Chemistry Reference
In-Depth Information
O
-
O
-
O
O
OH
O
OH
O
-
O
HO
O
OH
O
HO
O
O
OH
O
O
O
-
OH
O
OH
O
-
O
1-4-Linked
α
-
L
-Guluronate
1-4-Linked
β
-
D
-Mannuronate
Fig. 6.1
Molecular structure of alginate.
with respect to different growth conditions and seasons (Andresen
et al.
,
1977; Indergaard and SkjakBraek, 1987). Several publications have de-
scribed the process of alginate extraction in detail (Nussinovitch, 1997;
Owusu-Apenten, 2005). As a basic guide, however, extraction of al-
ginate from seaweed can be achieved using the following procedure:
Seaweed is milled (either wet or dry) and washed with acid before ex-
traction with hot alkali. The alginate is then filtered, precipitated with
calcium and acidified to produce alginic acid. The insoluble alginic acid
can afterwards be treated with metallic carbonate, hydroxide or oxide
to produce the desired salt form of alginate.
In addition to seaweed, it should also be mentioned that bacterial
exopolysaccharides similar to alginate can be produced by fermenta-
tion of several species of bacteria, which include
Azobacter vinelandii
(Gorin and Spencer, 1966) and
Pseudomonas aeruginosa
(Carlson and
Matthews, 1966; Linker and Jones, 1964). Indeed, these species have
helped scientists elucidate the mechanisms of alginate biosynthesis.
On a molecular level, alginate is described as a linear polysaccharide
consisting of two uronic acid residues 1,4-linked β-D-mannuronic (M)
acid and 1,4-linked α-L-guluronic acid (G), which occur along the
polymer chain as blocks of mannuronate, blocks of guluronate and
heteropolymeric blocks of both monomers (Fig. 6.1).
During the biosynthesis of alginate, polymer chains consisting solely
of mannuronate are initially produced, some of which are then con-
verted to α-L-guluronate by enzymatic epimerisation at C(5). With
regard to molecular conformation, this epimerisation results in convert-
ing the inter-residue linkages from 1,4 equatorial to 1,4 axial, having
the effect of altering the geometry throughout the polymer chain. Poly-
mannuronate blocks have what is described as flat, extended ribbon
geometry due to the 1,4 equatorial bond, with polyguluronate blocks
having 'buckled ribbon' geometry due to the 1,4 axial configuration
