O-(L-isoglutamyl)-L-serine (Glu-Ser) -18.010564804077148438 -18.020000457763671875 198.064056396484375 198.17999267578125 (2S)-2-amino-5-[(2S)-2-amino-2-carboxyethoxy]-5-oxopentanoic acid none artifact Cross-link 2; From DeltaMass: with no citation. O-gamma-Glutamyl- (Crosslink to Serine) MOD:00953 PSI-MOD A protein modification that effectively crosslinks an L-glutamic acid residue and an L-serine residue by an ester bond and releasing water to form O-(L-isoglutamyl)-L-serine. H -2 O -1 CROSSLNK isoglutamyl serine ester (Ser-Glu) C 8 H 10 N 2 O 4 O3-(isoglutamyl)-serine O-(L-isoglutamyl)-L-serine E, S O(beta)-(gamma-glutamyl)serine crosslinked residues with loss of water O-(L-isoglutamyl)-L-serine crosslinked L-glutamic acid residue