Color reactions of Proteins : Biochemistry series

Summary

The video provides a detailed overview of various biochemical tests used to detect the presence of amino acids and proteins. It covers tests like the Biuret test for peptide bonds, Ninhydrin test for alpha amino acids, and Millon’s test for tyrosine. Additional tests like the Sakaguchi test for arginine, the sulfur-containing amino acid test for cysteine, and Hopkins-Cole test for tryptophan are explained, showcasing color changes that occur due to specific chemical reactions. The video emphasizes accurate chemical handling to ensure correct results in these protein detection methods.

Takeaways

• 🧪 The Biuret test is used to detect peptide bonds in proteins, turning violet when peptide bonds are present.
• 🔵 Excess copper sulfate should be avoided as it can interfere with the Biuret test results.
• 🟣 Ninhydrin reacts with alpha amino acids to produce a purple color, a reaction used in forensics for detecting fingerprints.
• 🌡 The xanthoproteic test involves boiling protein with concentrated nitric acid, which can turn yellow due to aromatic amino acids like tyrosine and tryptophan, and orange upon alkalinization with sodium hydroxide.
• 💧 The Millon's test uses mercuric nitrate and sodium nitrite to detect tyrosine, forming a color due to the reaction with the hydroxyl group of tyrosine.
• 🌀 The Sakaguchi test is for detecting arginine, which contains a guanidine group, using alpha naphthol and bromine water or sodium hypochlorite to form a colored complex.
• 🍂 The lead acetate test detects the presence of sulfur-containing amino acids like cysteine, forming a brown to black precipitate of lead sulfide.
• 🧉 The Pauly test is for the imidazole group and phenolic hydroxyl group, reacting with benzene sulphonic acid to produce a colored product, indicating the presence of histidine or tyrosine.
• 🌀 The aldehyde test, also known as the Hopkins-Cole test, detects the indole nucleus present in tryptophan, forming a violet ring at the junction of two liquids with mercuric sulfate and concentrated sulfuric acid.
• ⏱ Each test has specific procedural steps and reagents that must be followed carefully to ensure accurate results.

Q & A

What is the purpose of the Biuret test?

The Biuret test is used to detect the presence of peptide bonds in proteins. A positive result indicates the presence of proteins with at least two peptide bonds.

What reaction occurs when copper sulfate is added in the Biuret test?

Initially, the addition of copper sulfate forms a blue precipitate of copper hydroxide. On mixing, this reacts with the peptide bonds, leading to the formation of a violet color, indicating a positive test for proteins.

Why should excess copper sulfate be avoided in the Biuret test?

Excess copper sulfate should be avoided because it can interfere with the reaction and potentially give misleading results, such as a false positive or overly intense color.

How does the ninhydrin test work, and what does it detect?

The ninhydrin test detects alpha amino acids. When heated with ninhydrin, these amino acids produce a purple color due to the formation of a complex called Ruhemann's purple. This test is also used in forensic science for fingerprint detection.

What is the result of the Xanthoproteic test, and which amino acids are detected?

The Xanthoproteic test results in a yellow color due to the nitration of aromatic amino acids like tyrosine and tryptophan. When sodium hydroxide is added, the color changes to orange, indicating the presence of these amino acids.

What is the principle behind the Millon’s test?

Millon’s test is used to detect tyrosine. The reaction forms a color due to the formation of nitrated mercuric phenolate, indicating the presence of the hydroxy phenyl group of tyrosine.

What does the Sakaguchi test detect in proteins?

The Sakaguchi test detects the presence of the guanidine group in the amino acid arginine. The test involves a reaction between arginine and alpha-naphthol, bromine water, or sodium hypochlorite, forming a colored complex.

How does the sulfur test for cysteine or cystine work?

In the sulfur test, proteins containing cysteine or cystine release sulfide ions, which react with lead acetate to form a brown to black precipitate of lead sulfide. This indicates the presence of sulfur-containing amino acids.

What does the Pauly test detect, and how does it work?

The Pauly test detects the presence of histidine and tyrosine. Benzene sulfonic acid reacts with the imidazole ring of histidine or the phenolic hydroxyl group of tyrosine to produce a red color, indicating their presence.

What is the principle of the Hopkins-Cole (Aldehyde) test?

The Hopkins-Cole test detects the presence of the indole nucleus in tryptophan. The reaction involves mercuric sulfate and an aldehyde, which condense with the indole group to form a violet-colored ring at the junction of the liquids.