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Introduction to Agar - Structure

Structure

The original structure of agar was was believed to be a simple sulphated poly galactose. However in in 1937 Araki showed that agar consisted of at least two separate polymers that could be fractionated. One was called agarose and the other agaropectin. Essentially agarose is the gelling fraction of agar. Later in 1957 agarose was assigned a linear polymer structure consisting of alternating D-galactose and 3,6 anhydro-L-galactose as shown in figure 1. However agar is actually a very complex polysaccharide and varies considerably depending on the source. In 1991 Lahaye showed that at least eleven different agarobiose structures could be identified in different agar bearing weeds depending on gender, species environmental conditions and time of the year. In summary agar can be considered to consist mainly of alternating β-(1-3)-D and α-(1-4)-L linked galactose residues. Most of the α-(1-4) residues are modified by the presence of a 3,6 anhydro bridge. The other modification that can be found are mainly substituents of sulphate, pyruvate, uronate or methoxyl groups. Modern alkalie treatment methods tend to increase the level of anhydro bridging in the molecule which subsequently improves the gelstrength. The level of methoxy content appears to be one of the main structural moieties that determines the gel setting temperature with very low methoxy contents giving the lower setting temperatures.

Figure 1. Original structure of agar repeat unit

Agarose is typically high in molecular weight and low in sulphate. Agaropectin is typically a lower molecular weight and also higher in sulphate at about 5-8%. Xylose has been found in some agars (Furneaux).