Cell Wall Polysaccharides of Marine Algae 22.4 Algal Polysaccharides such as Phycocolloids 559
Part D | 22.4
bridging) [22.202]. The strength of alginate gels depends on the M/G ratio and distribution of GG, MM,
and GM-blocks in the polysaccharide chain [22.203,
204]. Alginates with a low M/G ratio and a high content of G-blocks form dense and brittle gels, whereas
alginates with a high M/G ratio and a high content of
M-blocks give more elastic gels [22.12, 13, 205].
There are three distinct and successive steps in the
binding of Ca
2C to alginate with increased concentration of Ca
2C ions [22.206]: (i) interaction of Ca
2C
with a single GulpA unit forming mono-complexes,
(ii) propagation and formation of egg-box dimers via
pairing of these mono-complexes, and (iii) lateral association of these dimers, generating egg-box multimers.
In freshly prepared gels, junction zones are assumed to
consist of egg-box dimers only. At higher Ca
2C concentration upon drying, these dimers may associate laterally into ordered domains (egg-box multimers) by different ways, including the most probable electrostatic
interactions between Ca
2C and carboxylates [22.198,
207]. Lateral association of alginate chains is reduced
by removing Ca
2C excess from the gel beads in a washing step prior to air drying. The egg-box multimers
are disrupted by noncross-linking cations (Na
C , K
C or
Mg
2C ), which replace the Ca
2C bound to the alginate.
The result is swelling of the alginate gel. Ca
2C alginate
gelation has been described as random cross-linking
of multifunctional monochains [22.208]. Alginate gels
showed strain-hardening behavior at large deformations, which was explained by (i) the re-orientation of
chain segments longer than the distance between crosslinks [22.209], (ii) structure densification or highly
cross-linked polymer systems [22.205], and (iii) the deformation of rod-like junction zones [22.210].
22.4.2 Gels and Films Based
on Sulfated Galactans
Some red algal sulfated galactans (agars and carrageenans) are able to form films and thermally reversible hydrogels by cooling of their hot aqueous solutions. Agar films demonstrate a lower tensile strength,
water vapor permeability, swelling ratio, and water
solubility than Ä-carrageenan films. Each property of
the mixture agar/Ä-carrageenan films varies depending on the ratio of each component [22.211]. Gelation
of sulfated galactans is a complex process that depends on temperature, structure, and concentration of
polysaccharide, and on the presence of metal cations,
which can induce conformational changes in the polymer chain. As a result, the initial coil-to-helix transition is followed by subsequent aggregation of these
helices [22.212–215].
The final gel structure is the result of mutual interactions between conformational transition, molecular
cross-linking, and phase-separation processes [22.216].
A honeycomb-like network has been observed for aqueous solutions both of agars and carrageenans at various
temperatures. Its structure depends on the gelling stage,
the structural type (D- or L- absolute configuration of
the 3,6-anhydro-˛-Galp units), and the concentration of
sulfated galactan. Honeycomb-like structures are gradually replaced by more homogeneous networks during
the formation of brittle and strong agar-type hydrogels;
they are retained, however, in the case of highly elastic carrageenan-type hydrogels. Therefore, the gelation
mechanisms should be different for agars and carrageenans. In addition, sulfated galactan gels of high
polysaccharide concentration may be structurally different from those of low polysaccharide concentration.
Three possible mechanisms of carrageenan/agar gel formation focussing on the specificity of junction zones
have been proposed: (i) the double-helical model based
on intertwined double helices [22.217, 218], (ii) the domain model with cation-mediated aggregates of double
helices [22.212, 219], and (iii) the nested, single-helix
model with cation-specific salt bridges between ordered
chain segments [22.220, 221]. To date, the step-by-step
mechanism of gel formation as well as corresponding
conformational transition, i. e., coil-helix or coil-double helix are still unclear [22.216]. Metal cations may
be involved in gel formation depending on the presence
and distribution of sulfate semiester groups. The shielding effect of alkali metal cations prevents electrostatic
repulsion of sulfate groups in Ä-carrageenan gels and
thus stabilizes the double helices. Agarose gel is not influenced so much by these cations because of the lack
of sulfate groups [22.222].
Carrageenan films have randomly distributed sets
of pores [22.223]. Their mechanical properties depend
on the sulfate/anhydro substitution of carrageenan. Pure
Ä-carrageenan films are stronger, less flexible, and exhibit a higher tensile strength than pure Ã-carrageenan
films. The tensile strength of the mixture of Ä/ and Ä/Ãcarrageenan films decreases with an increasing contribution of Ä-carrageenan and is lower than that of pure
and Ã-carrageenan films. Reciprocal relations have
been demonstrated for the elongations and water vapor permeability [22.224]. The gelation properties of
carrageenans strongly depend on their structure, first
of all on the presence of sulfate semiester and 3,6anhydro groups. The high density of charged sulfates
Part D | 22.4
bridging) [22.202]. The strength of alginate gels depends on the M/G ratio and distribution of GG, MM,
and GM-blocks in the polysaccharide chain [22.203,
204]. Alginates with a low M/G ratio and a high content of G-blocks form dense and brittle gels, whereas
alginates with a high M/G ratio and a high content of
M-blocks give more elastic gels [22.12, 13, 205].
There are three distinct and successive steps in the
binding of Ca
2C to alginate with increased concentration of Ca
2C ions [22.206]: (i) interaction of Ca
2C
with a single GulpA unit forming mono-complexes,
(ii) propagation and formation of egg-box dimers via
pairing of these mono-complexes, and (iii) lateral association of these dimers, generating egg-box multimers.
In freshly prepared gels, junction zones are assumed to
consist of egg-box dimers only. At higher Ca
2C concentration upon drying, these dimers may associate laterally into ordered domains (egg-box multimers) by different ways, including the most probable electrostatic
interactions between Ca
2C and carboxylates [22.198,
207]. Lateral association of alginate chains is reduced
by removing Ca
2C excess from the gel beads in a washing step prior to air drying. The egg-box multimers
are disrupted by noncross-linking cations (Na
C , K
C or
Mg
2C ), which replace the Ca
2C bound to the alginate.
The result is swelling of the alginate gel. Ca
2C alginate
gelation has been described as random cross-linking
of multifunctional monochains [22.208]. Alginate gels
showed strain-hardening behavior at large deformations, which was explained by (i) the re-orientation of
chain segments longer than the distance between crosslinks [22.209], (ii) structure densification or highly
cross-linked polymer systems [22.205], and (iii) the deformation of rod-like junction zones [22.210].
22.4.2 Gels and Films Based
on Sulfated Galactans
Some red algal sulfated galactans (agars and carrageenans) are able to form films and thermally reversible hydrogels by cooling of their hot aqueous solutions. Agar films demonstrate a lower tensile strength,
water vapor permeability, swelling ratio, and water
solubility than Ä-carrageenan films. Each property of
the mixture agar/Ä-carrageenan films varies depending on the ratio of each component [22.211]. Gelation
of sulfated galactans is a complex process that depends on temperature, structure, and concentration of
polysaccharide, and on the presence of metal cations,
which can induce conformational changes in the polymer chain. As a result, the initial coil-to-helix transition is followed by subsequent aggregation of these
helices [22.212–215].
The final gel structure is the result of mutual interactions between conformational transition, molecular
cross-linking, and phase-separation processes [22.216].
A honeycomb-like network has been observed for aqueous solutions both of agars and carrageenans at various
temperatures. Its structure depends on the gelling stage,
the structural type (D- or L- absolute configuration of
the 3,6-anhydro-˛-Galp units), and the concentration of
sulfated galactan. Honeycomb-like structures are gradually replaced by more homogeneous networks during
the formation of brittle and strong agar-type hydrogels;
they are retained, however, in the case of highly elastic carrageenan-type hydrogels. Therefore, the gelation
mechanisms should be different for agars and carrageenans. In addition, sulfated galactan gels of high
polysaccharide concentration may be structurally different from those of low polysaccharide concentration.
Three possible mechanisms of carrageenan/agar gel formation focussing on the specificity of junction zones
have been proposed: (i) the double-helical model based
on intertwined double helices [22.217, 218], (ii) the domain model with cation-mediated aggregates of double
helices [22.212, 219], and (iii) the nested, single-helix
model with cation-specific salt bridges between ordered
chain segments [22.220, 221]. To date, the step-by-step
mechanism of gel formation as well as corresponding
conformational transition, i. e., coil-helix or coil-double helix are still unclear [22.216]. Metal cations may
be involved in gel formation depending on the presence
and distribution of sulfate semiester groups. The shielding effect of alkali metal cations prevents electrostatic
repulsion of sulfate groups in Ä-carrageenan gels and
thus stabilizes the double helices. Agarose gel is not influenced so much by these cations because of the lack
of sulfate groups [22.222].
Carrageenan films have randomly distributed sets
of pores [22.223]. Their mechanical properties depend
on the sulfate/anhydro substitution of carrageenan. Pure
Ä-carrageenan films are stronger, less flexible, and exhibit a higher tensile strength than pure Ã-carrageenan
films. The tensile strength of the mixture of Ä/ and Ä/Ãcarrageenan films decreases with an increasing contribution of Ä-carrageenan and is lower than that of pure
and Ã-carrageenan films. Reciprocal relations have
been demonstrated for the elongations and water vapor permeability [22.224]. The gelation properties of
carrageenans strongly depend on their structure, first
of all on the presence of sulfate semiester and 3,6anhydro groups. The high density of charged sulfates