unnatural residue naphthylalanine residue 197.0840606689453125 197.2400054931640625 A protein modification that inserts or replaces a residue with a naphthylalanine. C 13 H 11 N 1 O 1 From DeltaMass: Average Mass: 197 Formula: C13H11O1N1 Monoisotopic Mass Change: 197.084 Average Mass Change: 197.238. No citation provided. It is not obvious which isomer of naphthylalanine this DeltaMass entry is supposed to represent [JSG]. MOD:01098 PSI-MOD X artifact