Carrageenan is a linear polysaccharide, more specifically a galactan with the galactose residues linked with alternating

(13) linkages and

(14) linkages

In addition, the galactose units linked (13) in this general structure often occur as 3.6-anhydro-D-galactose and sulfate ester groups may be present on some or all galactose units.

It is evident that since carrageenan is a large molecule being made up of some 1000 residues, the possibility for structural variations are enormous. In order to simplify matters it is convenient to define three main types, kappa, iota, and lambda carrageenans, which are idealized molecules assigned definitive structures. Hardly any seaweed yields these ideal carrageenans. The normal situation is probably a range of intermediate structures.

Idealized Structures

Galactose residues in carrageenan
Kappa carrageenan is made up of

(14) D-galactose-4-sulfate and

(13) 3.6-anhydro-D-galactose.

In a kappa-type seaweed extract some of the D-galactose contains 6-sulfate ester groups and some of the 3.6-anhydro-D-galactose contains 2-sulfate ester groups.

6-sulfate ester groups reduce the gelling power considerably, but by alkali treatment it is possible to Tran eliminate 6-sulfate groups, which results in the formation of 3.6-anhydro-D-galactose imparting a higher degree of regularity to the molecule and thus the gelling power is increased.

Kappa-type extracts are made from Eucheuma cottonii and some Chondrus and Gigartina species. Iota carrageenan is characterized by having 4-sulfate ester groups on all D-galactose residues and 2-sulfate ester groups on all 3.6-anhydro-D-galactose residues. 2-sulfate ester groups are not removed by alkali treatment.

As in the case of kappa carrageenan, iota carrageenan also shows irregularities in the form of 6-sulfate ester groups on some D-galactose residues and through alkali treatment the iota carrageenan becomes more regular.

It follows that a lack of 2-sulfate ester groups on some 3.6-anhydro-D-galactose residues makes iota carrageenan more irregular. Lambda carrageenan differs from kappa and iota carrageenan by having a disulfated-D-galactose residue and no 4-sulfate in the -D-galactose residue. Instead of 4-sulfate ester groups lambda carrageenan contains variable amounts of 2-sulfate ester groups.

The position of sulfate ester groups in determined with an infrared spectrophotometer.

Typical infrared spectra are shown below:

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