CM-Cys vs PAM-Cys C From DeltaMass: Average Mass: 13 Formula:C2H2O2 vs C3H5ON Monoisotopic Mass Change:13.03 Average Mass Change:13.05 Notes:Residual acrylamide in SDS gels can partly label cysteine residues in proteins (propionamido-Cys, PAM-Cys, DeltaMass +71Da; see entry). Subsequent alkylation of protein bands with iodoacetic acid e.g. in preparation for proteomic analysis, will convert remaining free cysteines into carboxymethyl-Cys (CM-Cys, DeltaMass +58Da; see entry). Peptide mass fingerprinting may therefore potentially reveal the same cysteine-containing peptide in two forms, differing in mass by 13Da. The relative ratios of the peaks will depend on the initial degree of labelling with acrylamide. Use of high quality, deionised acrylamide in the SDS gel will minimise modification of cysteine through this route. Where it remains a problem, deliberate alkylation using acrylamide instead of iodoacetamide will ensure chemical homogeneity of the final product. MOD:00968 OBSOLETE because this isn't a protein modification, but rather the difference between two known modifications. modification from DeltaMass PSI-MOD artifact true