unnatural residue 2-aminoisobutyric acid residue (Aib) 2-amino-2-methylpropanoic acid 2-amino-2-methylpropionic acid 2-methylalanine 85.052764892578125 85.1100006103515625 A protein modification that inserts or replaces a residue with a 2-aminoisobutyric acid. Aib C 4 H 7 N 1 O 1 MOD:01001 Modification from DeltaMass: Average Mass: 85. PSI-MOD X alpha,alpha-dimethylglycine alpha-aminoisobutyric acid alpha-methylalanine artifact