A-L-Rhamnose

FIGURE 7.12 Several deoxy sugars and ouabain, which contains a-L-rhamnose (Rlia). Hydrogen atoms highlighted in red are deoxy positions. 

Pectin belongs to a family of plant polysaccharides in which the polymer backbone consists of (1— 4)-linked a-D-galacturonic acid repeating-units. Often, (1— 2)-linked a-L-rhamnose residues interrupt the regular polygalacturonate sequence. The high viscosity and gelling properties of pectins are exploited by the food and pharmaceutical industries. X-Ray studies on sodium pectate, calcium pectate, pectic acid, and pectinic acid (methyl ester of pectic acid) have disclosed their structural details. 

Fig. 7 A model of the cell-wall polysaccharide (CWPS) of group A Streptococcus, looking down the barrel of the helix. The core of the helix is composed of a-L-rhamnose residnes with the 8-d-G1cNAc moieties displayed on the periphery. Reprodnced with permission from [99]. 2004 Elsevier Science...

At first glance, the preferred conformation for L-hexoses, e.g. a-L-rhamnose, appears different from that of the o-hexoses. This is readily rationalized by considering the preferred conformation of a-L-glucose - the a-anomer is chosen simply because... 

Since A = 50.0 —> 64.7, only small quantities of furanoses can be present and the transition of the pyranoses is hardly disturbed. Accordingly the common or a-L-rhamnose is the imns-L-rhamnopyranose. 

Formation mechanisms of imidazoles in the Maillard reaction are not as well understood as those of other heterocyclic compounds. The role of a-amino carbonyl fragments as intermediates in imidazole formation was suggested in the reaction of sucrose and ammonia (43). In a study of a L-rhamnose/ammonia model system, which produced fifty-two imidazoles, it was proposed that an amino-hydroxy fragment was responsible for imidazole... 

Recent studies have revealed certain interesting features in the structure of a birch xylan (Fig. 3-16). The unit next to the reducing xylose end group is D-galacturonic acid, linked to a L-rhamnose unit through the C-2 position. The rhamnose unit, in turn, is connected through its C-3 position to thje xylan chain. 

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