Reactions of Amino Acids: Ninhydrin Test - Online Tutor, Practice Problems & Exam Prep

Amino acids react with Ninhydrin, an aromatic diketone hydrate, to form a purple dye known as Ruhem's purple, which is utilized in forensic science to detect fingerprints. The reaction involves two moles of Ninhydrin interacting with one amino acid, leading to deamination, decarboxylation, and aldehyde formation. The amino group contributes to the dye, while the carboxyl group is released as CO₂, and the alpha carbon forms an aldehyde. This process highlights the significance of amino acids in biochemical applications and forensic analysis.

Reactions of Amino Acids: Nihydrin Test Concept 1

Now we're going to say amino acids are curious compounds that become visibly detectable using ninhydrin. Ninhydrin itself is an aromatic diketone hydrate that reacts with the primary amines of amino acids. When we take a look here, we have our aromatic portion with this benzene, diketone from these two carbonyls, and a hydrate because we have a geminal diol. So, geminal meaning they're on the same carbon. Now, we're going to say this can interact with an amino acid and it creates a dye called Ruhemann's purple.

Now, where's the real-world application? Well, in forensics, it's used to detect fingerprints on surfaces. You might see TV shows or movies where detectives use strips and are able to lift fingerprints off of surfaces. This is ninhydrin interacting with the amino acids in sweat left behind by the person. Yes.

In our sweat, there are bits of amino acids. And the ninhydrin can interact with them leaving behind a fingerprint. Here we can see this purple fingerprint. Now, if we take a look at the reaction itself, we have 2 ninhydrins interacting with 1 amino acid. So it's a 2 to 1 relationship.

We have the use of our base Sodium Hydroxide that creates our ninhydrin here and we're going to say in our ninhydrin, we're going to have a single bond here to nitrogen and a double bond here. Okay. We're going to say this represents our ninhydrin. We're going to say the reaction outcomes of amino acid degradation, one is deamination where the amino group incorporates within the dye. So the nitrogen of the amino group of the amino acid is now here as part of the dye.

We're going to have decarboxylation. We're going to say here that the carboxyl group here is lost as CO₂. And then, finally, we have aldehyde formation. The alpha carbon, so this here gains a double bond O. So, here's the same alpha carbon and it gains a double bond O to create our aldehyde.

So this is the Ninhydrin test. It interacts with the primary amine of the amino acid and it basically breaks down the amino acid into Ruhemann's purple which we would see. And then aldehyde and carbon dioxide as additional byproducts. Right? So, just keep in mind when we're talking about the Ninhydrin test, this is what we mean.

2 moles of a ninhydrin molecule reacting with 1 amino acid to produce our dienaldehyde and carbon dioxide.

Reactions of Amino Acids: Nihydrin Test Example 1

Everyone. So here it says, draw the products formed when valine reacts with ninhydrin. So, we know ninhydrin here is going to help to make our dye initially. So let's draw this dye. And so we're gonna have a negative charge.

This is still a carbonyl. Alright. We're going to say here it's connected to our nitrogen. The nitrogen that we have comes from this amino group. It'll be double bonded.

So here goes our purple dye that we make initially, well, we make as a product, plus remember here that this carboxyl group changes into our carbon dioxide, so CO2 plus, we're going to say this alpha carbon here, that alpha carbon becomes the aldehyde group. It goes out from the alpha carbon, it becomes the aldehyde group; it's a carbonyl connected to an H. So these will represent our three products that are created when valine reacts with a ninhydrin molecule.

Which of the following amino acids would not create an aldehyde as a by-product?

• A. Cysteine
• B. Valine
• C. Proline
• D. Histidine
• E. Glycine

Name the aldehyde formed when isoleucine reacts with ninhydrin.

• A. (R)-2-methylbutanal
• B. Pentanal
• C. (S)-2-methylbutanal
• D. (S)-3-methylbutanal

The following aldehyde is obtained from the reaction between an amino acid and ninhydrin. Select the structure of the amino acid used initially. 

• A.
• B.
• C.
• D.

Provide the name of the amino acid used to create the following aldehyde after its reaction with ninhydrin. 

• A. Glycine
• B. Lysine
• C. Threonine
• D. Methionine
• E. Proline

Here’s what students ask on this topic:

What is the Ninhydrin test and how does it work?

The Ninhydrin test is a chemical reaction used to detect amino acids. Ninhydrin, an aromatic diketone hydrate, reacts with the primary amines of amino acids to form a purple dye known as Ruhem's purple. The reaction involves two moles of Ninhydrin interacting with one amino acid, leading to deamination, decarboxylation, and aldehyde formation. The amino group of the amino acid incorporates into the dye, the carboxyl group is released as CO₂, and the alpha carbon forms an aldehyde. This test is widely used in forensic science to detect fingerprints, as amino acids in sweat react with Ninhydrin to leave a visible purple mark.

What are the products of the Ninhydrin reaction with amino acids?

When Ninhydrin reacts with amino acids, the products include Ruhem's purple dye, carbon dioxide (CO₂), and an aldehyde. The reaction involves deamination, where the amino group of the amino acid becomes part of the dye, decarboxylation, where the carboxyl group is released as CO₂, and the formation of an aldehyde from the alpha carbon of the amino acid. This process highlights the breakdown of the amino acid into these specific products.

How is the Ninhydrin test used in forensic science?

In forensic science, the Ninhydrin test is used to detect fingerprints on surfaces. When a person touches a surface, amino acids from their sweat are left behind. Ninhydrin reacts with these amino acids to form a purple dye called Ruhem's purple, making the fingerprints visible. This technique is commonly seen in crime scene investigations, where detectives use Ninhydrin to lift and analyze fingerprints, aiding in the identification of individuals.

What is the chemical structure of Ninhydrin?

Ninhydrin is an aromatic diketone hydrate with the chemical formula C₉H₆O₄. It consists of a benzene ring (aromatic portion), two carbonyl groups (diketone), and a geminal diol (hydrate) on the same carbon. The structure allows Ninhydrin to react with the primary amines of amino acids, leading to the formation of Ruhem's purple dye, CO₂, and an aldehyde.

What is Ruhem's purple and how is it formed?

Ruhem's purple is a purple dye formed during the Ninhydrin test when Ninhydrin reacts with amino acids. The reaction involves two moles of Ninhydrin interacting with one amino acid, resulting in deamination, decarboxylation, and aldehyde formation. The amino group of the amino acid becomes part of the dye, while the carboxyl group is released as CO₂, and the alpha carbon forms an aldehyde. Ruhem's purple is significant in forensic science for detecting fingerprints.