Amino acids contain amino (–NH2) and carboxyl (–COOH) functional groups. Depending upon the relative position of amino group with respect to carboxyl group, the amino acids can be classified as α, β, γ, δ and so on. Only α-amino acids are obtained on hydrolysis of proteins. All α-amino acids have trivial names, which usually reflect the property of that compound or its source. Amino acids are classified as acidic, basic or neutral depending upon the relative number of amino and carboxyl groups in their molecule. Equal number of amino and carboxyl groups makes it neutral; more number of amino than carboxyl groups makes it basic and more carboxyl groups as compared to amino groups makes it acidic. The amino acids, which can be synthesized in the body, are known as nonessential amino acids. On the other hand, those which cannot be synthesized in the body and must be obtained through diet, are known as essential amino acids.
The carboxyl group of alanine combines with the amino group of glycine to form a dipeptide. Which of the following gives the dipeptide formed and the molecule eliminated, respectively?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \( \ce{H2N-CH(CH3)-CO-NH-CH2-COOH} \) and \( \ce{H2O} \)
A peptide bond is an amide link, –CO–NH–, formed when the –COOH group of one amino acid combines with the –NH2 group of another, with the elimination of a water molecule.
The amino acid that gives its carboxyl group keeps its free –NH2 group at one end of the dipeptide and is named first, with the ending “-yl”.
Here the –COOH group of alanine (side chain –CH3) joins the –NH2 group of glycine (side chain H):
\( \displaystyle \ce{H2N-CH(CH3)-COOH + H2N-CH2-COOH -> H2N-CH(CH3)-CO-NH-CH2-COOH + H2O} \)
The product is the dipeptide alanylglycine: alanine keeps the free –NH2 group, and glycine keeps the free –COOH group. It is named the same way as glycylalanine, where glycine gives the carboxyl group and is named first.
Why the other options are incorrect
Option (1) \( \ce{H2N-CH2-CO-NH-CH(CH3)-COOH} \) and \( \ce{H2O} \) — Incorrect. This is glycylalanine, formed when the carboxyl group of glycine combines with the amino group of alanine:
\( \displaystyle \ce{H2N-CH2-COOH + H2N-CH(CH3)-COOH -> H2N-CH2-CO-NH-CH(CH3)-COOH + H2O} \)
Option (2) \( \ce{H2N-CH(CH3)-CO-NH-CH2-COOH} \) and \( \ce{NH3} \) — Incorrect. The dipeptide is right, but the –OH of the carboxyl group and an H of the amino group leave as water; no ammonia is given off, and the nitrogen stays in the peptide bond.
NCERT (2026-27 edition):
10.2.3 Structure of Proteins – “…Chemically, peptide linkage is an amide formed between –COOH group and –NH2 group. The reaction between two molecules of similar or different amino acids, proceeds through the combination of the amino group of one molecule with the carboxyl group of the other. This results in the elimination of a water molecule and formation of a peptide bond –CO–NH–. The product of the reaction is called a dipeptide because it is made up of two amino acids. For example, when carboxyl group of glycine combines with the amino group of alanine we get a dipeptide, glycylalanine.”