Plasticity in the copper–thioether bond: Manifestation in blue Cu proteins and in synthetic analogs

Versatile Post-synthetic Modifications of Helical β-Peptide Foldamers Derived from a Thioether-Containing Cyclic β-Amino Acid

We introduce a novel cyclic β-amino acid, trans-(3S,4R)-4-aminotetrahydrothiophene-3-carboxylic acid (ATTC), as a versatile building block for designing peptide foldamers with controlled secondary structures. We synthesized and characterized a series of β-peptide hexamers containing ATTC using various techniques, including X-ray crystallography, circular dichroism, and NMR spectroscopy. Our findings reveal that ATTC-containing foldamers can adopt 12-helical conformations similar to their isosteres and offer the possibility of fine-tuning their properties via post-synthetic modifications. In particular, chemoselective conjugation strategies demonstrate that ATTC provides unique post-synthetic modification opportunities, which expand their potential applications across diverse research areas. Collectively, our study highlights the versatility and utility of ATTC as an alternative to previously reported cyclic β-amino acid building blocks in both structural and functional aspects, paving the way for future research in the realm of peptide foldamers and beyond.

Synthesis, Characterization, and Copper(II) Chelates of 1,11-Dithia-4,8-diazacyclotetradecane

Synthesis of 1,11-dithia-4,8-diazacyclotetradecane (L1), a constitutional isomer of the macrocyclic [14]aneN₂S₂ series, is accompanied with reaction and method optimization. Chelation of L1 with copper(II) provided assessment of lattice packing, ring contortion, and evidence of conformational fluxionality in solution through two unique crystal structures: L1Cu(ClO4)₂ and [(L1Cu)₂μ-Cl](ClO₄)₃. Multiple synthetic approaches are presented, supplemented with reaction methodology and reagent screening to access [14]aneN₂S₂ L1. Reductive alkylation of bis-tosyl-cystamine was integrated into the synthetic route, eliminating the use and isolation of volatile thiols and streamlining the synthetic scale-up. Late-stage cleavage of protecting sulfonamides was addressed using reductive N-S cleavage to furnish macrocyclic freebase L1.

Insights into the Molybdenum/Copper Heterometallic Cluster Assembly in the Orange Protein: Probing Intermolecular Interactions with an Artificial Metal-Binding ATCUN Tag

Orange protein (ORP) is a small bacterial protein, of unknown function, that contains a unique molybdenum/copper heterometallic cluster, [S₂Mo^VIS₂Cu^IS₂Mo^VIS₂]^3- (Mo/Cu), non-covalently bound. The native cluster can be reconstituted in a protein-assisted mode by the addition of Cu^II plus tetrathiomolybdate to apo-ORP under controlled conditions. In the work described herein, we artificially inserted the ATCUN ("amino terminus Cu and Ni") motif in the Desulfovibrio gigas ORP (Ala₁Ser₂His₃ followed by the native amino acid residues; modified protein abbreviated as ORP*) to increase our understanding of the Mo/Cu cluster assembly in ORP. The apo-ORP* binds Cu^II in a 1:1 ratio to yield Cu^II-ORP*, as clearly demonstrated by EPR (g_||,⊥ = 2.183, 2.042 and A^Cu_||,⊥ = 207 × 10^-4 cm^-1, 19 × 10^-4 cm^-1) and UV-visible spectroscopies (typical d-d transition bands at 520 nm, ε = 90 M^-1 cm^-1). The ¹H NMR spectrum shows that His₃ and His₅₃ are significantly affected upon the addition of the Cu^II. The X-ray structure shows that these two residues are very far apart (C_α-C_α ≈ 27.9 Å), leading us to suggest that the metal-induced NMR perturbations are due to the interaction of two protein molecules with a single metal ion. Docking analysis supports the metal-mediated dimer formation. The subsequent tetrathiomolybdate binding, to yield the native Mo/Cu cluster, occurs only upon addition of dithiothreitol, as shown by UV-visible and NMR spectroscopies. Additionally, ¹H NMR of Ag^I-ORP* (Ag^I used as a surrogate of Cu^I) showed that Ag^I strongly binds to a native methionine sulfur atom rather than to the ATCUN site, suggesting that Cu^II and Cu^I have two different binding sites in ORP*. A detailed mechanism for the formation of the Mo/Cu cluster is discussed, suggesting that Cu^II is reduced to Cu^I and transferred from the ATCUN motif to the methionine site; finally, Cu^I is transferred to the cluster-binding region, upon the interaction of two protein molecules. This result may suggest that copper trafficking is triggered by redox-dependent coordination properties of copper in a trafficking pathway.

Thioether Coordination Chemistry for Molecular Imaging of Copper in Biological Systems

Copper is an essential element in biological systems. Its potent redox activity renders it necessary for life, but at the same time, misregulation of its cellular pools can lead to oxidative stress implicated in aging and various disease states. Copper is commonly thought of as a static cofactor buried in protein active sites; however, evidence of a more loosely bound, labile pool of copper has emerged. To help identify and understand new roles for dynamic copper pools in biology, we have developed selective molecular imaging agents for this metal, drawing inspiration from both biological binding motifs and synthetic model complexes that reveal thioether coordination as a general design strategy for selective and sensitive copper recognition. In this review, we summarize some contributions, primarily from our own laboratory, on fluorescence- and magnetic resonance-based molecular-imaging probes for studying copper in living systems using thioether coordination chemistry.

Molecular recognition between adenine or 2,6-diaminopurine and copper(II) chelates with N,O2,S-tripodal tetradentate chelators having thioether or disulfide donor groups

Five novel ternary copper(II) complexes with the N,O2,S-tripodal tetradentate chelators N,N-bis(carboxymethyl)-S-benzylcysteaminate(2-) ion (BCBC) or N,N,N',N'-tetrakis(carboxymethyl)cystaminate(4-) ion (TCC) and adenine (Hade), 2,6-diaminopurine (Hdap), 2,2'-bipyridine (bpy) or 1,10-phenanthroline (phen) as co-ligand were synthesized and characterized by X-ray diffraction and other physical methods: [Cu2(BCBC)2(μ2-N3,N7-H(N9)ade)(H2O)2]·H2O (1), [Cu2(BCBC)2(μ2-N7,N9-H(N3)dap)(H2O)2]·4H2O (2), [Cu2(μ2-TCC)(H(N9)ade)2(H2O)2]·10H2O (3), [Cu2(μ2-TCC)(bpy)2]·15H2O (4) and [Cu2(μ2-TCC)(phen)2]·14H2O (5). The crystal structure of H4TCC·3H2O was also determined. All ternary Cu(II) complexes have molecular structures. The N-(2-mercaptoethyl)-iminodiacetate moieties of BCBC or TCC ligands play a NO2+S-tripodal tetradentate role, with the S-(thioether or disulfide) atom as the apical/distal donor of the copper(II) center. In 1-3, the iminodiacetate moiety exhibits a mer-NO2 conformation (two nearly coplanar chelate rings) while in 4 and 5 (with bpy or phen as coligand) it displays a fac-NO+O (apical/distal) conformation. We conclude that the formation of the Cu-S(thioether or disulfide) bonds is strongly favored by the N-branched topology of the S-ligands in the reported compounds.

A catalytic role for methionine revealed by a combination of computation and experiments on phosphite dehydrogenase

Novel role for methionine in enzyme catalysis.

Mechanism of copper incorporation in subunit II of cytochrome C oxidase from Thermus thermophilus: identification of intermediate species

Detailed spectroscopic and kinetic studies of incorporation of copper ion in the wild type (WT) and the D111AA (AA = K, N, or E) mutants of the metal ion binding site of the soluble fragment of subunit II of cytochrome c oxidase from Thermus thermophilus (TtCuA) showed the formation of at least two distinct intermediates. The global analyses of the multiwavelength kinetic results suggested a four-step reaction scheme involving two distinct intermediates in the pathway of incorporation of copper ions into the apoprotein forming the purple dinuclear CuA. An early intermediate similar to the red copper binding proteins was detected in the WT as well as in all the mutants. The second intermediate was a green copper species in the case of WT TtCuA. Mutation of Asp111, however, formed a second intermediate that is distinctly different from that formed in the case of the WT protein, suggesting that mutants follow pathways of copper ion incorporation different from that in the WT protein. The electrostatic interaction between Asp111 and the coordinating His114 possibly plays a subtle role in the mechanism of incorporation of metal ion into the protein. The overall Kd for WT TtCuA was found to be ~8 nM, which changed with mutation of the Asp111 residue. The activation and thermodynamic parameters were also determined from the temperature- and pH-dependent multiwavelength kinetics, and the results are discussed to unravel the role of Asp111 in the mechanism of formation of the dinuclear CuA center in cytochrome c oxidase.