Fig. 3. In vitro stability of XTEN-payload conjugate. (A) Structures of biotinylated XTEN-payload conjugates generated from an iodoacetamide or a maleimide Val–Cit-PAB-MMAE payload. Mass spectrometry data of (B) the iodoacetamide-derived conjugate or (C) the maleimide-derived conjugate before and after 48 or 96 hours of incubation at 37 °C in mouse serum. Peak marked with “*” corresponds to XTEN missing the C-terminal Arg residue. Peak marked with “#” is due to mass spectrometry-induced fragmentation after the Val–Cit dipeptide linker.