Reducing sugar

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Reducing form of glucose (the aldehyde group is on the far right) Glucose chain structure.svg
Reducing form of glucose (the aldehyde group is on the far right)

A reducing sugar is any sugar that is capable of acting as a reducing agent. [1] In an alkaline solution, a reducing sugar forms some aldehyde or ketone, which allows it to act as a reducing agent, for example in Benedict's reagent. In such a reaction, the sugar becomes a carboxylic acid.

Contents

All monosaccharides are reducing sugars, along with some disaccharides, some oligosaccharides, and some polysaccharides. The monosaccharides can be divided into two groups: the aldoses, which have an aldehyde group, and the ketoses, which have a ketone group. Ketoses must first tautomerize to aldoses before they can act as reducing sugars. The common dietary monosaccharides galactose, glucose and fructose are all reducing sugars.

Disaccharides are formed from two monosaccharides and can be classified as either reducing or nonreducing. Nonreducing disaccharides like sucrose and trehalose have glycosidic bonds between their anomeric carbons and thus cannot convert to an open-chain form with an aldehyde group; they are stuck in the cyclic form. Reducing disaccharides like lactose and maltose 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 the Tollens' test or Benedict's test. The cyclic hemiacetal forms of aldoses 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 the Maillard 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.

Terminology

Oxidation-reduction

A reducing sugar is one that reduces another compound and is itself oxidized ; that is, the carbonyl carbon of the sugar is oxidized to a carboxyl group. [2]

A sugar is classified as a reducing sugar only if it has an open-chain form with an aldehyde group or a free hemiacetal group. [3]

Aldoses and ketoses

Monosaccharides which contain an aldehyde group are known as aldoses, and those with a ketone group are known as ketoses. The aldehyde can be oxidized via a redox reaction in which another compound is reduced. Thus, aldoses are reducing sugars. Sugars with ketone groups in their open chain form are capable of isomerizing via a series of tautomeric shifts to produce an aldehyde group in solution. Therefore, ketones like fructose 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]

Reducing end

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 by glycosidic bonds, which consist of at least one anomeric 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 the reducing 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.

Examples

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 like galactose, glucose, glyceraldehyde, fructose, ribose, and xylose.

Many disaccharides, like cellobiose, lactose, and maltose, also have a reducing form, as one of the two units may have an open-chain form with an aldehyde group. [6] However, sucrose and trehalose, in which the anomeric carbon atoms of the two units are linked together, are nonreducing disaccharides since neither of the rings is capable of opening. [5]

Equilibrium between cyclic and open-chain form in one ring of maltose Maltose Gleichgewicht.svg
Equilibrium between cyclic and open-chain form in one ring of maltose

In glucose polymers such as starch and starch-derivatives like glucose syrup, maltodextrin and dextrin the macromolecule begins with a reducing sugar, a free aldehyde. When starch has been partially hydrolyzed 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 called dextrose 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 to glycogenin 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]

Characterization

Several qualitative tests are used to detect the presence of reducing sugars. Two of them use solutions of copper(II) ions: Benedict's reagent (Cu2+ in aqueous sodium citrate) and Fehling's solution (Cu2+ in aqueous sodium tartrate). [7] The reducing sugar reduces the copper(II) ions in these test solutions to copper(I), which then forms a brick red copper(I) oxide precipitate. Reducing sugars can also be detected with the addition of Tollen's reagent, which consist of silver ions (Ag+) in aqueous ammonia. [7] When Tollen's 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 form 3-amino-5-nitrosalicylic acid, which can be measured by spectrophotometry 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 be hydrolyzed using dilute hydrochloric 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 in Molisch's test. But the test has a faster rate when it comes to monosaccharides.

Importance in medicine

Fehling's solution was used for many years as a diagnostic test for diabetes, a disease in which blood 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]

Importance in food chemistry

Maillard reaction

The carbonyl groups of reducing sugars react with the amino groups of amino acids in the Maillard 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 is acrylamide, a neurotoxin and possible carcinogen that is formed from free asparagine 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]

Food quality

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 involves titrating the reducing sugar with copper(II) in Fehling's solution in the presence of methylene blue, a common redox indicator. However, it is inaccurate, expensive, and sensitive to impurities. [13]

Related Research Articles

<span class="mw-page-title-main">Carbohydrate</span> Organic compound that consists only of carbon, hydrogen, and oxygen

A carbohydrate is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 and thus with the empirical formula Cm(H2O)n, which does not mean the H has covalent bonds with O. However, not all carbohydrates conform to this precise stoichiometric definition, nor are all chemicals that do conform to this definition automatically classified as carbohydrates.

<span class="mw-page-title-main">Disaccharide</span> Complex sugar

A disaccharide is the sugar formed when two monosaccharides are joined by glycosidic linkage. Like monosaccharides, disaccharides are simple sugars soluble in water. Three common examples are sucrose, lactose, and maltose.

<span class="mw-page-title-main">Glucose</span> Naturally produced monosaccharide

Glucose is a sugar with the molecular formula C6H12O6. Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using energy from sunlight, where it is used to make cellulose in cell walls, the most abundant carbohydrate in the world.

<span class="mw-page-title-main">Ketone</span> Organic compounds of the form >C=O

In organic chemistry, a ketone is an organic compound with the structure R−C(=O)−R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group −C(=O)−. The simplest ketone is acetone, with the formula (CH3)2CO. Many ketones are of great importance in biology and in industry. Examples include many sugars (ketoses), many steroids, and the solvent acetone.

Monosaccharides, also called simple sugars, are the simplest forms of sugar and the most basic units (monomers) from which all carbohydrates are built. Simply, this is the structural unit of carbohydrates.

<span class="mw-page-title-main">Aldehyde</span> Organic compound containing the functional group R−CH=O

In organic chemistry, an aldehyde is an organic compound containing a functional group with the structure R−CH=O. The functional group itself can be referred to as an aldehyde but can also be classified as a formyl group. Aldehydes are a common motif in many chemicals important in technology and biology.

Benedict's reagent is a chemical reagent and complex mixture of sodium carbonate, sodium citrate, and copper(II) sulfate pentahydrate. It is often used in place of Fehling's solution to detect the presence of reducing sugars. The presence of other reducing substances also gives a positive result. Such tests that use this reagent are called the Benedict's tests. A positive test with Benedict's reagent is shown by a color change from clear blue to brick-red with a precipitate.

<span class="mw-page-title-main">Maltose</span> Chemical compound

Maltose, also known as maltobiose or malt sugar, is a disaccharide formed from two units of glucose joined with an α(1→4) bond. In the isomer isomaltose, the two glucose molecules are joined with an α(1→6) bond. Maltose is the two-unit member of the amylose homologous series, the key structural motif of starch. When beta-amylase breaks down starch, it removes two glucose units at a time, producing maltose. An example of this reaction is found in germinating seeds, which is why it was named after malt. Unlike sucrose, it is a reducing sugar.

An aldose is a monosaccharide with a carbon backbone chain with a carbonyl group on the endmost carbon atom, making it an aldehyde, and hydroxyl groups connected to all the other carbon atoms. Aldoses can be distinguished from ketoses, which have the carbonyl group away from the end of the molecule, and are therefore ketones.

Digestion is the breakdown of carbohydrates to yield an energy-rich compound called ATP. The production of ATP is achieved through the oxidation of glucose molecules. In oxidation, the electrons are stripped from a glucose molecule to reduce NAD+ and FAD. NAD+ and FAD possess a high energy potential to drive the production of ATP in the electron transport chain. ATP production occurs in the mitochondria of the cell. There are two methods of producing ATP: aerobic and anaerobic. In aerobic respiration, oxygen is required. Using oxygen increases ATP production from 4 ATP molecules to about 30 ATP molecules. In anaerobic respiration, oxygen is not required. When oxygen is absent, the generation of ATP continues through fermentation. There are two types of fermentation: alcohol fermentation and lactic acid fermentation.

<span class="mw-page-title-main">Fehling's solution</span> Chemical test for the reducibility of a sugar

In organic chemistry, Fehling's solution is a chemical reagent used to differentiate between water-soluble carbohydrate and ketone functional groups, and as a test for reducing sugars and non-reducing sugars, supplementary to the Tollens' reagent test. The test was developed by German chemist Hermann von Fehling in 1849.

<span class="mw-page-title-main">Caramelization</span> Process of liquifying sugar

Caramelization is a process of browning of sugar used extensively in cooking for the resulting rich, butter-like flavor and brown color. The brown colors are produced by three groups of polymers: caramelans (C24H36O18), caramelens (C36H50O25), and caramelins (C125H188O80). As the process occurs, volatile chemicals such as diacetyl (known for its intense, butter-like taste) are released, producing the characteristic caramel flavor.

<span class="mw-page-title-main">Ketose</span> Class of carbohydrates

A ketose is a monosaccharide containing one ketone group per molecule. The simplest ketose is dihydroxyacetone, which has only three carbon atoms. It is the only ketose with no optical activity. All monosaccharide ketoses are reducing sugars, because they can tautomerize into aldoses via an enediol intermediate, and the resulting aldehyde group can be oxidised, for example in the Tollens' test or Benedict's test. Ketoses that are bound into glycosides, for example in the case of the fructose moiety of sucrose, are nonreducing sugars.

<span class="mw-page-title-main">Barfoed's test</span>

Barfoed's test is a chemical test used for detecting the presence of monosaccharides. It is based on the reduction of copper(II) acetate to copper(I) oxide (Cu2O), which forms a brick-red precipitate.

<span class="mw-page-title-main">Tollens' reagent</span> Chemical reagent used to distinguish between aldehydes and ketones

Tollens' reagent is a chemical reagent used to distinguish between aldehydes and ketones along with some alpha-hydroxy ketones which can tautomerize into aldehydes. The reagent consists of a solution of silver nitrate, ammonium hydroxide and some sodium hydroxide. It was named after its discoverer, the German chemist Bernhard Tollens. A positive test with Tollens' reagent is indicated by the precipitation of elemental silver, often producing a characteristic "silver mirror" on the inner surface of the reaction vessel.

In chemistry, a chemical test is a qualitative or quantitative procedure designed to identify, quantify, or characterise a chemical compound or chemical group.

The Kiliani–Fischer synthesis, named for German chemists Heinrich Kiliani and Emil Fischer, is a method for synthesizing monosaccharides. It proceeds via synthesis and hydrolysis of a cyanohydrin, followed by reduction of the intermediate acid to the aldehyde, thus elongating the carbon chain of an aldose by one carbon atom while preserving stereochemistry on all the previously present chiral carbons. The new chiral carbon is produced with both stereochemistries, so the product of a Kiliani–Fischer synthesis is a mixture of two diastereomeric sugars, called epimers. For example, D-arabinose is converted to a mixture of D-glucose and D-mannose.

<span class="mw-page-title-main">Aldonic acid</span> Family of sugar acids

An aldonic acid is any of a family of sugar acids obtained by oxidation of the aldehyde functional group of an aldose to form a carboxylic acid functional group. Thus, their general chemical formula is HO2C(CHOH)nCH2OH. Oxidation of the terminal hydroxyl group instead of the terminal aldehyde yields a uronic acid, while oxidation of both terminal ends yields an aldaric acid.

<span class="mw-page-title-main">Seliwanoff's test</span> Chemical test

Seliwanoff’s test is a chemical test which distinguishes between aldose and ketose sugars. If the sugar contains a ketone group, it is a ketose. If a sugar contains an aldehyde group, it is an aldose. This test relies on the principle that, when heated, ketoses are more rapidly dehydrated than aldoses. It is named after Theodor Seliwanoff, the chemist who devised the test. When added to a solution containing ketoses, a red color is formed rapidly indicating a positive test. When added to a solution containing aldoses, a slower forming light pink is observed instead.

The suffix -ose is used in organic chemistry to form the names of sugars. This Latin suffix means "full of", "abounding in", "given to", or "like". Numerous systems exist to name specific sugars more descriptively. The suffix is also used more generally in English to form adjectives from nouns, with the sense "full of", as in "verbose": wordy, full of words.

References

  1. ↑ Pratt, Charlotte W.; Cornely, Kathleen (2013). Essential Biochemistry (Third ed.). Wiley. p. 626. ISBN   978-1118083505.
  2. 1 2 3 Nelson, David L.; Cox, Michael M. (2008). Lehnniger: Principles of Biochemistry (Fifth ed.). W.H. Freeman and Company. p.  241. ISBN   978-0716771081.
  3. 1 2 3 Campbell, Mary K.; Farrell, Shawn O. (2012). Biochemistry. Cengage Learning. p. 459. ISBN   978-0840068583.
  4. ↑ Nelson, David L.; Cox, Michael M. (2008). Lehnniger: Principles of Biochemistry (Fifth ed.). W.H. Freeman and Company. p.  243. ISBN   978-0716771081.
  5. 1 2 Davidson, Eugene A. (2015). "Carbohydrate". Encyclopædia Britannica.
  6. ↑ Klein, David. (2012). Organic Chemistry (First ed.). John Wiley & Sons. pp. 1162–1165. ISBN   978-0471756149.
  7. 1 2 Klein, David. (2012). Organic Chemistry (First ed.). John Wiley & Sons. p. 1159. ISBN   978-0471756149.
  8. ↑ Leung, David W. M.; Thorpe, Trevor A. (April 1984). "Interference by edta and calcium ions of the 3,5-dinitrosalicylate reducing sugar assay". Phytochemistry. Pergamon Press. 23 (12): 2949–2950. doi:10.1016/0031-9422(84)83048-4. ISSN   0031-9422.
  9. ↑ Dills, William L. Jr. (November 1993). "Protein fructosylation: fructose and the Maillard reaction". The American Journal of Clinical Nutrition. American Society for Nutrition. 58 (5): 779S–87. doi: 10.1093/ajcn/58.5.779s . ISSN   0002-9165. PMID   8213610.
  10. ↑ Jiang, Zhanmei; Wang, Lizhe; Wu, Wei; Wang, Yu (June 2013). "Biological activities and physicochemical properties of Maillard reaction products in sugar–bovine casein peptide model systems". Food Chemistry. Elsevier. 141 (4): 3837–3845. doi:10.1016/j.foodchem.2013.06.041. ISSN   0308-8146. PMID   23993556.
  11. ↑ Pedreschi, Franco; Mariotti, María Salomé; Granby, Kit (August 2013). "Current issues in dietary acrylamide: formation, mitigation and risk assessment". Journal of the Science of Food and Agriculture. Society of Chemical Industry. 94 (1): 9–20. doi:10.1002/jsfa.6349. hdl: 10533/127076 . ISSN   0022-5142. PMID   23939985.
  12. ↑ "Acrylamide and Cancer Risk". American Cancer Society. 11 February 2019.
  13. ↑ Leotério, Dilmo M.S.; Silva, Paulo; Souza, Gustavo; Alves, Aline de A.; Belian, Mônica; Galembeck, André; Lavorante, André F. (November 2015). "Copper–4,4′-dipyridyl coordination compound as solid reagent for spectrophotometric determination of reducing sugar employing a multicommutation approach". Food Control. European Federation of Food Science and Technology; International Union of Food Science and Technology. 57: 225–231. doi:10.1016/j.foodcont.2015.04.017. ISSN   0956-7135.
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