What is Adamkiewicz reaction?
The Adamkiewicz reaction, also known as the Hopkins-Cole test, is a biochemical test used to identify the presence of tryptophan, an essential amino acid, in proteins. This test, named after Edward Hopkins and Otto Adamkiewicz, is significant in protein chemistry and is still used in educational settings to demonstrate protein composition.

Table of Contents
Objective
The goal of the Adamkiewicz reactionis to detect tryptophan within a protein sample. A positive reaction produces a violet or purple ring at the interface between two layers of solutions, indicating the presence of tryptophan in the protein.
Principle
The test works by exploiting the reaction between tryptophan’s indole ring and glyoxylic acid (formed from oxalic acid and concentrated sulfuric acid) under highly acidic conditions.

In the presence of concentrated sulfuric acid, tryptophan’s indole ring reacts with glyoxylic acid, producing a purple or violet-colored complex. The sulfuric acid serves two roles: it enhances the reaction by creating a strong acidic environment and helps to separate the layers, making the resulting color change at the interface more visible.
The test is highly specific because only tryptophan’s indole structure reacts with glyoxylic acid, forming a distinct colored product that confirms the presence of this amino acid in the sample.
Reagents of Adamkiewicz reaction
The reagents required for the Adamkiewicz reaction are:
- Protein sample: Any protein solution that potentially contains tryptophan.
- Oxalic acid solution (10%): This is used to create glyoxylic acid.
- Concentrated sulfuric acid (H₂SO₄): This acid creates the necessary reaction conditions and forms the visible interface for the reaction.
- Water: Used to dilute or prepare the reagents.
Procedure of Adamkiewicz reaction
Reagent preparation
a. Prepare a 10% oxalic acid solution by dissolving 10 g of oxalic acid in 100 mL of distilled water.
b. Carefully handle concentrated sulfuric acid, as it is highly corrosive.

Add the protein sample
Place 2-3 mL of the protein solution in a clean test tube. A 1-2% protein concentration typically gives clear results.
Add glyoxylic acid
Add 1-2 drops of the 10% oxalic acid solution to the protein solution. Glyoxylic acid will form during the next step when sulfuric acid is added.
Add sulfuric acid
Slowly add 2 mL of concentrated sulfuric acid along the side of the test tube to form a separate layer beneath the protein solution, without mixing. Hold the tube at an angle and let the acid slide down gently.
Observe the reaction
After a few minutes, a purple or violet ring should form at the interface if tryptophan is present. The color may vary in intensity depending on the amount of tryptophan in the sample.
Result and Interpretation
A positive Hopkins-Cole test is confirmed by the appearance of a purple or violet ring at the interface of the protein solution and the sulfuric acid layer, indicating the presence of tryptophan.
Positive result: A purple or violet ring confirms the presence of tryptophan in the sample.
Negative result: No color change indicates that tryptophan is absent.
Conclusion
The Adamkiewicz reaction or Hopkins-Cole test is an effective method to detect tryptophan in proteins. The test relies on the specific interaction between glyoxylic acid and tryptophan’s indole ring, which produces a visible color change. Although newer methods, such as spectrometry and chromatography, have largely replaced this qualitative test, the Hopkins-Cole test remains a valuable tool for educational purposes and to quickly assess the presence of tryptophan in a protein solution.
While the test cannot quantify tryptophan levels or differentiate between various proteins, it serves as a simple and efficient way to confirm the presence of tryptophan in protein samples.
Frequently Asked Questions(FAQ)
What is Adamkiewicz reaction?
The Adamkiewicz reaction, also known as the Hopkins-Cole test, is a biochemical test used to identify the presence of tryptophan, an essential amino acid, in proteins.
What is glyoxylic acid?
An organic substance is oxoacetic acid, also known as glyoxylic acid. Glyoxylic acid is one of the C2 carboxylic acids, along with oxalic, glycolic, and acetic acids. It is a naturally occurring, colorless solid with industrial use.
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