Читать книгу Polysaccharides - Группа авторов - Страница 106

The ability to gel and solubilize for agar polysaccharides solely depends upon the comparative hydrophobicity of structural repeating units of agarobiose. Gelling property of agar also alters when the 1,4-linked 3,6-anhydro-α-L-galactopyranoses gets substituted with hydrophobic groups like methoxyl and polar groups like sulfates and pyruvates.

In 1977 Rees and Welsh [18] stated that agar gels are created only when agar polysaccharide reaches it helical conformation and these helical conformations accumulate at one place. According to Arnott et al. 1974, the hydrogen bond between water molecule and the O₂ of galactose and O₅ anhydrogalactose maintain this structure. The detailed structure of agar was explained by X-ray diffraction studies, which have revealed that extended single helices with a lead of 0.888–0.973 nm are created predominantly during the gelling or drying of the agar. These structures were verified later with the help of molecular modelling of agaro-oligosaccharides, showing a triple folded left handed single helix with a lead of 0.95 nm and 2.85 nm pitch. This confirms that clustering of single helices is also a possible step during agar gelation. X-ray diffraction study of agarose by Chandrasekaran in 1998 revealed that it is a double helical structure which is stabilized by the hydrogen bonds between agarose and water molecules at O-2 and O-5 of galactose unit. The helix is left handed with 1.9 nm pitch. The inner and outer dimensions are 0.42 and 1.36 nm. The length of repeating unit is 0.633 nm built only with sugar in ⁴C₁ conformation. However it consists of three-fold symmetry. Therefore we can say agarose is a crystallized, oriented, double helical left handed compound with repeating units of interconnecting galactose units. Since only O-2 of galactose unit is needed in gelling process, anything that changes its conformation, abrupt the gelling process.

Polymerization of agar gel takes place when hexagonal fibbers of 6 double helices club together to make bigger clusters as shown in Figure 5.2. L-galactose 6-sulfate and L-galactose both having ¹C₄ conformation, tend to replace their biological precursor 3,6-anhydrogalactose, which has ⁴C₁ conformation. This switching between precursors, prevent the helix formation in agar or introduce kinks or interrupt it otherwise. These kinks would help in the formation of 3-D structure of the gel. Moreover, when replacement of these chemicals on O₂ of anhydrogalactose and O₆ and O₄ galactose does not disturb the helical conformation of agar, they help in lessen the clustering of helices hence stops the gel formation. It can be prevented by increasing the temperature so that ordered conformation can take place. In 1970, Guiseley [19] stated that methoxyl content of agar partially controls the gelling temperature of agar. Agar with higher methoxyl content needs higher temperature to form gel. However, an opposite association was obtained with synthetically methylated agar and revealing that synthetic methylation takes place at random sites in agar compared to its counterparts which naturally occurs at O₆ of galactose and/or O₂ of 3,6anhydrogalactose in agar.

Figure 5.2 Chemical construct of different agarose units [20].

Commercially prepared agar seems to have molecular weight ranging from 35.7 to 144 kD. In sequential solvent extraction method, the molecular weight of agar doesn’t appear to determine its differential solubility. Like the other polymers, the solubility of agar depends upon its nature of solvent, like if solvent is capable of distorting its helices and conformations, it will likely disturb the gelling process of agar. If the aggregation and helical confirmation of agar (mixed with ethanol and water) is melted, the solubility of agar shows its affinity for different proportions of ethanol, as well as it also reflects the aggregation and strength of polysaccharides in different concentration of ethanol. The higher concentrations of methoxyl and 3,6-anhydrogalactose elevate the hydrophobicity in agar, therefore alleviate their solubility in 40–80% of ethanol–water solution at high temperature. In conclusion, agar substituted by 3,6-anhydrogalactose and/or by any electrically charged groups, has elevated hydrophobic property with their associated solubility in polar solvents at lesser temperatures. However these solvents are highly diluted with water. Therefore it is really important to precisely understand the defined chemical structure of agar, to better understand not only the physico-chemistry of the agar molecule but also the other related polysaccharide molecules.