Helical structures from neutral biopolymers - Physical Gels from Biological and Synthetic Polymers - page 210

Figure 7.20.

Schematic representation of agarose network structure composed of bundles of double helices.
Adapted with permission from Arnott et al.( 1974 ) © 1974 Elsevier.

would explain the appearance of the gel, usually slightly hazy or turbid because of the
light scattered from aggregated helical regions. There is a view that the gelation of
agarose is actually driven by the of the sulphated form. Without these

acting to terminate the double helices, a 100% pure agarose would most likely precipitate
out on cooling. However, a more detailed explanation in terms of a spinodal decom-
position mechanism is described below.
Consequently the large hysteresis loop between the gelation and melting temperatures
of the gels is suggested as indicating the substantial amount of helix aggregation accom-
panying the network building. The bundles of double helices create a rigid framework,
shown in Figure 7.20 . SAXR measurements support this view (Djabourov et al., 1989 ). It
is observed by this technique that the scattered intensity scales exactly with concentration
between 0.1 and 5 wt% and is not sensitive to time or temperature between 34°C and 4°C,
provided the sample is left to

' rogue residues '

2 h. The angular distribution of the scattered
intensity was found to be consistent with a model based on a

' set ' for 1 -

fibrous structure containing

rod-like fibres of variable thickness. Computed intensity curves assuming two populations

of 3 and 9 nm diameters with roughly equal weights (on average 7 times as many thin rods
as thick ones) are supported by TEM imaging, which mainly shows the population of
thin rods. Using structural parameters (molecular dimensions and mass per unit length)
proposed by Arnott et al.( 1974 ), it was found that the thin fibres are consistent with a

hexagonal packing of double helices, while the larger aggregates consist of assemblies of
the thin fibres. The distribution of the aggregation number is likely to induce the thermal

hysteresis which is well established in these gels. Because these measurements were not
time-resolved, the two steps, consisting of growth of double helices followed by sub-
stantial aggregation, could not be separated in this experiment.
DSC work on agarose gels seems to have concentrated on the low-temperature behav-
iour and on the sol - gel transition. A low-temperature peak was detected below 0°C and