Background:

Bis-PEG4-acid is a PEG PROTAC linker. PROTAC linker[1]

[1]. Maniaci C, et al. Homo-PROTACs: bivalent small-molecule dimerizers of the VHL E3 ubiquitin ligase to induce self-degradation. Nat Commun. 2017 Oct 10;8(1):830.