Gracilaria brevis (Rhodophycae) from Baker, A.L. et al. 2012. Phycokey -- an image based key to Algae (PS Protista), Cyanobacteria, and other aquatic objects. University of New Hampshire Center for Freshwater Biology. http://cfb.unh.edu/phycokey/phycokey.htm 17 May 2014 — *Gracilaria brevis*

Agar

Agar dissolves in hot water to form gels on cooling.

Source

Structural unit

Molecular structure

Functionality

Source

Agar (E406) is a scaffolding polysaccharide prepared from the same family of red seaweeds (Rhodophycae) as the carrageenans. It is commercially obtained from species of Gelidium and Gracilariae

Structural unit

-(1-3)-beta-D-galactopyranose-(1-4)-3,6-anhydro-alpha-L-galactopyranose unit

Agar consists of a mixture of agarose and agaropectin. Agarose is a linear polymer, of relative molecular mass (molecular weight) about 120,000, based on the -(13)-β-D-galactopyranose-(14)-3,6-anhydro-α-L-galactopyranose unit; the major differences from carrageenans being the presence of L-3,6-anhydro-α-galactopyranose rather than D-3,6-anhydro-α-galactopyranose units and the lack of sulfate groups. Agaropectin is a heterogeneous mixture of smaller molecules that occur in lesser amounts. Their structures are similar but slightly branched and sulfated, and they may have methyl and pyruvic acid ketal substituents. They gel poorly and may be removed easily from the excellent gelling agarose molecules by using their charge. Alkaline treatment that converts any L-galactose-6-sulfate to 3,6-anhydro-L-galactose improves the quality of the agar. [Back to Top ]

Molecular structure

The gel network of agarose contains double helices formed from left-handed threefold helices. The presence of water molecules bound to the inside of the double-helical cavity stabilizes these double helices [508 ]. Exterior hydroxyl groups allow aggregation of up to 10,000 of these helices to form suprafibers. Spectrophotometric investigation of agar gel formation has suggested that the gel formation occurs over a much narrower temperature range than its collapse [4048 ]. [Back to Top ]

Functionality

Agar is insoluble in cold water but dissolves in boiling water to give random coils. Gelation is reported to follow a phase separation process [501a] (although these findings are disputed [501b]) and association on cooling (≈ 35 °C), forming gels with up to 99.5% water and remaining solid up to about 85 °C. Agar has significant use in microbiological media. It is not easy for microorganisms to metabolize and forms clear, stable, and firm gels, but it finds use in icings, glazes, processed cheese, jelly sweets, and marshmallows. It may be used in tropical countries and by vegetarians as a substitute for gelatin. Agar gel can be used to selectively clean the surface of sensitive artworks and historical artifacts by the gradual removal of any soiling layer through the slow release of water (syneresis), diffusion of water into the porous substrate, and the withdrawal of the dirty water (evaporation, capillarity, and osmosis) [3343 ].

Interactive structures are available (Jmol). [Back to Top ]