Reagents

Barfoed's Reagent: This looks like Benedict's but differs somewhat. The reagent is prepared by dissolving 70 g copper acetate monohydrate and 9 mL glacial acetic acid in water to a final volume of one liter. The reagent is stable for years.

When 1 mL of reagent is heated with 5 drops of sample in a boiling water bath, a positive test for monosaccharides is formation of a brick-red precipitate within five minutes. Disaccharides generally don't give any reaction even for ten minutes. The precipitate isn't nearly as voluminous as that seen with Benedict's test and tends to adhere to the walls of the test tube.

Benedict's Reagent: We generally use a commercial reagent, but to make it from scratch, first dissolve 100 g sodium carbonate and 173 g sodium citrate dihydrate in a final volume of 850 mL water. Slowly, with stirring, add a solution of 17.3 g copper sulfate pentahydrate in 100 mL of water. Bring the final volume to one liter. The commercial reagent, at least, seems to be stable for years.

When 1 mL of reagent is heated with 5 drops of sample in a boiling water bath, a positive test for reducing sugars is formation of a precipitate within five minutes. The color ranges from green to yellow to orange to brick-red depending on the amount of reducing sugar in the sample; with a sample containing 1% glucose, the precipitate is usually brick-red.

Bial's Reagent: Dissolve 3 g orcinol in 500 mL concentrated HCl, add 2.5 mL of a 10% solution of ferric chloride hexahydrate, and dilute to one liter with water; this is approximately 6 M HCl. The reagent is stable for months, but its yellow color gradually darkens and some precipitate forms; this doesn't seem to affect its reactivity. The "classical" Bial's reagent is made with a liter of concentrated HCl, undiluted with water. It gives a slightly stronger reaction, and considerably faster (30-60 seconds), but is much less stable than the recipe we've come up with, and the fumes are much more a problem with concentrated than with 6 M HCl. The reaction even seems to work, more slowly and with less intense color, if the final HCl concentration is only 4 M.

When 1 mL of reagent is heated with 5 drops of sample in a boiling water bath, a positive test for pentoses is formation of a green to blue color (not precipitate) in less than five minutes.

Biuret Reagent: Add, with stirring, 300 mL of 10% (w/v) NaOH to 500 mL of a solution containing 0.3% copper sulfate pentahydrate and 1.2% sodium potassium tartrate, then dilute to one liter. The reagent is stable for a few months but not a year. Adding one gram of potassium iodide per liter and storing in the dark makes it stable indefinitely.

The reagent can be used either qualitatively or quantitatively.  In a typical reaction, one volume of sample is mixed with two to five volumes of reagent; the optimal ratio depends on the maximum protein concentrations you want to be able to resolve.  The presence of protein gives a violet color with maximum absorbance around 550-555 nm; we typically read absorbances at 540 nm.

Bradford's Reagent: The original published recipe [see Analyt. Biochem. 72, 248-254 (1976)] calls for dissolving 100 mg Coomassie Blue G-250 in 50 mL of 95% ethanol, add 100 mL of 85% phosphoric acid, and dilute to one liter. The reagent needs to be filtered at least once and perhaps more, since it seems to precipitate dye over time. "Bradford reagents" are available commercially that use more stable formulations. I heard from someone that Sigma's formula uses 40 mL of methanol (final 4%) in place of ethanol and about 120 mL of phosphoric acid (final 10%); I tried this and I couldn't say it worked any better than the original. This reagent is said to be unstable, but I think I've used the same batch over a year or two without any problems.

To quantify protein, mix 0.25 mL of sample with 2.5 mL of Bradford reagent. After 5 minutes, measure the absorbance at 595 nm.  One disadvantage to the reagent is that it gives a high blank which may affect subsequent readings because some reagent adheres to the cuvette.  Another is that it is very sensitive to the presence of detergent, either from poorly-rinsed glassware or, heaven forbid, in the event you are studying detergent-solubilized membrane proteins.

DNSA Reagent: This reagent detects reducing ends of carbohydrates and I find it useful in many experiments. Its composition is 1% 3,5-dinitrosalicylic acid (DNSA), 30% sodium potassium tartrate, and 0.4 M NaOH. It appears to be stable for a year or so; there is some darkening on longer storage, though older reagent still seems to function adequately.

In a typical reaction, equal volumes of sample and the reagent are mixed and heated in a boiling water bath for 10 minutes.  The resulting solution is cooled and diluted with about ten volumes of water, and absorbance is determined at 540 nm.  I typically use about 0.4 mL each of sample and DNSA reagent, then dilute after heating with 4 mL of water, giving a reasonable volume for absorbance determination.  When there are no reducing ends present, the final color is yellow and the absorbance ranges from 0.03 to 0.05. A positive result is formation of a red color with absorbances that may range upward to well over 1.0.

Lowry Reagents: Reagent 1: Mix one volume of reagent B (0.5% copper sulfate pentahydrate, 1% sodium or potassium tartrate) with 50 volumes of reagent A (2% sodium carbonate, 0.4% NaOH). Both reagents A and B are supposed to be stable for a long time but I have had a problem with precipitation in reagent B that seems to be remedied by adding a little NaOH.

Reagent 2: Dilute commercial Folin-Ciocalteu phenol reagent with an equal volume of water. Stable for a few days or weeks.

To quantify protein, mix 0.25 mL of protein with 2.5 mL of Lowry reagent 1. After 10 minutes, add 0.25 mL of Lowry reagent 2 and mix well immediately. After 30 minutes, measure the absorbance at 750 nm (if you're using a Spectronic 20 with a normal phototube, 750 is too long; 600 nm gives lower absorbances but works okay).

Seliwanoff's Reagent:  Dissolve 1 g resorcinol in 330 mL concentrated HCl, dilute to one liter (approx. 4 M HCl final). This reagent seems to be stable for more than a year, though we usually make less than the recipe specifies. 

When 1 mL of reagent is heated with 5 drops of sample in a boiling water bath, a positive test for ketoses (sucrose works, too) is formation of an orange to red color (not precipitate) within five minutes. Some sources say an apricot color is negative, but it's a judgment call. It depends on the concentration in the sample, and sugars like glucose give essentially no color even after ten minutes.

This reaction is also quantitative; absorbances can be read at around 480 nm. I haven't carefully documented the linear ranges in terms of amount of ketose and incubation times.

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Revised 6/14/07