Combinatorial design of multimeric chelating peptoids for selective metal coordination

The combinatorial synthesis of a new library of tetrameric peptoid ligands is introduced, enabling coordination and characterization of f-block metals.

Current methods for metal chelation are generally based on multidentate organic ligands, which are generated through cumbersome multistep synthetic processes that lack flexibility for systematically varying metal-binding motifs. Octadentate ligands incorporating hydroxypyridinone or catecholamide binding moieties onto a spermine scaffold are known to display some of the highest affinities towards f-elements. Enhancing binding affinity for specific lanthanide or actinide ions however, necessitates ligand architectures that allow for modular and high throughput synthesis. Here we introduce a high-throughput combinatorial library of 16 tetrameric N-substituted glycine oligomers (peptoids) containing hydroxypyridinone or catecholamide chelating units linked via an ethylenediamine bridge and, for comparison, we also synthesized the corresponding mixed ligands derived from the spermine scaffold: 3,4,3-LI(1,2-HOPO)₂(CAM)₂ and 3,4,3-LI(CAM)₂(1,2-HOPO)₂. Coordination-based luminescence studies were carried out with Eu³⁺ and Tb³⁺ to begin probing the properties of the new ligand architecture and revealed higher sensitization efficiency with the spermine scaffold as well as different spectroscopic features among the structural peptoid isomers. Solution thermodynamic properties of selected ligands revealed different coordination properties between the spermine and peptoid analogues with Eu³⁺ stability constants log β₁₁₀ ranging from 28.88 ± 3.45 to 43.97 ± 0.49. The general synthetic strategy presented here paves the way for precision design of new specific and versatile ligands, with a variety of applications tailored towards the use of f-elements, including separations, optical device optimization, and pharmaceutical development.

Polypeptide gels incorporating the exotic functional aromatic amino acid 4-amino-l-phenylalanine

High-molecular-weight polypeptides with functional aromatic side chains, poly(4-amino-l-phenylalanine), were prepared by the metal-initiated polymerization of the N^α-carboxyanhydride of the corresponding amino acid, which is a microbial derivative of phenylalanine.

Engineered Recognition of Tetravalent Zirconium and Thorium by Chelator-Protein Systems: Toward Flexible Radiotherapy and Imaging Platforms

Targeted α therapy holds tremendous potential as a cancer treatment: it offers the possibility of delivering a highly cytotoxic dose to targeted cells while minimizing damage to surrounding healthy tissue. The metallic α-generating radioisotopes ²²⁵Ac and ²²⁷Th are promising radionuclides for therapeutic use, provided adequate chelation and targeting. Here we demonstrate a new chelating platform composed of a multidentate high-affinity oxygen-donating ligand 3,4,3-LI(CAM) bound to the mammalian protein siderocalin. Respective stability constants log β₁₁₀ = 29.65 ± 0.65, 57.26 ± 0.20, and 47.71 ± 0.08, determined for the Eu^III (a lanthanide surrogate for Ac^III), Zr^IV, and Th^IV complexes of 3,4,3-LI(CAM) through spectrophotometric titrations, reveal this ligand to be one of the most powerful chelators for both trivalent and tetravalent metal ions at physiological pH. The resulting metal-ligand complexes are also recognized with extremely high affinity by the siderophore-binding protein siderocalin, with dissociation constants below 40 nM and tight electrostatic interactions, as evidenced by X-ray structures of the protein:ligand:metal adducts with Zr^IV and Th^IV. Finally, differences in biodistribution profiles between free and siderocalin-bound ²³⁸Pu^IV-3,4,3-LI(CAM) complexes confirm in vivo stability of the protein construct. The siderocalin:3,4,3-LI(CAM) assembly can therefore serve as a "lock" to consolidate binding to the therapeutic ²²⁵Ac and ²²⁷Th isotopes or to the positron emission tomography emitter ⁸⁹Zr, independent of metal valence state.