(18)O monosubstituted residue modified L-serine residue (18)O monosubstituted L-serine (16)O -1 (18)O 1 2.0 2.0042459964752197266 89.03627777099609375 89.04000091552734375 C 3 H 5 N 1 (16)O 1 (18)O 1 Label:18O(1) MOD:01234 O18 Labeling PSI-MOD S Unimod:258 artifact modification from Unimod Isotopic label - alkaline phosphatase to dephosphorylate