Biomedical Engineering Reference
In-Depth Information
The agarose molecule consists of
β
(1
→
3)-linked d-galactose and
α
(1
→
4)-linked
3,6-anhydro-
α
L-galactose residues.
Figure 7.19
form (Norton et al.,
1986
), just like the carrageenans. That said, some
data in
the literature has actually been taken using agar rather than agarose, and so care has to be
taken when comparing results obtained by various workers.
As mentioned above, the partly puri
'
agarose
'
ed agar is widely employed as a medium for
microbial growth; in this application a mixture of agar and appropriate electrolyte and/or
low molecular mass nutrients is dissolved by autoclaving, poured into a Petri dish and
allowed to cool and gel; then the microbes of interest can be
by gently
scratching the agar plate with a needle dipped in the appropriate solution. The agar in
the dish therefore serves both to encourage and sustain growth and as a solid support. From
this application, some of the mechanical properties of the agarose can already be deduced,
i.e. that it gels on cooling and that the resultant gel has suf
'
plated out
'
,i.e.modulus,to
act as a rigid support, but that it is brittle (easily scratched) (Matsuhashi,
1990
).
cient
'
strength
'
7.3.1
Gelation mechanism
Early work on establishing the gelation mechanism and macromolecular structure of
agarose gels (Dea et al.,
1972
) used the technique of optical rotation to investigate what
was happening. There is a large and relatively sharp change in the OR, occurring
at around 35
40°C, on cooling a heated solution (100°C or higher, in an autoclave) of
0.2 wt% agarose. On reheating the solution (and 0.2 wt% is quite close to the critical gel
concentration) there is a very large hysteresis and the OR does not change very signifi-
-
-
cantly until the temperature exceeds 80°C. This clearly suggests that some ordered
structure is formed and that, once formed, it is quite stable. However, without the help
of other techniques, OR cannot establish the form of the ordered structure unequivocally,
although parallels with gelatin and other gelling polysaccharides would suggest some
helical structure. That said, a more quantitative re-examination of the OR and its
behaviour at lower temperatures (Schafer and Stevens,
1995
) strongly supports the
double helix picture found fromX-ray measurements and described in more detail below.
Wide-angle X-ray measurements on oriented
bres by Rees, Arnott and co-workers
(Arnott et al.,
1974
) indicated that the ordered structure was due to a double helix, so that
this was the origin of the gelation process. The particular double helix proposed has 0.95
nm axial periodicity. Each chain in the double helix forms a left-handed, three-fold helix
of pitch 1.90 nm and is translated axially relative to its partner by half this distance. This
model accounts for the sign and magnitude of the optical rotation shift that accompanies
the sol
gel transitions. However, the agarose network is described as arising by double
helix formation and the subsequent aggregation of these helices into bundles. This alone
-
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