Which amino acid (cystine or cysteine) is in the AccQ•Tag standard? - WKB75858

ENVIRONMENT

• AccQ•Tag
• Amino acid analysis solution
• Cysteine
• Cystine

ANSWER

WAT088122 - AMINO ACID STD contains cystine, the dimer of cysteine, at a concentration of ~1.25 µMol.

ADDITIONAL INFORMATION

You are actually measuring or assaying cystine (the dimer in the standard at 1/2 the concentration of all other amino acids in the standard), but you are reporting results for cysteine, and you must take care to double your results in the processing calculations.

Cysteine is the monomer amino acid, however at basic pH it forms a dimer, cystine. Since the AccQTag derivatization reaction needs a high (basic)  pH to occur, cysteine will dimerize when it is in the basic buffer used for the derivatization.
However, there are instances when a large excess of the monomer will dimerize at a slower rate than it will be derivatized, resulting in the derivatization of both the monomer and dimer. If one is only looking at the dimer, yes, you multiply by 2 to get the concentration of the monomer. 

Please see the attached chromatogram of the derivitized standard with the hydrolysate standard below. While that separation was performed on a Classic UPLC system, similar results can be expected on an H-Class.