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JEE MAIN Chemistry QUESTION #5563
Question 1
The thiol (–SH) functional group is present in which amino acid?
  • Cystine
  • Cysteine✔️
  • Methionine
  • Cytosine
Correct Answer Explanation
Cysteine is an amino acid with the structure $HSCH_2CH(NH_2)COOH$ — it contains a free thiol (–SH) group. Cystine is formed by oxidative coupling of two cysteine molecules (disulphide bond, –S–S–). Methionine contains a thioether (–S–) group, not a free –SH. Cytosine is a nucleobase, not an amino acid.