. Author manuscript; available in PMC: 2013 Jul 21.

Published in final edited form as: Anal Chem. 2011 Jan 6;83(3):701–707. doi: 10.1021/ac100775s

Table 1.

Relevant amino acid modifications that interfere with iTRAQ quantitation.

Modification	Amino acid	Δm, Da	Reason	Abbreviation
iTRAQ	Lys, N terminal	144	iTRAQ labeling (intended)	144K, 144Nt
iTRAQ	Tyr	144	iTRAQ labeling (side reaction)	144Y
iTRAQ	Cys	144	iTRAQ labeling (side reaction, un-blocked Cys)	144C
Acetylation	Lys	42	Enzymatic; Sample prep (CH₃COOH)	42K
Carbamylation	Lys, Arg	43	Sample prep (urea)	43K, 43R
Methylation	Lys	14	Enzymatic (methyl transferases)	14K
Oxidation	Cys to cysteic acid	48	Enzymatic; Sample prep (reactive oxygen species)	48C
Oxidation	Met to sulphoxide	16	Enzymatic; Sample prep (reactive oxygen species)	16M