Carboxymethylation of Cysteine Using Iodoacetamide/ Iodoacetic Acid

  • Alastair Aitken
  • Michèle Learmonth
Part of the Springer Protocols Handbooks book series (SPH)


If cysteine or cystine is identified in a protein it requires modification in order to be quantified. Thiol groups may be blocked by a variety of reagents including iodoacetic acid and iodoacetamide. Iodoacetate produces the S-carboxymethyl derivative of cysteine, effectively introducing new negative charges into the protein. Where such a charge difference is undesirable, iodoacetamide may be used to derivatize cysteine to S-carboxyamidomethylcysteine (on acid hydrolysis, as for amino acid analysis, this yields S-carboxymethylcysteine). The charge difference between these two derivatives has been utilized in a method to quantify the number of cysteine residues in a protein ([1], see  Chapter 89).


Thiol Group Ammonium Bicarbonate Urea Solution Iodoacetic Acid Charge Difference 
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  1. 1.
    Creighton, T. E. (1980) Counting integral numbers of amino acid residues per polypeptide chain. Nature 284, 487–488.PubMedCrossRefGoogle Scholar

Copyright information

© Humana Press Inc., Totowa, NJ 2002

Authors and Affiliations

  • Alastair Aitken
    • 1
  • Michèle Learmonth
    • 2
  1. 1.Division of Biomedical and Clinical Laboratory Sciences, Membrane Biology GroupUniversity of EdinburghScotland, UK
  2. 2.Department of Biomedical SciencesUniversity of EdinburghScotland

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