Anodic oxidation of diaryl sulphides—I. diphenyl sulphide in sulphate and perchlorate media

Copper Coordination Chemistry of Sulfur Pendant Cyclen Derivatives: An Attempt to Hinder the Reductive-Induced Demetalation in 64/67Cu Radiopharmaceuticals

The Cu²⁺ complexes formed by a series of cyclen derivatives bearing sulfur pendant arms, 1,4,7,10-tetrakis[2-(methylsulfanyl)ethyl]-1,4,7,10-tetraazacyclododecane (DO4S), 1,4,7-tris[2-(methylsulfanyl)ethyl]-1,4,7,10-tetraazacyclododecane (DO3S), 1,4,7-tris[2-(methylsulfanyl)ethyl]-10-acetamido-1,4,7,10-tetraazacyclododecane (DO3SAm), and 1,7-bis[2-(methylsulfanyl)ethyl]-4,10-diacetic acid-1,4,7,10-tetraazacyclododecane (DO2A2S), were studied in aqueous solution at 25 °C from thermodynamic and structural points of view to evaluate their potential as chelators for copper radioisotopes. UV-vis spectrophotometric out-of-cell titrations under strongly acidic conditions, direct in-cell UV-vis titrations, potentiometric measurements at pH >4, and spectrophotometric Ag⁺-Cu²⁺ competition experiments were performed to evaluate the stoichiometry and stability constants of the Cu²⁺ complexes. A highly stable 1:1 metal-to-ligand complex (CuL) was found in solution at all pH values for all chelators, and for DO2A2S, protonated species were also detected under acidic conditions. The structures of the Cu²⁺ complexes in aqueous solution were investigated by UV-vis and electron paramagnetic resonance (EPR), and the results were supported by relativistic density functional theory (DFT) calculations. Isomers were detected that differed from their coordination modes. Crystals of [Cu(DO4S)(NO₃)]·NO₃ and [Cu(DO2A2S)] suitable for X-ray diffraction were obtained. Cyclic voltammetry (CV) experiments highlighted the remarkable stability of the copper complexes with reference to dissociation upon reduction from Cu²⁺ to Cu⁺ on the CV time scale. The Cu⁺ complexes were generated in situ by electrolysis and examined by NMR spectroscopy. DFT calculations gave further structural insights. These results demonstrate that the investigated sulfur-containing chelators are promising candidates for application in copper-based radiopharmaceuticals. In this connection, the high stability of both Cu²⁺ and Cu⁺ complexes can represent a key parameter for avoiding in vivo demetalation after bioinduced reduction to Cu⁺, often observed for other well-known chelators that can stabilize only Cu²⁺.

Selective Site-Specific Fenton Oxidation of Methionine in Model Peptides: Evidence for a Metal-Bound Oxidant

The metal-catalyzed oxidation (MCO) of proteins represents an important pathway for protein degradation. Although many mechanistic details of MCO are currently unknown, such mechanistic information would greatly benefit formulation scientists in the rational design and analysis of protein formulations. Here, we describe the Fenton oxidation (by Fe(2+)/H(2)O(2)) of several Met-, Tyr-, and His containing model peptides, including one derivative containing a conformationally restricted norbornyl Met analogue (Nor), Nor-Gly-His-Met-NH(2). Our results will provide evidence for a metal-bound reactive oxygen species selectively oxidizing Met to Met sulfoxide, indicating a Met-specific oxidant and arguing against the involvement of freely diffusible hydroxyl radicals. The Fenton oxidation of Nor-Gly-His-Met-NH(2) yields a 2:1 preference for sulfoxide formation at the C-terminal Met versus the N-terminal Nor residue, respectively, while incubation of the peptide with H(2)O(2) alone results in a 1:1 ratio. These results are rationalized by the better access of the thioether side chain of the flexible C-terminal Met residue to the peptide-bound iron compared with the conformationally restricted Nor residue. It is commonly believed that Fenton oxidation reactions involve hydroxyl radicals, and that Met oxidation in proteins is predominantly controlled by the surface-accessibility of the respective Met residues. However, occasionally protein oxidation in formulations shows selectivities, which are not consistent with these paradigms. Our results demonstrate additional features of the Fenton reaction such as the formation of a metal-bound oxidant specific for Met (and not Tyr or His), which may assist formulation scientists in the rationalization of unexpected oxidation selectivities.