pyridinyl ring crosslinked residues isodesmosine -58.13497161865234375 -58.15000152587890625 1+ 2-[(4S)-4-amino-4-carboxybutyl]-1-[(5S)-5-amino-5-carboxypentyl]-3,5-bis[(3S)-3-amino-3-carboxypropyl]pyridinium 454.244873046875 454.54998779296875 6-[2-(4-amino-4-carboxybutyl)-3,5-bis(3-amino-3-carboxypropyl)pyridinio]norleucine A protein modification that effectively cross-links four L-lysine residues to form isodesmosine. C 0 H -16 N -3 O 0 C 24 H 32 N 5 O 4 Cross-link 4. K, K, K, K MOD:01934 PSI-MOD isodesmosine natural crosslinked L-lysine residue