Molecular structure, κ-carrageenan, ι-carrageenan, λ-carrageenan, Helices
is a collective term for polysaccharides prepared by alkaline extraction
(and modification) from red seaweed (Rhodophycae) , mostly
of genus Chondrus, Eucheuma, Gigartina and Iridaea. Different seaweeds produce different carrageenans.
consists of alternating 3-linked-β-D-galactopyranose
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Carrageenans are linear polymers of about 25,000 galactose derivatives
with regular but imprecise structures, dependent on the source and
extraction conditions .
Idealized structures are given below and κ-carrageenan,
for example, has been found to contain a small proportion of the
dimer associated with ι-carrageenan.
is produced by alkaline elimination from μ-carrageenan
 isolated mostly from the tropical seaweed Kappaphycus
alvarezii (also known as Eucheuma cottonii).
The experimental charge/dimer is 1.03 rather than 1.0 with
0.82 molecules of anhydrogalactose rather than one.
is produced by alkaline elimination from ν-carrageenan
isolated mostly from the Philippines seaweed Eucheuma
denticulatum (also called Spinosum). The experimental
charge/dimer is 1.49 rather than 2.0 with 0.59 molecules
of anhydrogalactose rather than one. The three-dimensional
structure of the ι-carrageenan
double helix has been determined 
as forming a half-staggered, parallel, threefold, right-handed
double helix, stabilized by interchain O2-H···O-5
and O6-H···O-2 hydrogen bonds between
units (see structure). The structure of some calcium iota-carrageenans have been determined 
(isolated mainly from Gigartina pistillata or Chondrus
crispus) is converted into θ-carrageenan
(theta-carrageenan) by alkaline elimination, but at a much
slower rate than causes the production of ι-carrageenan
The experimental charge/dimer is 2.09 rather than 3.0 with
0.16 molecules of anhydrogalactose rather than zero.
The torsion angles phi (φH,
H1C1OC4 or H1C1OC3), psi (ψH,
C1OC4H4 or C1OC3C3) have been determined for λ-, μ- and ν-carrageenans . All carrageenans are highly flexible molecules which, at
higher concentrations, wind around each other to form double-helical
zones. Gel formation in κ-
involves helix formation on cooling from a hot solution together
with gel-inducing and gel-strengthening K+ or Ca2+ cations respectively (not Na+, although Na+ does take part in an aggregation process to form weak gels
due to phase separation ),
which not only aid helix formation but subsequently support
aggregating linkages between the helices so forming the junction
zones. The strongest gels of κ-carrageenan are formed with K+ rather than Li+, Na+, Mg2+, Ca2+, or Sr2+ . Incomplete formation of 1C4 3,6-anhydro-links
will reduce the extent of helix formation as the unbridged α-linked galactose residues may
flip to the 4C1 conformation.
κ-Carrageenan forms stronger gels in D2O rather than H2O. This has been attributed to an increase in the number of double-helix aggregates rather than the fraction of helix formed, which differs little between the two solvents . This is probably due to the increase in the affinity of D2O molecules for each other relative to polysaccharide water linkages..
Note that the gelling hydrocolloid agar is produced from the same family of seaweeds, the major difference
being the presence of L- rather than D-3,6-anhydro-α-galactopyranose
units but still forming double helical junction zones. [Back to Top ]
Carrageenans are used mainly for thickening, suspending and gelling. κ- and ι-carrageenans
form thermoreversible gels on cooling in the presence of appropriate
counter ions. κ-Carrageenan
forms a firm clear, if brittle, gel with poor freeze-thaw stability;
the coil-double helix transition being followed by a K+-induced
aggregation of the helices . κ-Carrageenan
gels may be softened (and is generally regarded to be synergistically
strengtheneda) with locust
bean gum. ι-Carrageenan
has less specific ionic binding but increased ionic strength allows
helices to form junction zones in soft elastic gels with good freeze-thaw
is non gelling as the lack of the 1C4 3,6-anhydro-link
allows the galactose residues to revert to their 4C1 conformation which does not allow the initial double helix formation
required for gelling. Additionally, the high density of charged
sulfate groups encourages an extensive conformation. λ-Carrageenan
has been found to act as a cryoprotectant and improves the freeze-thaw
behavior of locust bean gum.
stabilizes milk κ-casein
products due to its charge interaction with the casein micelles
(~200 nm diameter); their incorporation into the network preventing
whey separation. Such complexes are soluble when both have same
charge and are held together by counter ions or oppositely charged
patches. Carrageenan is also used as a binder in cooked meats, to
firm sausages and as a thickener in toothpaste and puddings.
It may be noted that a cancer health scare concerning degraded
carrageenan has recently been examined by the European
Commission Scientific Committee on Food which found no evidence
in support and states that carrageenan is safe to use in foods. It is believed that carrageenan is safe to use in foods, within current regulations .
Interactive structures are available (Jmol). [Back to Top ]
a However, a recent paper found no such
synergy . [Back]