Which of the following α-amino acids has a sulphur atom in its side c…

The correct answer is Option (4) → Methionine

The side chain of methionine is H₃C–S–CH₂–CH₂–. The sulphur atom sits between two carbon atoms, so it carries no hydrogen and there is no –SH group.

Option (1) Cysteine — Incorrect. Its side chain, HS–CH₂–, does contain sulphur, but as an –SH group.

Option (2) Serine — Incorrect. Its side chain is HO–CH₂–; it has an –OH group and no sulphur.

Option (3) Threonine — Incorrect. Its side chain is H₃C–CHOH–; it also has an –OH group and no sulphur.

NCERT (2026-27 edition):
10.2.1 Amino Acids – “…Structures of some commonly occurring amino acids along with their 3-letter and 1-letter symbols are given in Table 10.2.”

Table 10.2: Natural Amino Acids