modified L-histidine residue residue modified by radical hydroxylation N2-Formyl-L-asparagine (2S)-4-amino-2-formamido-4-oxobutanoic acid 142.037841796875 142.0399932861328125 4.9699997901916503906 4.9789299964904785156 A protein modification that effectively transforms an L-histidine residue to N2-formyl asparagine C 5 H 6 N 2 O 3 Formylasparagine H H -1 C -1 N -1 O 2 MOD:02100 Nω-formyl asparagine PSI-MOD Unimod:1917 artifact