Localization of labile posttranslational modifications by electron capturedissociation: The case of gamma-carboxyglutamic acid

Tandem mass spectrometry (MS/MS) of 28 residue peptides harboring gamma-car boxylated glutamic acid residues, a posttranslational modification of sever al proenzymes of the blood coagulation cascade, using either collisions or infrared photons results in complete ejection of the gamma-CO2 moieties (-4 4 Da) before cleavage of peptide-backbone bonds. However, MS/MS using elect ron capture dissociation (ECD) in a Fourier transform mass spectrometer cle aves backbone bonds without ejecting CO2, allowing direct localization of t his labile modification. Sulfated side chains are also retained in ECD back bone fragmentations of a 21-mer peptide, although CAD causes extensive SO3 loss. ECD thus is a unique complement to conventional methods for MS/MS, ca using less undesirable loss of side-chain functionalities as well as more d esirable backbone cleavages.