Reducing sugars refer to sugars with reducing properties.In sugars, molecules contain freeAldehyde grouporKeto groupBoth monosaccharides containing free aldehyde groups and disaccharides containing free aldehyde groups have reducibility.Reducing sugars mainly include glucosefructose、Galactose、lactose、malt dustEtc.
Fehling's solution And citric acid, copper sulfate andsodium hydroxideFormulatedBenedet reagent（Bannister's reagent）It often reacts with aldose and ketose under the condition of water bath heatingCuprous oxideBrick red precipitation means that the reagent itself is reduced, so all sugars that can react with the above reagents are called reducing sugars, and all sugars that cannot react with the above reagents are called non reducing sugars, and glycosides cannot react with the above reagents[1]。Glucose molecules contain free aldehyde groups, fructose molecules contain free ketone groups, lactose and maltose molecules contain free aldehyde groups, so they arereducing sugar。
Non reducing sugars include sucrose, starch, cellulose, etc., but they can be hydrolyzed to produce corresponding reducing propertiesmonosaccharide。
nature
Capable of reducing H.von Fehling reagent（Benedet reagent）OrTorranceThe sugar of (B. Tollens) reagent is calledreducing sugar, all monosaccharides (except dihydroxyacetone）, regardless ofaldoseAnd ketose are reducing sugars.grossdisaccharideIt is also reducing sugar, sucrose andtrehalose[4]Exceptions.Fehling's solution It is a solution containing Cu2+complex, which is reduced to brick red CutwoO precipitation.Torrance reagentAfter reduction, simple silver can be generated and "silver mirror reaction" occurs.
The molecular structure contains reducibilityGroupSugar (such as free aldehyde group or free ketone group) is called reducing sugar.Such as glucose.
Fructose contains free ketone group, so fructose is also a reducing sugar[2]。
If the sugar isKetose, the carbonyl group will break down into two smaller molecules, such asfructose。
All monosaccharides (except dihydroxyacetone) and most disaccharides (except sucrose, etc.) showedpositiveReaction, so most monosaccharides and disaccharides are reductive[2]。
reducing sugarIt can react with Fehling reagent to form brick red precipitate.
Reagents required
Fehling reagent (mainly prepared from NaOH solution with mass concentration of 0.1g/mL and CuSO4 solution with mass concentration of 0.05g/mL)
be careful: ready to use
Preparation of experimental materials
Plant tissue is a commonly used experimental material, but it must be selected.The most ideal experimental material for this experiment is the one with high sugar contentbiological tissue (or organ), and the color of the tissue is lighter or nearly white, such as the fruit of apples and pears.By comparison, the obvious degree of color reaction is apple, pearWhite cabbage leaves, white radish.
Operation method
① Inject 2mL of tissue drug solution to be tested into the test tube
② Inject 1mL into the test tubeFehling's solution (Liquid A and liquid B shall be mixed equally before injection)
③ Put the test tube into a large beaker containing 50~65 ℃ warm water and heat it for about 2min
④ Observe the color change in the test tube
Other instructions
Fehling reagent andBannister's reagentThey are all reagents for testing reducing sugars, and their use methods, principles and components are different.The following is a brief summary of the use principle, composition and use method of these reagents.
Fehling reagent and biuret reagent are both composed of NaOH solution and CuSO4 solution, but they are different in the following three aspects:
(1) Different solution concentrations
The solution in Fehling reagent is Fehling reagent A (NaOH solution) with a concentration of 0.1g/ml, and Fehling reagent B (CuSO4 solution) with a concentration of 0.05g/ml;Biuret reagent: the concentration of biuret reagent A (NaOH solution) is 0.1g/ml, and the concentration of biuret reagent B (CuSO4 solution) is 0.01g/ml.
(2) Different principles of use
Fehling reagent is a newly prepared solution. It reacts with aldehyde group under heating conditions and is reduced to brick red precipitate, which can be used to identify the solubilityreducing sugarThe exists of.When Fihlin reagent was used to identify soluble reducing sugar, the color change process of the solution was light blue → brown → brick red (precipitation).
appraisalbiological tissue When there is protein inBiuret method, usingBiuret reagent, what happened wasBiuret reaction。The essence of biuret reaction is that it occurs with biuret reagent in alkaline environmentPurple reaction。The protein molecule contains a lot ofBiuretPeptide bond with similar structure, so all proteins can occur with biuret reagentColor response, can use biuret reagent to identify the existence of protein[3]。
(3) The two reagents are stored in different ways.Fehling's solution A and Fehling reagent B can be produced strongly and precipitated easily, so Fehling reagent is usually prepared on the spot;andBannister's reagentIn the formula of,Sodium citrateAs a pairBuffer materialThe amount is limited, and the concentration is relatively low after mixing with the solution, so it is not easy to precipitate, so the reagent can be stored for a long time[3]。
Of course, it doesn't matterBannister's reagentstillFehling's solution In the final analysis, it is the reaction with aldehyde group under the heating condition of boiling water bath to form brick red precipitate. The reaction phenomenon of the two is the same, which is the similarity of the two.
(4) Remarks:
If in solutionreducing sugarLow content, resultingCuprous oxideThere will be less, and only green, turbid yellow or orange will appear after the test.
In acidic environment, Cu2 will become more stable and not easy to react, so the test cannot be carried out.
Alcohols and aldehydes will also produce brick red precipitates in this test, because both have functional groups that play a role in this test.
