modified residue that can arise from different natural residues ester crosslinked residues O-(L-isoglutamyl)-L-serine 111.0320281982421875 111.09999847412109375 198.064056396484375 198.17999267578125 O-(L-isoglutamyl)-L-serine O(beta)-(gamma-glutamyl)serine PSI-MOD S, X none (2S)-2-amino-5-[(2S)-2-amino-2-carboxyethoxy]-5-oxopentanoic acid MOD:01977 Cross-link 2. natural A protein modification that effectively crosslinks either an L-glutamine residue or an L-glutamic acid residue with an L-serine residue by an ester bond to form O-(L-isoglutamyl)-L-serine. C 8 H 10 N 2 O 4 C 5 H 5 N 1 O 2 O3-(isoglutamyl)-serine crosslinked L-serine residue