are produced by brown seaweeds (Phaeophyceae, mainly Laminaria).
Alginates are linear unbranched polymers containing β-(14)-linked
D-mannuronic acid (M) and α-(14)-linked
L-guluronic acid (G) residues. Although these residues
are epimers (D-mannuronic acid residues being enzymatically converted
to L-guluronic after polymerizationa)
and only differ at C5, they possess very different conformations;
D-mannuronic acid being 4C1 with diequatorial links between them and L-guluronic acid being 1C4 with diaxial links between them. Bacterial
alginates are additionally O-acetylated on the 2 and/or 3 positions
of the D-mannuronic acid residues. The bacterial O-acetylase may
be used to O-acetylate the algal alginates, so increasing their
water binding. [Back to Top ]
Alginates are not random copolymers but,
according to the source algae, consist of blocks of similar
and strictly alternating residues (that is, MMMMMM, GGGGGG and GMGMGMGM), each
of which have different conformational preferences and behavior.
As examples, the M/G ratio
of alginate from Macrocystis pyrifera is about 1.6
whereas that from Laminaria hyperborea is about 0.45.
Alginates may be prepared with a wide range of average molecular
weights (50 - 100000 residues) to suit the application.
D-mannuronate prefers forming a 3-fold left-handed helix with (weak)
intra-molecular hydrogen bonding between the hydroxyl group in the
3 position and the subsequent ring oxygen (that is, O3-HO').
L-guluronate forms stiffer (and more acid-stable) 2-fold screw helical
chains, preferring intra-molecular hydrogen bonding between the
carboxyl group and the 2-OH group of the prior residues and (weaker)
the 3-OH group of the subsequent residues. The diaxial links also
inherently allow less flexibility. Alternating poly α-(14)-linked
D-mannuronate contains both equatorial-axial and axial-equatorial
links and take up dissimilar rather disorderly conformations. They
have hydrogen bonds between the carboxyl group on the mannuronate
and the 2-OH and 3-OH groups of the subsequent guluronate but the
differing degrees of freedom of the two residues gives greater overall
flexibility than the poly β-(14)-linked
D-mannuronate chains. The free carboxylic acids (without counter ion)
have a water molecule H3O+ firmly hydrogen
bound to carboxylate (pKa M 3.38, pKa G 3.65). Ca2+ ions can replace this
hydrogen bonding, zipping guluronate, but not mannuronate, chains
together stoichiometrically in a supposedly egg-box like conformation
(the ions being the eggs in the puckered box formed by sequential
saccharides; the box possibly consists of six oxygen ligands from
the 2-OH and 3-OH plus a carboxylate oxygen of the subsequent residue,
supplied by each poly-guluronate chain) with 7th and
8th ligands from the (14)-O-linkages
slightly further away. The chains are stabilized by hydrogen bonding
between the other carboxylate oxygen and 2-OH groups on the subsequent
residues. Poly-guluronate has specific binding sites for calcium
consisting of five oxygen ligands from the 2-OH and 3-OH, (14)-O-linkage
and carboxylate and ring oxygen of the subsequent residue, so holding
the calcium ready for this junction zone formation. Initially dimers are formed .
zone optimally requires 10-12 residues (depending on parameterization)
to form half a complete revolution (as optimized using the AMBER-96 force field )
of the parallel left-handed double helix (see below) and consequent
permanent junction zone formation. Interactions with further poly-guluronate
segments favor an unwound sheet-like structure; the winding -unwinding
only requiring changes in the anomeric
linkage angles (φ and ψ)
of about 10° whilst retaining the hydrogen bonding and ionic
linkages. A possibly-related two-stage junction zone formation has
been recently proposed to occur in alginic acid gels, based on X-ray
scattering and rheological characterization .
Curiously, calcium poly-guluronate also forms a (only slightly less)
stable parallel right-handed helix (φ and ψ further changing by about 10°) of about the same number of residues
per helix where the calcium ions sit in a pocket approximately equispaced
from 10 oxygen ligands (from the 2-OH and 3-OH, (14)-O-linkage
and a carboxylate and ring oxygen of the subsequent residue from
both chains) and where hydrogen bonds are found from alternative
carboxyl groups and both the prior 2-OH group and the 3-OH group
of the prior residues on the parallel strand. Under similar conditions,
poly-mannuronic acid blocks take up a less-gelling ribbon conformation,
where carboxylate groups on sequential residues may bind calcium
intra- or inter-molecularly.
Possible helix formation from egg-box dimers.
view down axis
view along axis, showing the hydrogen bonding and
calcium binding sites.
'Designer' alginates may be available in the future by the 5-epimerization
D-mannuronic acid residues to α-(14)-linked
L-guluronic acid residues in algal alginates using bacterial epimerases.
An available natural alternative is to harvest the seaweed from
exposed seaboards (more G giving the kelp strength)
or sheltered bays (more M). [Back to Top ]
The primary function of the alginates are as thermally stable
cold setting gelling agents in the presence of calcium ions;
gelling at far lower concentrations than gelatin.
Such gels can be heat treated without melting, although they
may eventually degrade. Gelling depends on the ion binding
(Mg2+ << Ca2+ < Sr2+ < Ba2+) with the control of the di-cation addition
being important for the production of homogeneous gels (for example, by ionic diffusion or controlled acidification of CaCO3).
High G content produces strong brittle gels with good heat stability
(except if present in low molecular weight molecules) but
prone to water weepage (syneresis) on freeze-thaw, whereas
high M content produces weaker more-elastic gels with good freeze-thaw
behavior and high MGMG content zips with
Ca2+ ions to reduces shear .
However, at low or very high Ca2+ concentrations
high M alginates produce the stronger gels. So long as the average
chain lengths are not particularly short, the gelling properties
correlate with average G block length (optimum block size ~12; see also the similarity
to pectin gelling) and not necessarily
with the M/G ratio which may be primarily due to alternating MGMG chains. The future prospects are excellent as recombinant
epimerases with different specificities may be used to produce
novel designer alginates.
Alginate's solubility and water-holding capacity depend on
pH (precipitating below about pH 3.5), molecular weight (lower
molecular weight calcium alginate chains with less than 500
residues showing increasing water binding with increasing
size), ionic strength (low ionic strength increasing the extended
nature of the chains) and the nature of the ions present.
Generally alginates show high water absorption and may be
used as low viscosity emulsifiers and shear-thinning thickeners.
They can be used to stabilize phase separation in low
fat fat-substitutes for example, as alginate/caseinate blends
in starch three-phase systems. Alginate
is used in a wide variety of foodstuff such as pet food chunks,
onion rings, stuffed olives, low fat spreads, sauces and pie
fillings. The health roles of alginates have been reviewed .
Propylene glycol alginates have widespread use as
acid-stable stabilizers for uses such as preserving the head
Interactive structures are available (Jmol). [Back to Top ]
a Algal, but not bacterial, alginates
can also add L-guluronic acid residues directly to the biosynthesizing