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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3bm261

Which of the following statements about the amino acid shown is/are correct?

  1. AIt has exactly one stereocentre.
  2. BIts stereocentre has R configuration although it belongs to the L series.
  3. CIt is an essential amino acid.
  4. DTwo molecules can be joined by a disulfide bond on mild oxidation.

Answer: A, B, D

Cysteine has one stereocentre (the alpha carbon). Its CH2SH group outranks COOH in CIP priority, so the L-amino acid is R. Humans synthesise it from methionine and serine, so it is not essential. Mild oxidation of two thiol groups gives the S-S bond of cystine.
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