Identify the correct statement among the following:

• A

  All naturally occurring amino acids except glycine contain one chiral centre.
• B

  All naturally occurring amino acids are optically active.
• C

  Glutamic acid is the only amino acid that contains a-COOH group at the side chain.
• Correct answer
  D

  Amino acid, cysteine easily undergo dimerization due to the presence of free SH group.

Hence, the correct answer is option: 'D'.