Читать книгу Polysaccharides - Группа авторов - Страница 107

Ever since, Araki discovered the fundamental structure of agar and agarose, in 1966, so many alternative forms of this basic structure of agar, have been proposed by scientists. 4,6-O-(1-carboxyethylidene)-D-galactose, an acetal group of pyruvic acid of agar extracted from G. amansii, was discovered by Hirase 1957. Later it was also found in Gracilaria agar by Duckworth and his team. On the other hand Rees discovered in 1961 that L-galactose 6-sulfate works as a precursor of 3,6-anhydogalactose.

Hirase in 1957 [21], Araki and Peat in 1961 described the functions and the physicochemical properties of some of the methylated galactose units like, 6-O-methyl-D-galactose and 4-O-methyl-L-galactose, L-galactose, methyl-pentose, and xylose, even though the precise site of methyl-pentose is still unidentified. Craigie, Jurgens and Karamanos in 1989 stated that 4-O-methyl-L-galactose works as a subdivision on galactose in polymer chains.

Hook and Harvey proposed that xylose is also similarly located in agar (extracted from Curdiea flabellate and Melanthalia abscissa). Turvey and Williums in 1976, stated that Ceramium rubrum agar has neutral repeating unit, where the 3,6-anhydrogalactose was exchanged by L-galactose. The 2-O-methyl-L-galactose 6-sulfate, and D-galactose 2-sulfate in agar (extracted from Laurencia pinnatifiada) were identified by Bowker and Turvey in 1968. In 1990, Furneaux [22], reported and described these type of repeating di-methylated agarose unit in the agar from Curdiea coriaceea. Natural as well as commercially produced agar contains repeating methylation units of either 6-O-/2-O-methylated units or it may contain both. In 1975, Batey and Turvey studied and analyzed the agar from Polysiphonia lanosa and introduced the 6-O-methyl-D-galactose 4-sulfate. Later it was discovered that Odonthalia corymbifera and Gracilaria tikvahiae also have 4-sulfate units in their agar gel. The unmethylated sulfated derivative was identified in Gracilaria agar. The other derivatives like D-galactose 6-sulfate and 3,6-anhydogalactose 2-sulfate were recognized in agar-like polysaccharides extracts of Gloiopeltis furcata.

Except the biological precursor, mostly innate chemical alterations take place on those sites that don’t upset the helical conformation of the polysaccharides. These sites are O₆ and O₄ of galactose and O₂ of 3,6-anhydrogalactose. Conversely these natural chemical modifications may disturb the aggregation of helices, which will result in disturbance in gelation.