Experiment 03Amino Acid & Protein Chemistry • B.Sc. / M.Sc.
Qualitative Color Reactions & Differentiation of Amino Acids and Proteins
๐ฏ Aim: To conduct specific qualitative color reactions on amino acid and protein samples (Biuret, Ninhydrin, Xanthoproteic, Millon’s, Sakaguchi, and Lead Acetate tests) to identify peptide linkages and functional side-chain residues.
๐ฌ Theoretical Principle & Side-Chain Specificity
Amino acids and proteins can be identified by reactions targeting either their shared backbone components ($alpha$-amino, carboxyl, peptide bond) or specific functional R-groups:
- Biuret Reaction: Requires a minimum of two peptide bonds (tripeptides and above). In alkaline solution ($text{NaOH}$), cupric ions ($text{Cu}^{2+}$) form a tetradentate coordination complex with four peptide nitrogen atoms, yielding an intense violet/purple chromophore ($lambda_{max} = 540,text{nm}$).
- Ninhydrin Reaction: Ninhydrin (triketohydrindene hydrate) oxidatively deaminates and decarboxylates free $alpha$-amino acids, releasing $text{NH}_3$, $text{CO}_2$, and an aldehyde. The reduced hydrindantin condenses with another ninhydrin and ammonia to form Ruhemann’s purple ($lambda_{max} = 570,text{nm}$). Secondary amino acids like proline and hydroxyproline yield a characteristic yellow derivative ($lambda_{max} = 440,text{nm}$).
- Xanthoproteic Reaction: Concentrated nitric acid undergoes electrophilic aromatic substitution on benzene rings of aromatic amino acids (Tyrosine and Tryptophan; Phenylalanine reacts slowly), producing yellow nitro derivatives that turn dark orange in alkali.
- Millon’s Reaction: Mercuric nitrate in nitric acid reacts specifically with the phenolic hydroxyl group of Tyrosine, yielding a brick-red mercurated adduct.
- Sakaguchi Reaction: Under alkaline conditions, $alpha$-naphthol and sodium hypobromite react specifically with the guanidino group of Arginine to form a brilliant red dye complex.
- Lead Acetate Test (Sulfur-Containing AAs): Boiling cysteine or methionine with concentrated $text{NaOH}$ hydrolyzes organic sulfur to inorganic sulfide ($text{S}^{2-}$), which precipitates with lead acetate as black lead sulfide ($text{PbS}$).
Figure 3.1: Systematic Qualitative Color Reactions for Specific Amino Acid Side Chains
๐ Experimental Observation Matrix
| Assay Name | Target Moiety | Reagent Formulation | Positive Color / Result | Diagnostic For |
|---|---|---|---|---|
| Biuret Test | Peptide Bonds ($ge 2$) | $10% text{NaOH} + 0.5% text{CuSO}_4$ | Violet / Purple coloration | Proteins, Gelatin, Albumin, Peptones |
| Ninhydrin Test | Free $alpha$-amino group | $0.2% (text{w/v})$ in acetone/ethanol | Ruhemann’s Purple (Yellow: Proline) | All $alpha$-amino acids & peptides |
| Xanthoproteic Test | Aromatic phenyl ring | Conc. $text{HNO}_3$, followed by $40% text{NaOH}$ | Yellow, turning bright orange | Tyrosine, Tryptophan |
| Millon’s Test | Phenolic —OH group | $10% text{HgSO}_4$ in $10% text{H}_2text{SO}_4 + 1% text{NaNO}_2$ | White ppt turning brick-red on heating | Tyrosine (Phenolic residue) |
| Sakaguchi Test | Guanidinium group | $alpha$-Naphthol in ethanol $+ text{NaOBr}$ | Bright cherry red color | Arginine exclusively |
| Lead Acetate Test | Sulfhydryl / Sulfide | $40% text{NaOH} + 5% text{Pb(CH}_3text{COO)}_2$ | Black precipitate ($text{PbS}$) | Cysteine & Cystine |
๐ฌ Viva-Voce Questions & Model Answers
Q1: Why does free glycine give a positive Ninhydrin test but a negative Biuret test?
Ninhydrin reacts with any free primary $alpha$-amino group, which free glycine possesses. However, the Biuret test requires a minimum of two peptide linkages ($-text{CO}-text{NH}-$) separated by a single carbon atom to coordinate cupric ions. Free amino acids possess zero peptide bonds and thus cannot form the purple Biuret complex.
Q2: Why does proline give a yellow color instead of purple with ninhydrin?
Proline is a cyclic secondary amino acid (imino acid) where the $alpha$-amino nitrogen is incorporated into a 5-membered pyrrolidine ring. Reaction with ninhydrin cannot proceed through normal oxidative deamination to generate free ammonia; instead, it yields a yellow-orange condensation iminium adduct ($lambda_{max} = 440,text{nm}$).
Q3: Why does methionine give a negative or very weak lead acetate test compared to cysteine?
In cysteine, the sulfur exists as a reactive, terminal sulfhydryl ($-text{SH}$) group that is readily cleaved by boiling with concentrated alkali to release sulfide ions ($text{S}^{2-}$). In methionine, the sulfur is locked in an unreactive, internal thioether linkage ($-text{S}-text{CH}_3$) that resists alkaline hydrolysis under standard laboratory conditions.
