.,
2000
). Aggregation of the
-
water mixtures
-
β
-Lg H-state has been investigated
-
mostly at neutral pH, and in ethanol
water mixtures.
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