Part D | 22.4
558 Part D Marine Algal Biotechnology
by the treatment with oxidative agents. Fucoidan was
purified from its natural complex with polyphenols by
the treatment with 5% H 2 O 2 (pH 8:38:7, 18
ı C), and
this treatment did not lead to marked desulfation of
the polysaccharide [22.83, 186]. Fucoidans treated with
H 2 O 2 have not demonstrated any significant changes in
backbones or sulfation degrees compared with the raw
polysaccharide [22.83, 187]. However, a decrease in M w
is possible at higher reaction temperatures, H 2 O 2 concentration, and reaction time.
22.4 Algal Polysaccharides such as Phycocolloids
Algal matrix cell wall polysaccharides can be assigned
as phycocolloids (or phycohydrocolloids) because they
are soluble in water and can create colloid systems in
aqueous media or participate in their formation. Some
of these polysaccharides can form gels and films at appropriate conditions; the films are commonly obtained
by solution casting and solvent evaporation. The physical and chemical properties of phycocolloids, including
gel formation, have been reviewed in connection with
their structure [22.188]. The geometry of glycosidic
linkages and the ability to form intra and intermolecular junctions are prerequisites of gel and film formation.
These associations may be affected by (i) negatively
charged groups (sulfates and carboxylates), which tend
to expend a polysaccharide chain due to mutual electrostatic repulsion; (ii) various substituents (methyls,
acetyls, pyruvate ketals, glycosylation, etc.), which
cause steric hindrance and/or participate in various intra and intermolecular interactions; and (iii) dissolved
small molecules or metal cations that influence the
hydratation property of water. Conditions such as temperature, pH, and ionic strength may also influence gel
formation.
22.4.1 Alginic Acid/Alginate Gels and Films
Alginates of alkali metals and alginic acids form transparent, flexible, and water soluble films by casting of
their aqueous solutions. Films prepared from GulpA
rich alginate demonstrate lower plasticity and have
proved to be better moisture barriers than those prepared from ManpA rich alginate under the same conditions [22.189, 190]. Alginate films are effective cation
exchangers. Their solubility may significantly decrease
in the presence of multivalent metal cations [22.191,
192]. The Ca
2C and Zn
2C cations make alginate film
water insoluble, increase its tensile strength, and decrease elongation. These effects are less pronounced
for Cu
2C and Al
3C cations and little pronounced for
Fe
3C and Mg
2C cations. Metal cations of smaller
ionic radius cause tighter cross-linking of alginate
films [22.193]. Cross-linking by Ca
2C produces significantly thicker and stronger, but less elastic alginate
films; water vapor permeability decreases significantly
only for highly cross-linking films [22.194]. The incorporation of multivalent metal cations into the alginate
films may partially replace the weak polar interactions
between polysaccharide chains by stronger cation mediated electrostatic junctions [22.195].
Alginates are able to form two types of gels: (i) alginic acid gel at acidic conditions (pH < pK a of uronic
carboxyls) and (ii) cross-linking thermostable gels at
neutral conditions in the presence of Ca
2C . Acidic gels
are stabilized by intermolecular hydrogen bonds and
the junction zones of these gels are probably composed of random aggregates [22.196]. G-blocks are the
most important fragments for both acidic and Ca
2C gel
formation, whereas M-blocks support only acidic gel
formation. Alternating or random sequences destabilize
alginic acid gel formation due to their lack of repeating structure. In the case of neutral gels, cations of
other alkaline earth metals, excluding Mg
2C or other
divalent metal cations, can be used instead of Ca
2C . Gblocks, but not GM or M-blocks, of alginates interact
strongly and cooperatively with these cations, forming
egg-box type structures [22.47, 197]. Ca
2C ions can replace carboxylic protons in the G-blocks, which are
zipped into cavities [22.198, 199]. The Ca
2C binding
capacity does not depend on the M/G ratio of alginates, but GulpA exhibit much stronger affinity to these
cations than ManpA [22.200]. In this model, the cations
(eggs) are situated in the cavities inside puckered boxes
formed by four GulpA units of the two superimposed
chains. The individual boxes potentially possess oxygen ligands from COO
, 2-OH, 3-OH, O-5 pyranoid
rings, and O-4 of glycosidic bonds in the G-blocks;
some of these ligands directly interact with Ca
2C , others participate in intra and intermolecular hydrogen
bonds stabilizing the egg-box complex [22.201]. Based
on FTIR (Fourier transformed infrared) spectroscopic
analysis of ionic alginate gels with various divalent
metal cations, different coordination geometry was proposed for the metal-carboxylate complexes in G-blocks
(pseudo-bridged unidentate) and M-blocks (bidentate
558 Part D Marine Algal Biotechnology
by the treatment with oxidative agents. Fucoidan was
purified from its natural complex with polyphenols by
the treatment with 5% H 2 O 2 (pH 8:38:7, 18
ı C), and
this treatment did not lead to marked desulfation of
the polysaccharide [22.83, 186]. Fucoidans treated with
H 2 O 2 have not demonstrated any significant changes in
backbones or sulfation degrees compared with the raw
polysaccharide [22.83, 187]. However, a decrease in M w
is possible at higher reaction temperatures, H 2 O 2 concentration, and reaction time.
22.4 Algal Polysaccharides such as Phycocolloids
Algal matrix cell wall polysaccharides can be assigned
as phycocolloids (or phycohydrocolloids) because they
are soluble in water and can create colloid systems in
aqueous media or participate in their formation. Some
of these polysaccharides can form gels and films at appropriate conditions; the films are commonly obtained
by solution casting and solvent evaporation. The physical and chemical properties of phycocolloids, including
gel formation, have been reviewed in connection with
their structure [22.188]. The geometry of glycosidic
linkages and the ability to form intra and intermolecular junctions are prerequisites of gel and film formation.
These associations may be affected by (i) negatively
charged groups (sulfates and carboxylates), which tend
to expend a polysaccharide chain due to mutual electrostatic repulsion; (ii) various substituents (methyls,
acetyls, pyruvate ketals, glycosylation, etc.), which
cause steric hindrance and/or participate in various intra and intermolecular interactions; and (iii) dissolved
small molecules or metal cations that influence the
hydratation property of water. Conditions such as temperature, pH, and ionic strength may also influence gel
formation.
22.4.1 Alginic Acid/Alginate Gels and Films
Alginates of alkali metals and alginic acids form transparent, flexible, and water soluble films by casting of
their aqueous solutions. Films prepared from GulpA
rich alginate demonstrate lower plasticity and have
proved to be better moisture barriers than those prepared from ManpA rich alginate under the same conditions [22.189, 190]. Alginate films are effective cation
exchangers. Their solubility may significantly decrease
in the presence of multivalent metal cations [22.191,
192]. The Ca
2C and Zn
2C cations make alginate film
water insoluble, increase its tensile strength, and decrease elongation. These effects are less pronounced
for Cu
2C and Al
3C cations and little pronounced for
Fe
3C and Mg
2C cations. Metal cations of smaller
ionic radius cause tighter cross-linking of alginate
films [22.193]. Cross-linking by Ca
2C produces significantly thicker and stronger, but less elastic alginate
films; water vapor permeability decreases significantly
only for highly cross-linking films [22.194]. The incorporation of multivalent metal cations into the alginate
films may partially replace the weak polar interactions
between polysaccharide chains by stronger cation mediated electrostatic junctions [22.195].
Alginates are able to form two types of gels: (i) alginic acid gel at acidic conditions (pH < pK a of uronic
carboxyls) and (ii) cross-linking thermostable gels at
neutral conditions in the presence of Ca
2C . Acidic gels
are stabilized by intermolecular hydrogen bonds and
the junction zones of these gels are probably composed of random aggregates [22.196]. G-blocks are the
most important fragments for both acidic and Ca
2C gel
formation, whereas M-blocks support only acidic gel
formation. Alternating or random sequences destabilize
alginic acid gel formation due to their lack of repeating structure. In the case of neutral gels, cations of
other alkaline earth metals, excluding Mg
2C or other
divalent metal cations, can be used instead of Ca
2C . Gblocks, but not GM or M-blocks, of alginates interact
strongly and cooperatively with these cations, forming
egg-box type structures [22.47, 197]. Ca
2C ions can replace carboxylic protons in the G-blocks, which are
zipped into cavities [22.198, 199]. The Ca
2C binding
capacity does not depend on the M/G ratio of alginates, but GulpA exhibit much stronger affinity to these
cations than ManpA [22.200]. In this model, the cations
(eggs) are situated in the cavities inside puckered boxes
formed by four GulpA units of the two superimposed
chains. The individual boxes potentially possess oxygen ligands from COO
, 2-OH, 3-OH, O-5 pyranoid
rings, and O-4 of glycosidic bonds in the G-blocks;
some of these ligands directly interact with Ca
2C , others participate in intra and intermolecular hydrogen
bonds stabilizing the egg-box complex [22.201]. Based
on FTIR (Fourier transformed infrared) spectroscopic
analysis of ionic alginate gels with various divalent
metal cations, different coordination geometry was proposed for the metal-carboxylate complexes in G-blocks
(pseudo-bridged unidentate) and M-blocks (bidentate