Color Reaction Tests for Amino Acids and Proteins

A complete set of vocabulary flashcards covering qualitative color reaction tests for amino acids and proteins, including reagents, functional groups, and visual results.

Biuret's Test

A test specific for proteins that uses alkaline copper sulfate ($\text{CuSO}_4$ and $\text{NaOH}$) to react with peptide bonds, producing a purple to violet color solution.

Biuret Complex

The violet coordination complex formed when copper ions ($\text{Cu}^{2+}$) bind with at least two peptide linkages in proteins under basic conditions.

Ninhydrin's Test

A test specific for amino acids and proteins that reacts with $\alpha$-amino acids ($-\text{NH}_2$) to form a purple-colored product.

Ruhemann's Purple

The characteristic purple-colored complex produced when ninhydrin reacts with $\alpha$-amino groups in proteins and free amino acids.

Proline (Ninhydrin Reaction)

An imino acid lacking a free $\alpha$-amino group ($-\text{NH}_2$) that produces a yellow color instead of purple in the Ninhydrin test.

Xanthoproteic Test

A test for aromatic amino acids (tyrosine and tryptophan) using concentrated nitric acid ($\text{HNO}_3$) to yield a yellow precipitate or orange solution upon nitration of the aromatic ring.

Phenylalanine (Xanthoproteic Test)

An aromatic amino acid that requires a catalyst for nitration of its ring, unlike tyrosine and tryptophan which react directly with concentrated nitric acid ($\text{HNO}_3$).

Millon's Test

A test specific for tyrosine that uses Millon's reagent ($\text{Hg}/\text{HNO}_3$ or mercuric sulfate in sulfuric acid) to produce an old-rose, red, or brick-red product upon heating.

Phenolic Group

The specific aromatic functional group in tyrosine responsible for forming a red product in Millon's test.

Lead Acetate Test (Sulfur Test)

A test for sulfur-containing amino acids (cysteine and cystine) using lead acetate ($\text{Pb}(\text{CH}_3\text{COO})_2$) and $\text{NaOH}$ to form a dark brown or black precipitate of lead sulfide ($\text{PbS}$).

Methionine (Sulfur Test)

A sulfur-containing amino acid that gives a negative result in the sulfur test because its sulfur atom is not freely available to react.

Hopkins-Cole Test

A test (also called Adamkiewicz-Hopkins' test) specific for tryptophan that uses glyoxalic acid and concentrated $\text{H}_2\text{SO}_4$ to form a violet ring at the junction of two liquid layers.

Indole Ring

The heterocyclic functional group present in tryptophan that reacts with glyoxalic acid and concentrated $\text{H}_2\text{SO}_4$ in the Hopkins-Cole test.

Imidazole Ring

The five-membered nitrogenous ring system present in the chemical structure of histidine.

Bromine Water Test

A detection reaction for tryptophan using bromine water and amyl alcohol to yield a pink or lavender alcohol layer.

Pauly's Test

A test for histidine (cherry red) and tyrosine (orange red) using sulfanilic acid in sodium nitrite and concentrated $\text{HCl}$ to form a fuschia or red-colored complex.

Sakaguchi Test

A test for arginine using $\alpha$-naphthol and bromine water ($\text{NaOBr}$) to detect its guanidine group, producing an intense red or red-orange solution.

Forensic Application of Ninhydrin

The chemical visualization of fingerprints, where amines from skin proteins in fingerprint residue react with ninhydrin to form a characteristic purple pattern.

Peptide Bond Threshold (Biuret Test)

The requirement of at least two peptide bonds for complex formation with alkaline copper sulfate, causing single amino acids to give negative Biuret test results.

Summary Matrix of Amino Acid Tests

A comprehensive overview table matching each color test (Ninhydrin, Xanthoproteic, Millon's, Bromine Water, Pauly, Lead Acetate, Sakaguchi, Biuret) with its reagent, targeted functional group, and visual product color.