misincorporation of norleucine for methionine -17.9564208984375 -18.030000686645507812 113.0840606689453125 113.160003662109375 C 1 H 2 N 0 O 0 S -1 C 6 H 11 N 1 O 1 From DeltaMass: Average Mass: -18 Average Mass Change:-18 Notes: It has the same mass as leucine or isoleucine and can be charged on the methionyl t-RNA. This often happens in minimal media-prepared fermentations that are not supplemented with enough free methionine. It gives a mass change of -18 and can often be confused with dehydration. M MOD:00956 PSI-MOD modification from DeltaMass norleucine residue (Nle)