Page 117 - PG 504-theoretical notes-phyto-1-2024-2025..

Clinical pharmacy PharmD program Third level Phytochemistry-1 (PG-504)

Constituents and structure:

Agar is predominately the calcium salt of strongly ionized acidic

polysaccharide that can be resolved into two major fractions, agarose and

agaropectin.

1. Agarose:

• Neutral polymer (free from sulfate), of repeated disaccharide unit, -

agarobiose, which is composed of -D- galactose and 3,6-anhydro--L-

galactose.

• Simply agarose is alternating units of galactose and 3,6-dehydrogalactose.

• Agarose is responsible for the gel strength.

2. Agaropectin

• It is a sulfonated polysaccharide in which galactose and uronic acid units are

partly esterified with sulfuric acid.

Identity tests:

1) Acid hydrolysis:

On boiling with dilute acid (HCl), it yields galactose and sulfate ions that are

recognized by the following:

a) Galactose: 1) Reduction of Fehling’s reagent.

b) Sulfate ions: 2) Precipitated with BaCl 2 solution.

2) It gives a pink color with ruthenium red (Gums arabic and tragacanth give no

color or only very faint).

3) It gives a pink deep crimson color with iodine solution (Gum Arabic is also

negative, while gum tragacanth gives blue spots due to starch granules under

microscope).

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