IMID d4 200.1575164794921875 200.160003662109375 72.05999755859375 72.06255340576171875 C 3 (2)H 4 N 2 C 9 (1)H 12 (2)H 4 N 4 O 1 IMID d4 IMID:2H(4) K MOD:00473 PSI-MOD Unimod:95 artifact modification from Unimod Isotopic label isotope tagged reagent derivatized residue