modified residue that can arise from different natural residues O-(L-isoaspartyl)-L-threonine (2S)-2-amino-4-([(1S,2R)-1-amino-1-carboxypropan-2-yl]oxy)-4-oxobutanoic acid (2S,3R)-2-amino-3-([(4S)-3-amino-3-carboxypropanoyl]oxy)propanoic acid 216.074615478515625 216.19000244140625 A protein modification that effectively converts an L-threonine residue to O-(L-isoaspartyl)-L-threonine, using either free L-asparagine and releasing ammonia or using a peptidyl L-aspartic acid residue and releasing water. C 8 H 12 N 2 O 5 MOD:01978 O(beta)-(beta-aspartyl)threonine O-(L-isoaspartyl)-L-threonine O3-(isoaspartyl)-threonine PSI-MOD T, X This is a threonine active intermediate and not an ester cross-link of peptides [JSG]. natural crosslinked L-threonine residue