Gellans

Gellan m [71010-52-1] Gelling agent Gelling agents Gelling reaction... 

Gellan Gum. GeUan gum is the generic name for the extraceUular polysaccharide produced by the bacterium. Pseudomonas elodea (ATCC 31461). Proprietary to Kelco Division of Merck Co., Inc., geUan gum is manufactured in an aerobic, submerged fermentation (76). 

Polysaccharides can also be used to immobilise cells or enzymes, permitting the re-use of the catalyst and continuous flow systems. Alginates have the advantage that gel formation occurs under mild conditions, therefore cells remain viable and enzymes are not denatured but calcium gradually leaches out and the gel dissolves. Gellan or other combinations may prove superior for this application. 

This group includes a set of anionic polysaccharides secreted by unrelated bacteria. The common theme, however, is that their main chains have the same tetrasaccharide repeat. Although (high acyl) native gellan (42) and (deacylated) gellan (40,41) are linear polymers, welan (43) is a branched polymer in which a monosaccharide side chain is regularly attached to each repeat. Other members of this family, such as S-657 and rhamsan, are also branched, like welan."0... 

The side chains in the latter are flexible disaccharides on account of poor-quality diffraction patterns, their tentative molecular structures are known only from computer modeling.1" On the other hand, well-defined crystal structures are available for gellan and welan, and they can be correlated with the physical properties of the polysaccharides the details are presented here. Their conformation angles are listed in Table VI. 

Helical Parameters and Conformation Angles in the Gellan Family of Polysaccharides... 

Fig. 34.—X-Ray diffraction pattern from a polycrystalline and the well-oriented fiber of the potassium salt of gellan (41) shows 3-fold helix symmetry.

Fig. 35.—(a) Stereo view of about a turn of the 3-fold double helix of potassium gellan (41). The two chains are drawn in open and filled bonds for distinction. Both intra- and inter-chain hydrogen bonds stabilize the helix. The vertical line is the helix axis. Octahedrally coordinated potassium ions (crossed circles) and triply hydrogen-bonded water molecules (open circles) located above the ions are integral components of the structure of 41. 

Three welan helices pass through the unit cell, as shown in Fig. 37b. Two of them, I and II, at (2A A0) and ( A2A0), are antiparallel as in gellan. A third helix, III, at (000) is new and parallel to the first. They are equilaterally 12.0 A apart from each other, 2.9 A farther than in 41. The unit cell contains a total of nine pentasaccharides surrounded by ordered guest molecules, which are accounted by six calcium ions and 75 water molecules. The two up and down helices interact via side chains, calcium ions, and/or water molecules. Specifically, helices I and II are linked only by side chain side chain hydrogen bonds... 

Galactoglucan, 362-364, 417-418 Galactomannans, 395-396 y-radiation, effects on saccharides, 17 Gellan, 385 - 391... 

Lactones, nomenclature, 105 Linked-Atom Least-Squares analysis, 319 Lithium gellan, 386 Luteic acid, 8... 

Potassium native gellan, 389-391,430-431 Pseudo-oligosaccharides, spirodioxanyl, 220-221 Pyramine... 

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