Editing Reducing sugar

{{Short description|Sugars that contain free OH group at the anomeric carbon atom}}
[[File:D-glucose lineal.svg|thumb|class=skin-invert-image|Reducing form of [[glucose]] (the [[aldehyde|aldehyde group]] is on the far right)]]

A '''reducing sugar''' is any [[sugar]] that is capable of acting as a [[reducing agent]].<ref>{{cite book |last1=Pratt |first1=Charlotte W. |last2=Cornely |first2=Kathleen  |date=2013 |title=Essential Biochemistry |publisher=Wiley |edition=Third |page=626 |isbn= 978-1118083505}}</ref> 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]].

All [[monosaccharide]]s are reducing sugars, along with some [[disaccharide]]s, some [[oligosaccharide]]s, and some [[polysaccharide]]s. The monosaccharides can be divided into two groups: the [[aldose]]s, which have an aldehyde group, and the [[ketose]]s, which have a ketone group. Ketoses must first [[tautomerization|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 bond]]s between their [[anomeric carbon]]s 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' reagent|Tollens' test]] or [[Benedict's test]]. The cyclic [[hemiacetal]] forms of [[aldose]]s can open to reveal an aldehyde, and certain ketoses can undergo tautomerization to become aldoses. However, [[acetal]]s, 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 [[Chemical compound|compound]] and is itself ''[[Redox|oxidized]]''; that is, the [[Carbonyl group|carbonyl]] carbon of the [[sugar]] is oxidized to a [[carboxyl]] group.<ref name=Lehninger>{{cite book |last1=Nelson |first1=David L. |last2=Cox |first2=Michael M. |date=2008 |title=Lehnniger: Principles of Biochemistry |publisher=W.H. Freeman and Company |edition=Fifth |page=[https://archive.org/details/lehningerprincip00lehn_1/page/241 241] |isbn=978-0716771081 |url-access=registration |url=https://archive.org/details/lehningerprincip00lehn_1/page/241 }}</ref>

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.<ref name=Campbell>{{cite book |last1=Campbell |first1=Mary K. |last2=Farrell |first2=Shawn O.  |date=2012 |title=Biochemistry |publisher=Cengage Learning |page=459 |isbn=978-0840068583}}</ref>

===Aldoses and ketoses===
[[Monosaccharide]]s 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 [[tautomerism|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.<ref name=Campbell/>

===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 bond]]s, 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.<ref>{{cite book |last1=Nelson |first1=David L. |last2=Cox |first2=Michael M. |date=2008 |title=Lehnniger: Principles of Biochemistry |publisher=W.H. Freeman and Company |edition=Fifth |page=[https://archive.org/details/lehningerprincip00lehn_1/page/243 243] |isbn=978-0716771081 |url-access=registration |url=https://archive.org/details/lehningerprincip00lehn_1/page/243 }}</ref>

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).<ref name=Britannica>{{cite encyclopedia |last=Davidson |first=Eugene A. |encyclopedia=Encyclopædia Britannica |title=Carbohydrate |year=2015}}</ref> This includes common monosaccharides like [[galactose]], [[glucose]], [[glyceraldehyde]], [[fructose]], [[ribose]], and [[xylose]].

Many [[disaccharide]]s, 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.<ref>{{cite book |last1=Klein |first1=David.  |date=2012 |title=Organic Chemistry |edition=First |publisher=John Wiley & Sons |pages=1162–1165 |isbn=978-0471756149}}</ref> 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.<ref name=Britannica/>

[[File:Maltose Gleichgewicht.svg|thumb|center|upright=3|class=skin-invert|Equilibrium between cyclic and open-chain form in one ring of maltose]]

In glucose [[polymer]]s such as [[starch]] and starch-[[Derivative (chemistry)|derivatives]] like [[corn syrup|glucose syrup]], [[maltodextrin]] and [[dextrin]] the [[macromolecule]] begins with a reducing sugar, a free aldehyde. When starch has been partially [[hydrolysis|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.<ref name=Lehninger/>

==Characterization==
Several [[qualitative organic analysis|qualitative tests]] are used to detect the presence of reducing sugars. Two of them use solutions of [[cupric|copper(II)]] ions: [[Benedict's reagent]] (Cu<sup>2+</sup> in aqueous sodium citrate) and [[Fehling's solution]] (Cu<sup>2+</sup> in aqueous sodium tartrate).<ref name=Klein>{{cite book |last1=Klein |first1=David.  |date=2012 |title=Organic Chemistry |edition=First |publisher=John Wiley & Sons |page=1159 |isbn=978-0471756149}}</ref> The reducing sugar reduces the [[copper|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<sup>+</sup>) in aqueous ammonia.<ref name=Klein/> When Tollen's reagent is added to an aldehyde, it precipitates silver metal, often forming a silver mirror on clean glassware.<ref name=Campbell/>

[[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.<ref>{{cite journal |last1=Leung |first1=David W. M. |last2=Thorpe |first2=Trevor A. |date=April 1984 |title=Interference by edta and calcium ions of the 3,5-dinitrosalicylate reducing sugar assay |journal=Phytochemistry |publisher=Pergamon Press  |volume=23 |issue=12 |pages=2949–2950 |doi=10.1016/0031-9422(84)83048-4  |bibcode=1984PChem..23.2949L |issn=0031-9422 }}</ref>

Some sugars, such as sucrose, do not react with any of the reducing-sugar test solutions. However, a non-reducing sugar can be [[Hydrolysis#Polysaccharides|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 level]]s 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.<ref name=Lehninger/>

==Importance in food chemistry==

===Maillard reaction===
{{main|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.<ref>{{cite journal 
 |last=Dills |first=William L. Jr. |date=November 1993 |title=Protein fructosylation: fructose and the Maillard reaction |journal=The American Journal of Clinical Nutrition |publisher=American Society for Nutrition |volume=58 |issue=5 |pages=779S–87 |issn=0002-9165 | doi=10.1093/ajcn/58.5.779s |pmid=8213610 |doi-access=free }}</ref> 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.<ref>{{cite journal |last1=Jiang |first1=Zhanmei |last2=Wang |first2=Lizhe |last3=Wu |first3=Wei |last4=Wang |first4=Yu |date=June 2013 |title=Biological activities and physicochemical properties of Maillard reaction products in sugar–bovine casein peptide model systems |journal=Food Chemistry |publisher=Elsevier |volume=141 |issue=4 |pages=3837–3845 |doi=10.1016/j.foodchem.2013.06.041 |pmid=23993556 |issn=0308-8146 }}</ref> 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&nbsp;°C).<ref>{{cite journal |last1=Pedreschi |first1=Franco
|last2=Mariotti |first2=María Salomé |last3=Granby |first3=Kit |date=August 2013 |title=Current issues in dietary acrylamide: formation, mitigation and risk assessment |journal=Journal of the Science of Food and Agriculture |publisher=Society of Chemical Industry  |volume=94 |issue=1 |pages=9–20 |doi=10.1002/jsfa.6349 |pmid=23939985
|issn=0022-5142 |hdl=10533/127076 |hdl-access=free }}</ref> However, evidence from epidemiological studies suggest that dietary acrylamide is unlikely to raise the risk of people developing cancer.<ref name=acs>{{cite web |url=http://www.cancer.org/cancer/cancercauses/othercarcinogens/athome/acrylamide |title=Acrylamide and Cancer Risk|date=11 February 2019 |publisher=[[American Cancer Society]] }}</ref>

===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.<ref>{{cite journal |last1=Leotério |first1=Dilmo M.S. |last2=Silva |first2=Paulo |last3=Souza |first3=Gustavo |last4=Alves |first4=Aline de A. |last5=Belian |first5=Mônica |last6=Galembeck |first6=André  |last7=Lavorante |first7=André F. |date=November 2015 |title=Copper–4,4′-dipyridyl coordination compound as solid reagent for spectrophotometric determination of reducing sugar employing a multicommutation approach |journal=Food Control |publisher=European Federation of Food Science and Technology; International Union of Food Science and Technology |volume=57 |pages=225–231 |doi=10.1016/j.foodcont.2015.04.017 |issn=0956-7135 }}</ref>

==References==
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[[Category:Carbohydrate chemistry]]
[[Category:Biomolecules]]
[[Category:Carbohydrates]]