EXP2 (Color Reaction of Amino Acids)

Which amino acid gives a positive Sakaguchi test?

Arginine

What functional group of arginine is responsible for the Sakaguchi test?

The guanidino group.

What reaction principle is involved in the Sakaguchi test?

The guanidino group of arginine reacts with α-naphthol under alkaline oxidative conditions to form a colored complex.

What positive color is produced in the Sakaguchi test?

Red / cherry-red

Which amino acids are detected by Pauly’s test?

Histidine and Tyrosine

What reaction principle is involved in Pauly’s test?

Diazotized sulfanilic acid undergoes azo coupling with the imidazole group of histidine or phenolic ring of tyrosine.

What positive color is produced in Pauly’s test?

Red to orange-red

Which amino acid gives a positive Nitroprusside test?

Cysteine

What functional group is responsible for the Nitroprusside test?

The free sulfhydryl (-SH) group.

What occurs in the Nitroprusside test?

The free sulfhydryl (-SH) group reacts with sodium nitroprusside under alkaline conditions.

What positive color is produced in the Nitroprusside test?

Red-purple / violet

Which amino acids are detected by the Isatin test?

Proline and Hydroxyproline

What group of imino acids reacts characteristically with Isatin?

The secondary amino (imino) group.

What positive color is produced in the Isatin test?

Blue

Which amino acid is detected by Ehrlich’s test?

Tryptophan

What structural feature of tryptophan is responsible for Ehrlich’s test?

The indole ring.

What reagent reacts with the indole ring of tryptophan in Ehrlich’s test?

p-dimethylaminobenzaldehyde

Under what conditions does tryptophan react in Ehrlich’s test?

Under acidic conditions.

What type of product forms in Ehrlich’s test?

A colored condensation product.

What positive color is produced in Ehrlich’s test?

Red-violet / purple

What reagent is used in the Ninhydrin test?

Triketohydrin Hydrate

What group is responsible for the Ninhydrin reaction?

The free α-amino group.

What positive result is produced by most α-amino acids in the Ninhydrin test?

Blue-violet / purple

What exception to the usual Ninhydrin color response is shown for Proline and Hydroxyproline?

They produce Yellow.

What does the Ninhydrin test detect?

Free amino groups.

What reaction flow is shown for the Ninhydrin test?

Free α-amino group → Reaction with Ninhydrin → Ruhemann’s Purple

What reagents compose the Biuret test reagent?

NaOH + CuSO₄

What group is responsible for the Biuret test?

Peptide linkages.

What positive result is produced by the Biuret test?

Violet

What substances are detected by the Biuret test?

Peptides & Proteins

What reaction flow is shown for the Biuret test?

Peptide bonds + Cu²⁺ → Violet Coordination Complex

What is the key distinction between the Ninhydrin and Biuret tests?

Ninhydrin targets free amino groups, while the Biuret test specifically targets peptide bonds.

What reagent is used in Millon’s test?

Hg in HNO₃

What group is responsible for Millon’s test?

The Phenolic Group.

Which amino acid gives a positive Millon’s test?

Tyrosine

What positive result is produced by Millon’s test?

Red

What reaction target is identified for Millon’s test?

Phenolic group → Chemical reaction

What reagent is used in the Xanthoproteic test?

Conc. HNO₃

What group is responsible for the Xanthoproteic test?

The Benzene ring.

Which amino acids are associated with a positive Xanthoproteic test?

Tyr, Phe, Trp

What reaction occurs in the Xanthoproteic test?

Aromatic rings → Nitration

What initial positive color is produced in the Xanthoproteic test?

Yellow

What happens to the Xanthoproteic test color after addition of base?

Yellow → Orange

How do Tyrosine, Tryptophan, and Phenylalanine differ in the Xanthoproteic test?

Tyrosine is Strong Positive, Tryptophan is Positive, and Phenylalanine is Weak / Negative.

Why does Tyrosine give a strong positive Xanthoproteic test?

Its –OH group activates the aromatic ring, making nitration much easier and yielding a rapid reaction.

Why does Tryptophan give a positive Xanthoproteic test?

Its electron-rich indole ring undergoes nitration readily under test conditions.

Why is Phenylalanine weak or negative in the Xanthoproteic test?

Its simple benzene ring is less reactive toward nitration under the standard test conditions.

Why do aromatic amino acids not react equally in the Xanthoproteic test?

Their aromatic rings differ in chemical reactivity.

Which aromatic amino acids are listed for the Xanthoproteic test, and how is Phenylalanine qualified?

Tyr, Trp; Phe (weak)

What reagents are used in the Hopkins-Cole test?

Glyoxylic acid and conc. H₂SO₄

What group is responsible for the Hopkins-Cole test?

The Indole group.

Which amino acid is detected by the Hopkins-Cole test?

Tryptophan

What positive result is produced by the Hopkins-Cole test?

Violet ring

What reaction is associated with the Hopkins-Cole test?

Indole group → Condensation

What relationship is shown between aromatic amino acids and more selective tests?

Aromatic → Specific Functional Group → More Selective Test

What reagents are used in the Unoxidized Sulfur test?

NaOH, Lead acetate

What group is responsible for the Unoxidized Sulfur test?

Sulfur

Which amino acids are positive in the Unoxidized Sulfur test?

Cystine and Cysteine

What positive observation is produced by the Unoxidized Sulfur test?

Black or black precipitate.

Under what conditions does the Unoxidized Sulfur test reaction pathway occur?

Under strong alkaline conditions.

What happens to cysteine or cystine sulfur under strong alkaline conditions in the Unoxidized Sulfur test?

Cysteine / cystine sulfur → Sulfide formed

What reaction occurs when the sulfide formed in the Unoxidized Sulfur test reacts with Pb²⁺?

Sulfide + Pb²⁺ → PbS ↓

What visual observation is specifically described for the Unoxidized Sulfur test reaction pathway?

Brown/black precipitate

What is the key specificity of the Unoxidized Sulfur test for cysteine, cystine, and methionine?

Cysteine / cystine → positive; Methionine → generally negative.