Carboxy-PEG4-phosphonic acid ethyl ester
CAS No.: 1964503-39-6
Cat No.: BP-501604
4.5 
Carboxy-PEG4-phosphonic acid ethyl ester is a PEG-based bifunctional linker featuring a terminal carboxylic acid for conjugation and a phosphonic acid ethyl ester handle that can be used to introduce or stabilize phosphate/phosphonate interactions in PROTAC architectures. The PEG4 chain provides a flexible, hydrophilic spacer that helps tune effective distance and reduce steric mismatch between the binding moieties attached at either end, which is critical for efficient formation of the ternary complex. In targeted protein degradation design, this linker can be incorporated to connect an E3 ligase ligand and a target-binding ligand while maintaining conformational freedom for simultaneous engagement. The phosphonate functionality is particularly useful for strategies that benefit from negatively charged or metal/biomolecular interaction motifs, potentially improving linker orientation and degradation potency. Overall, it is a valuable building block for systematic linker optimization and for constructing degraders with improved solubility and reduced aggregation.
Structure of 1964503-39-6
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Carboxy-PEG4-phosphonic acid ethyl ester is a versatile PROTAC linker building block designed to connect targeting ligands while providing a hydrophilic, conformationally flexible PEG spacer and a reactive phosphonate handle. Its dual functional motifs support robust conjugation strategies that are commonly used to tune linker length, solubility, and degradation-relevant geometry in targeted protein degradation workflows. The following sections describe the structure and the practical reactivity considerations for PROTAC assembly in detail below.
Structure: The linker contains a PEG-based polyether chain that confers water-compatible flexibility, flanked by a carboxylic acid group and a phosphonate moiety masked as an ethyl ester. It features ester and acid functionalities, along with stable C–O and P–O linkages that support controlled chemical coupling.
Reactivity: For PROTAC construction, the carboxylic acid can be activated for amide or ester bond formation using standard coupling chemistries, while the phosphonic acid ethyl ester can be used as a protected phosphonate intermediate for subsequent transformations. Typical approaches rely on activation reagents and base/acid conditions compatible with PEG stability, followed by deprotection or phosphonate coupling steps under conditions that preserve the ether backbone.
* Our calculator is based on the following equation:
Concentration (start) x Volume (start) = Concentration (final) x Volume (final)
It is commonly abbreviated as: C1V1 = C2V2
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