Biuret Reagent Prep and Use
Biuret solution is the standard reagent for detecting peptide bonds in proteins. The unreacted liquid is blue. When it encounters proteins, it shifts to violet or purple depending on concentration. If there are no peptide bonds, the solution stays blue. That is the basic principle.What Color Is Biuret Solution
Unreacted biuret reagent is light blue. This comes from the copper(II) ions in copper sulfate dissolved in a strongly alkaline solution. The deep blue color of the complex itself gives it away. Once you add a protein sample, the copper ions coordinate with the nitrogen atoms in the peptide bonds, and the color shifts through pink to violet to purple as concentration increases. A negative result means it stays blue. That is the whole test in one sentence. I mix my own batches because commercial stuff is overpriced and sometimes sits on shelves too long. My recipe is straightforward: dissolve 1.5 grams of copper sulfate pentahydrate and 6 grams of potassium sodium tartrate in about 500 milliliters of distilled water, then carefully add 300 milliliters of 10% sodium hydroxide solution and top up to one liter. The tartrate keeps the copper in solution so it does not precipitate out as copper hydroxide when the NaOH hits. Without it, you get a messy suspension and bad results. The solution is stable for months if stored in a tightly sealed bottle away from light. I label it with the prep date and discard anything older than six months just to be safe. I have seen people keep it for years and wonder why their controls give weird readings. Carbon dioxide from the air slowly neutralizes the alkali over time, which degrades the reagent's effectiveness. Keep the cap on.
When you run the test, you add about 2 milliliters of reagent to 1 milliliter of your sample and let it sit for five minutes at room temperature. Then compare the color against a known standard or a blank. The timing matters. Read too early and the color has not fully developed. Read too late and you may pick up background interference from non-protein substances that slowly reduce copper under those alkaline conditions. I ran into a problem once with a plant extract that kept giving false violet readings even though my Bradford assay showed negligible protein. The issue was free amino acids and small peptides in the extract. Biuret only detects bonds, so anything with two or more peptide linkages triggers a positive. That plant sample was full of them. I ended up using the Lowry method instead, which gave me a clearer picture, though it is slower and more finicky with interfering compounds like detergents and reducing agents. One thing beginners consistently mess up is the order of addition. Do not add the sample to the concentrated reagent and then try to mix. Add the reagent to the sample. It gives better contact and more consistent results, especially with viscous or cloudy solutions that resist mixing.
Also, the test is not quantitative on its own. You need a calibration curve with known protein concentrations if you want actual numbers. Bovine serum albumin is the usual standard because it is cheap and well-characterized. I prepare standards at 0, 0.5, 1, 2, and 4 milligrams per milliliter and plot absorbance at 540 nanometers against concentration. The curve is linear up to about 2 mg/mL, then it starts flattening out. If your sample falls in that higher range, dilute it and run it again. There are limitations worth knowing. High concentrations of ammonium salts interfere because they compete with the peptide nitrogens for copper coordination. Ammonia-based cleaning solutions on glassware can throw off readings if you do not rinse thoroughly. And amide groups without peptide bonds, like in urea at high concentrations, can produce a faint pink color that someone might mistake for a weak positive. It is not a real protein signal, but it is easy to misinterpret if you are not expecting it. If you need something faster for a quick check, a few drops of biuret reagent on a spot plate will tell you whether your isolation worked within a minute. No spectrophotometer needed. It is not precise, but it saves you from continuing down a dead-end purification route. I use this trick all the time before committing to a full column run.
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