Printer friendly versionIntroduction to Carrageenan
Properties
Solution properties
Most of the work on carrageenans has focused attention on their properties in the ordered state. However work on carrageenan in its disordered state (Vreeman)has been looked at using light scattering, viscosity and sedimentation experiments. Disordered carrageenans are shown to be random coils with similar chain flexibilities to other polysaccharides but with a slightly expanded structure due to the higher charge density from the sulphate side groups.
The intrinsic viscosity of a disordered carrageenan solution can be related to the molecular weight with the well known Mark-Houwink equation.
The value of the K and α parameters vary with solvent quality and with the type of molecular weight average used in the determination.At low salt concentrations the exponential factor is slightly below 1 suggesting a slightly expanded chain conformation.
Gel Properties
Xray diffraction studies have shown that kappa, iota and the mixed kappa/beta
carrageenan from Furcellaran all give x-ray diffraction patterns that show
that they form ordered helices in the ordered state.
(Anderson,
Cairns
Arnott)
According
to recent evidence
(Millane)
The best fit for kappa carrageenan was a parallel double helix with a pitch of
about 2.5nm. The iota helix appear to be generally similar to the kappa helix.
There is some debate about the exact conformation of the kappa carrageenan
helix with some evidence of the chains in kappa carrageenan being rotate and
translated relative to each other. however this is disputed by some other authors
(Cairns).
Early work on the thermal transition of carrageenan showed that the transition was accompanied by a change in the optical rotation.(Rees). Rees also showed that the calculated molar rotation for iota carrageenan in the ordered state corresponded closely to a double helix formation whereas in the disordered state it closely corresponded to a random coil.
Further direct evidence of the conformational transition has been obtained from carbon-13 and proton NMR studies of the of the carrageenan chain. The disordered conformation gives a well resolved NMR indicative of a highly mobile system with long relaxation times. However the ordered conformation has broad NMR peaks due to quicker relaxation times and this suggests a much more ordered conformation. Other methods employed to analyse the transition have included light scattering, osmometry,intrinsic viscosity, calorimetry, counter ion activity, counter ion diffusion and ion NMR.
Oligomers of carrageenan can also be forced to undergo the transition. This has been shown for kappa and iota. It has been found that the minimum length of kappa chain required to undergo the transition is four units.
Theoretically it is possible to relate the degree of polymerisation and intrinsic co operativity to the distribution of helical units at any degree of helix formation. However in practise this approach is complicated for carrageenan by the high level of structural heterogeneity along the polymer chain and the level of polydispersity of the samples. Reid(Reid) has shown that the transition curve can be rationalised in terms of molecular weight distribution by assuming an "all or nothing" process involving the dimerisation of chains of matching length. NMR studies have shown the peak broadening is due to superposition of many peaks rather than actual being due to averaging and this supports the idea of the "all or one" proposition.
The kinetics of the transitions has been shown to be second order which once again fits in with the bimolecular double helix formation theory (Norton)
As a general rule the addition of extra ions to a gelling system increases the stability of the helix.Rochas has shown that generally for any pure ionic form of carrageenan Tm is a linear function relative to the log of the total cation concentration.It appears that cations can be divided into three categories with respect to their helix promoting abilities. The specific cations show much stronger helix promotion than the non specific ions.
It appears that some cations actually bind to the helix of the kappa carrageenan polymer and this promotes helix formation. Studies on Furcellaran have shown very similar results to kappa carrageenan. Iota is different in that strong helix stabilisation is shown for divalent ions such as calcium. There is not thought to be any site specific binding in the calcium / iota interaction but merely a stronger affect due to the higher charged density of the polymer and the cation. This lack of ion site binding in iota carrageenan suggests that the binding seen in kappa carrageenan is due to the specifics of the chain conformation rather than a binding between sulphate oxygen atoms.
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