Arrange the following amino acids in increasing order of N-atoms in their side chain.
(A) Histidine.
(B) Alanine.
(C) Lysine.
(D) Arginine.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (B), (C), (A), (D)
α-Amino acids have the general formula \( \ce{R-CH(NH2)-COOH} \). The α-\( \ce{NH2} \) group belongs to the common backbone; the side chain R is what makes each amino acid different. So only the nitrogen atoms in R are counted:
(B) Alanine: R = –\( \ce{CH3} \): 0 N
(C) Lysine: R = –\( \ce{(CH2)4-NH2} \), one extra amino group: 1 N
(A) Histidine: R = –CH2–(imidazole ring); the imidazole ring has 2 N
(D) Arginine: R = \( \ce{-(CH2)3-NH-C(=NH)-NH2} \), with the guanidino group: 3 N
Shortcut: Every option starts with alanine (B). Arginine (D), with three N atoms, must be last, which leaves Options (1) and (2); lysine (1 N) comes before histidine (2 N) only in Option (1).
NCERT (2026-27 edition):
Amino Acids – …
