124
6 Carrageenans
6.3 Chemistry of Carrageenan
The structure of carrageenan is the sulfated polysaccharide polymer chain made up
of alternating disaccharide units of 1, 3 linked beta galactose linked to either 1, 4
alpha galactopyranose or 3,6 anhydrogalactose. It can therefore be said to be an
alternating copolymer of these two units. Carrageenans are linear polymer structures characterized by the monomeric units and also importantly sulfate ester groups
attached to some of the repeating units. Carrageenans can be differentiated from the
other sulfated polysaccharide present in red algae (agarans) by the stereochemistry
of the 1-4 linked alpha galactopyranose which is D form in carrageenan but L form
in agarans (Jiao et al. 2011). They are classified as either λ, κ, ι, ε, μ, depending on
the degree of sulfation which could range from 22 to 35%. The most common types
are λ, κ and ι, while κ is the most used of the three types. While k carrageenan has
alternating units of 1, 3 beta-d-galactose with a sulfate group at C4 and 1-4 linked
anhydrogalactose unit, iota carrageenan has a similar structure but with further sulfation on the C2 carbon of the anhydrogalactose unit such that the iota carrageenan
has two sulfate groups on the repeating disaccharide unit. The lambda carrageenan is
further sulfated by having a third sulfate group attached to the C6 carbon of the alpha
1-4 linked galactose unit, such that it has 3 sulfate groups per disaccharide unit. The
lambda carrageenan further differs from the other forms by having no 3,6 anhydride
bridge on the 1-4 linked galactopyranose (Funami et al. 2007). The types present
vary in different species of red alga; for example, K. alvarezii is the most common
source of k carrageenan and lambda carrageenan is more abundant in species such
as Gigartina and Chondrus genera (Zhou et al. 2006). The presence of the sulfate
group makes carrageenans anionic polymers. Extraction methods have significant
effect on the properties of the carrageenan obtained. The degree of sulfation in turn
affects properties such as the gel strength.
Other than the natural form in which they occur within different species, carrageenan of different forms can be obtained by chemically processing the original
forms. For example, theta carrageenan can be obtained from lambda carrageenan
through alkali treatment to form anhydride bridges between the units. This alkali
treatment usually follows the extraction process in order to convert the extracted
from into the desired form of carrageenan (Doyle et al. 2010). In nature, carrageenans
occur as a mix or hybrid of different forms of carrageenan. In addition to this, the
carrageenan structure can be further complicated by the presence of methyl groups,
pyruvic acetal or other sugars attached to the main chain (Yu et al. 2010). Figure 6.1
shows the different forms of carrageenan and how they can be transformed from one
form to the other.
In FTIR spectrometry, carrageenan is characterized by absorption peaks at
1234 cm
−1 which is indicative of the presence of sulfate ester (S = 0) functional
group, 3,6, anhydrogalactose at 926 cm
−1 and glycosidic linkages at 1072 cm
−1 .
The band occurs around 840–850 cm
−1 which indicates the presence of sulfated
anhydrogalactose for k type carrageenan. The vibration of the O–H hydroxyl groups
6 Carrageenans
6.3 Chemistry of Carrageenan
The structure of carrageenan is the sulfated polysaccharide polymer chain made up
of alternating disaccharide units of 1, 3 linked beta galactose linked to either 1, 4
alpha galactopyranose or 3,6 anhydrogalactose. It can therefore be said to be an
alternating copolymer of these two units. Carrageenans are linear polymer structures characterized by the monomeric units and also importantly sulfate ester groups
attached to some of the repeating units. Carrageenans can be differentiated from the
other sulfated polysaccharide present in red algae (agarans) by the stereochemistry
of the 1-4 linked alpha galactopyranose which is D form in carrageenan but L form
in agarans (Jiao et al. 2011). They are classified as either λ, κ, ι, ε, μ, depending on
the degree of sulfation which could range from 22 to 35%. The most common types
are λ, κ and ι, while κ is the most used of the three types. While k carrageenan has
alternating units of 1, 3 beta-d-galactose with a sulfate group at C4 and 1-4 linked
anhydrogalactose unit, iota carrageenan has a similar structure but with further sulfation on the C2 carbon of the anhydrogalactose unit such that the iota carrageenan
has two sulfate groups on the repeating disaccharide unit. The lambda carrageenan is
further sulfated by having a third sulfate group attached to the C6 carbon of the alpha
1-4 linked galactose unit, such that it has 3 sulfate groups per disaccharide unit. The
lambda carrageenan further differs from the other forms by having no 3,6 anhydride
bridge on the 1-4 linked galactopyranose (Funami et al. 2007). The types present
vary in different species of red alga; for example, K. alvarezii is the most common
source of k carrageenan and lambda carrageenan is more abundant in species such
as Gigartina and Chondrus genera (Zhou et al. 2006). The presence of the sulfate
group makes carrageenans anionic polymers. Extraction methods have significant
effect on the properties of the carrageenan obtained. The degree of sulfation in turn
affects properties such as the gel strength.
Other than the natural form in which they occur within different species, carrageenan of different forms can be obtained by chemically processing the original
forms. For example, theta carrageenan can be obtained from lambda carrageenan
through alkali treatment to form anhydride bridges between the units. This alkali
treatment usually follows the extraction process in order to convert the extracted
from into the desired form of carrageenan (Doyle et al. 2010). In nature, carrageenans
occur as a mix or hybrid of different forms of carrageenan. In addition to this, the
carrageenan structure can be further complicated by the presence of methyl groups,
pyruvic acetal or other sugars attached to the main chain (Yu et al. 2010). Figure 6.1
shows the different forms of carrageenan and how they can be transformed from one
form to the other.
In FTIR spectrometry, carrageenan is characterized by absorption peaks at
1234 cm
−1 which is indicative of the presence of sulfate ester (S = 0) functional
group, 3,6, anhydrogalactose at 926 cm
−1 and glycosidic linkages at 1072 cm
−1 .
The band occurs around 840–850 cm
−1 which indicates the presence of sulfated
anhydrogalactose for k type carrageenan. The vibration of the O–H hydroxyl groups