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  Derivatives of Monosaccharides, Chemistry tutorial

Introduction:

The reactions of monosaccharides result most of the significant derivatives that ranges from sugar alcohols, amino sugars and uronic acids. Such derivatives serve as significant components of numerous organisms and biologically significant substances.   

Types of Sugar derivatives:

Sugars might be modified through natural or laboratory methods into compounds which retain the fundamental configuration of saccharides, however encompass various functional groups.

Sugar alcohols:

Sugar alcohols, as well termed as polyols, polyhydric alcohols or polyalcohols, are the hydrogenated forms of the aldoses or ketoses. For illustration - glucitol, as well termed as sorbitol consists of the similar linear structure as the chain form of glucose, however the aldehyde (-CHO) group is substituted by a -CH2OH group. The other common sugar alcohols comprise the monosaccharides erythritol and xylitol and the disaccharides lactitol and maltitol. Sugar alcohols have around half the calories of sugars and are often utilized in low-calorie or 'sugar-free' products.

45_Glucitol or sorbitol.jpg

Fig: Glucitol or sorbitol

Xylitol that consists of the hydroxyl groups oriented similar to xylose, is a very common ingredient in 'sugar-free' candies and gums as it is around as sweet as sucrose, however includes 40 percent less food energy. However this sugar alcohol appears to be safe for humans, xylitol in comparatively small doses can cause seizures, liver failure and death in dogs.

Amino sugars:

Amino sugars or aminosaccharides substitute a hydroxyl group having an amino (-NH2) group. Glucosamine is the amino sugar employed to treat cartilage damage and decrease the pain and progression of arthritis.

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Fig: Amino sugars or aminosaccharides

Uronic acids:

The Uronic acids encompass a carboxyl group (-COOH) on the carbon which is significant part of the ring. The aldehyde at C1 or the hydroxyl on the terminal carbon is oxidized to a carboxylic acid. Their names maintain the root of the monosaccharides, however the -ose sugar suffix is changed to -uronic acid. For illustration, galacturonic acid has the similar configuration as galactose and the structure of glucuronic acid corresponds to glucose.

1798_Uronic acids.jpg

Fig: Uronic acids

N-acetylneuraminic acid:

N-acetylneuraminate, (that is, N-acetylneuraminic acid, as well known as sialic acid) is often found as the terminal residue of oligosaccharide chains of glycoproteins. Sialic acid passes on negative charge to glycoproteins, as its carboxyl group tends to dissociate a proton at physiological pH, as illustrated here.

1521_N-acetylneuraminic acid.jpg

Fig: N-acetylneuraminic acid

Reactions of Monosaccharides:

Carbohydrates have been provided non-systematic names; however the suffix ose is usually employed. The most general carbohydrate is glucose (C6H12O6). Applying the terms stated above, glucose is a monosaccharide, an aldohexose (it will be noted that the function and size of categorization are joined in one word) and a reducing sugar. The general structure of glucose and most of the other aldohexoses was established via simple chemical reactions. The given diagram describes the type of proof considered, however some of the reagents illustrated here are dissimilar from those employed by the original scientists.

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Fig: Glucose-Reactions of Monosaccharides

Hot hydriodic acid (HI) was frequently employed to reductively take away oxygen functional groups from a molecule, and in the case of glucose this treatment provides hexane (in low yield). From this it was summarized that the six carbons are in an unbranched chain. The presence of an aldehyde carbonyl group was deduced from the formation of cyanohydrin, its reduction to the hexa-alcohol sorbitol, as well known as glucitol, and mild oxidation to the mono-carboxylic acid, glucuronic acid. Rather stronger oxidation via dilute nitric acid provides the diacid, glucaric acid, supporting the proposal of a six-carbon chain. The five oxygen remaining in glucose subsequent to the aldehyde was accounted for were thought to be in hydroxyl groups, as a penta-acetate derivative could be made. Such hydroxyl groups were assigned, one each, to the last five carbon atoms, as geminal hydroxyl groups are usually unstable relative to the carbonyl compound made by loss of water. The four middle carbon atoms in the glucose chain are centers of chirality and are colored red. Glucose and other saccharides are widely cleaved through periodic acid, thanks to the plenty of vicinal diol moieties in their structure. This oxidative cleavage, termed as the Malaprade reaction is specifically helpful for the analysis of selective O-substituted derivatives of saccharides, as ether functions don't react. The Stoichiometry of aldohexose cleavage is illustrated in the given equation.

HOCH2(CHOH)4CHO + 5 HIO4 → H2C=O + 5 HCO2H + 5 HIO3

The Configuration of Glucose:

The four Chiral centers in glucose point out that there might be as many as sixteen (24) stereoisomers having this constitution. These would exist as eight diastereomeric pairs of enantiomers, and the initial challenge was to find out which of the eight corresponded to glucose. This challenge was accepted and met up in the year 1891 by the German chemist Emil Fischer. His successful negotiation of the stereochemical maze represented via the aldohexoses was a logical tour de force, and it is fitting that he received the Nobel Prize for chemistry in the year 1902 for this accomplishment. One of the primary tasks faced by Fischer was to formulate a process of representing the configuration of each and every Chiral center in an unambiguous way. To this end, he discovered a simple method for drawing chains of chiral centers, that we now state the Fischer projection formula. At the time Fischer undertook the glucose project it was not possible to set up the absolute configuration of enantiomers. As a result, Fischer made a random choice for (+)-glucose and established a network of associated aldose configurations which he stated as the D-family. The mirror images of such configurations were then designated the L-family of aldoses. To demonstrate by employing present day knowledge, Fischer projection formulas and names for the D-aldose family (that is, three to six-carbon atoms) are illustrated below, having the asymmetric carbon atoms (that is, Chiral centers) colored red. The last Chiral center in an aldose chain (that is, farthest from the aldehyde group) was selected by Fischer as the D/L designator site. Whenever the hydroxyl group in the projection formula pointed to the right, it was stated as a member of the D-family. A left directed hydroxyl group (that is, the mirror image) then symbolized the L-family. 

Fischer's primary assignment of the D-configuration had a 50:50 possibility of being right, however all his following conclusions regarding the relative configurations of different aldoses were soundly based. In the year 1951 x-ray fluorescence studies of  (+)-tartaric acid, taken out in the Netherlands via Johannes Martin Bijvoet (pronounced 'buy foot'), confirmed that Fischer's choice was right. It is significant to be familiar with that the sign of a compound's specific rotation (that is, an experimental number) doesn't correlate by its configuration (D or L). This is a simple matter to measure an optical rotation by a polarimeter. Finding out an absolute configuration generally needs chemical interconversion by known compounds via stereo specific reaction paths.

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