Carrageenans. Part IV. Variations in the structure and gel properties of κ-carrageenan, and the characterisation of sulphate esters by infrared spectroscopy
Structures of the ‘masked repeating’ type seem to occur generally in κ-carrageenans from Chondrus crispus and Gigartina species and will account for the analytical variation between one sample and another. Most—possibly all—of the variation is in the 4-linked units which occur as 3,6-anhydrogalactose, 3,6-anhydrogalactose 2-sulphate, galactose 6-sulphate, and galactose 2,6-disulphate. In κ-carrageenan from G. stellata, as from C. crispus, all galactose units are believed to have the D-configuration. Samples from Chondrus were similar to each other and had a lower content of both types of 2-sulphate than those from Gigartina. The separation of a Chondrus sample into a series of subfractions which differed in the relative proportions of the various 4-linked units, suggests that κ-carrageenan might normally contain a mixture of related molecules. Evidence is given that absorption bands shown by sulphated 3,6-anhydrogalactose units in the 800–850 cm.–1 region of the infrared spectrum cannot be interpreted in the simple stereochemical terms used previously. However, many carrageenans show a band at about 805 cm.–1 which is characteristic of 3,6-anhydrogalactose sulphate and potentially useful because it is easily recognised, even in the presence of other sulphate esters. Gel formation by κ-carrageenan is enhanced by removal of 6-sulphate with conversion into 3,6-anhydrogalactose, but is much less sensitive to an excess of 2-sulphate. The variation of gel strength with polysaccharide structure provides some evidence on the molecular basis of gel formation.