Areducing sugar is anysugar that is capable of acting as areducing agent.[1] In analkaline solution, a reducing sugar forms somealdehyde orketone, which allows it to act as a reducing agent, for example inBenedict's reagent. In such a reaction, the sugar becomes acarboxylic acid.
Allmonosaccharides are reducing sugars, along with somedisaccharides, someoligosaccharides, and somepolysaccharides. The monosaccharides can be divided into two groups: thealdoses, which have an aldehyde group, and theketoses, which have a ketone group. Ketoses must firsttautomerize to aldoses before they can act as reducing sugars. The common dietary monosaccharidesgalactose,glucose andfructose are all reducing sugars.
Disaccharides are formed from two monosaccharides and can be classified as either reducing or nonreducing. Nonreducing disaccharides likesucrose andtrehalose haveglycosidic bonds between theiranomeric carbons and thus cannot convert to an open-chain form with an aldehyde group; they are stuck in the cyclic form. Reducing disaccharides likelactose andmaltose have only one of their two anomeric carbons involved in the glycosidic bond, while the other is free and can convert to an open-chain form with an aldehyde group.
The aldehyde functional group allows the sugar to act as a reducing agent, for example, in theTollens' test orBenedict's test. The cyclichemiacetal forms ofaldoses can open to reveal an aldehyde, and certain ketoses can undergo tautomerization to become aldoses. However,acetals, including those found in polysaccharide linkages, cannot easily become free aldehydes.
Reducing sugars react with amino acids in theMaillard reaction, a series of reactions that occurs while cooking food at high temperatures and that is important in determining the flavor of food. Also, the levels of reducing sugars in wine, juice, and sugarcane are indicative of the quality of these food products.
Areducing sugar is one thatreduces anothercompound and is itselfoxidized; that is, thecarbonyl carbon of thesugar is oxidized to acarboxyl group.[2]
A sugar is classified as a reducing sugar only if it has anopen-chain form with an aldehyde group or a freehemiacetal group.[3]
Monosaccharides which contain an aldehyde group are known asaldoses, and those with a ketone group are known asketoses. The aldehyde can be oxidized via aredox reaction in which another compound is reduced. Thus, aldoses are reducing sugars. Sugars withketone groups in their open chain form are capable of isomerizing via a series oftautomeric shifts to produce an aldehyde group in solution. Therefore, ketones likefructose are considered reducing sugars but it is the isomer containing an aldehyde group which is reducing since ketones cannot be oxidized without decomposition of the sugar. This type of isomerization is catalyzed by the base present in solutions which test for the presence of reducing sugars.[3]
Disaccharides consist of two monosaccharides and may be either reducing or nonreducing. Even a reducing disaccharide will only have one reducing end, as disaccharides are held together byglycosidic bonds, which consist of at least oneanomeric carbon. With one anomeric carbon unable to convert to the open-chain form, only the free anomeric carbon is available to reduce another compound, and it is called thereducing end of the disaccharide. A nonreducing disaccharide is that which has both anomeric carbons tied up in the glycosidic bond.[4]
Similarly, most polysaccharides have only one reducing end.
All monosaccharides are reducing sugars because they either have an aldehyde group (if they are aldoses) or can tautomerize in solution to form an aldehyde group (if they are ketoses).[5] This includes common monosaccharides likegalactose,glucose,glyceraldehyde,fructose,ribose, andxylose.
Manydisaccharides, likecellobiose,lactose, andmaltose, also have a reducing form, as one of the two units may have an open-chain form with an aldehyde group.[6] However,sucrose andtrehalose, in which theanomeric carbon atoms of the two units are linked together, are nonreducing disaccharides since neither of the rings is capable of opening.[5]
In glucosepolymers such asstarch and starch-derivatives likeglucose syrup,maltodextrin anddextrin themacromolecule begins with a reducing sugar, a free aldehyde. When starch has been partiallyhydrolyzed the chains have been split and hence it contains more reducing sugars per gram. The percentage of reducing sugars present in these starch derivatives is calleddextrose equivalent (DE).
Glycogen is a highly branched polymer of glucose that serves as the main form of carbohydrate storage in animals. It is a reducing sugar with only one reducing end, no matter how large the glycogen molecule is or how many branches it has (note, however, that the unique reducing end is usually covalently linked toglycogenin and will therefore not be reducing). Each branch ends in a nonreducing sugar residue. When glycogen is broken down to be used as an energy source, glucose units are removed one at a time from the nonreducing ends by enzymes.[2]
Severalqualitative tests are used to detect the presence of reducing sugars. Two of them use solutions ofcopper(II) ions:Benedict's reagent (Cu2+ in aqueous sodium citrate) andFehling's solution (Cu2+ in aqueous sodium tartrate).[7] The reducing sugar reduces thecopper(II) ions in these test solutions to copper(I), which then forms a brick redcopper(I) oxide precipitate. Reducing sugars can also be detected with the addition ofTollens' reagent, which consist of silver ions (Ag+) in aqueous ammonia.[7] When Tollens' reagent is added to an aldehyde, it precipitates silver metal, often forming a silver mirror on clean glassware.[3]
3,5-dinitrosalicylic acid is another test reagent, one that allows quantitative detection. It reacts with a reducing sugar to form3-amino-5-nitrosalicylic acid, which can be measured byspectrophotometry to determine the amount of reducing sugar that was present.[8]
Some sugars, such as sucrose, do not react with any of the reducing-sugar test solutions. However, a non-reducing sugar can behydrolyzed using dilutehydrochloric acid. After hydrolysis and neutralization of the acid, the product may be a reducing sugar that gives normal reactions with the test solutions.
All carbohydrates are converted to aldehydes and respond positively inMolisch's test. But the test has a faster rate when it comes to monosaccharides.
Fehling's solution was used for many years as a diagnostic test fordiabetes, a disease in whichblood glucose levels are dangerously elevated by a failure to produce enough insulin (type 1 diabetes) or by an inability to respond to insulin (type 2 diabetes). Measuring the amount of oxidizing agent (in this case, Fehling's solution) reduced by glucose makes it possible to determine the concentration of glucose in the blood or urine. This then enables the right amount of insulin to be injected to bring blood glucose levels back into the normal range.[2]
The carbonyl groups of reducing sugars react with the amino groups of amino acids in theMaillard reaction, a complex series of reactions that occurs when cooking food.[9] Maillard reaction products (MRPs) are diverse; some are beneficial to human health, while others are toxic. However, the overall effect of the Maillard reaction is to decrease the nutritional value of food.[10] One example of a toxic product of the Maillard reaction isacrylamide, aneurotoxin and possiblecarcinogen that is formed from freeasparagine and reducing sugars when cooking starchy foods at high temperatures (above 120 °C).[11] However, evidence from epidemiological studies suggest that dietary acrylamide is unlikely to raise the risk of people developing cancer.[12]
The level of reducing sugars in wine, juice, and sugarcane are indicative of the quality of these food products, and monitoring the levels of reducing sugars during food production has improved market quality. The conventional method for doing so is the Lane-Eynon method, which involvestitrating the reducing sugar with copper(II) in Fehling's solution in the presence ofmethylene blue, a commonredox indicator. However, it is inaccurate, expensive, and sensitive to impurities.[13]
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