Mono-nuclear copper complexes mimicking the intermediates for the binuclear copper center of the subunit II of cytochrome oxidase: a peptide based approach

Synthesis and photophysical properties of dinuclear N-heterocyclic carbene (NHC) copper(I) complexes and their application to photoluminescent light-emitting diodes and anti-counterfeiting

Here, a series of dinuclear N-heterocyclic carbene (NHC) copper(I) complexes having 3,3'-(1,4-phenylenebis(methylene))bis(1-(pyridin-2-yl)-1H-imidazolylidene as bis-NHC ligand and bis[(2-diphenylphosphino)phenyl]ether (POP) as auxiliary ligand have been successfully prepared, and their photophysical properites were investigaged experimentally and theocitcally. The resulting complexes all exhibited intense green to yellow emission that originated from the thermally activated delayed fluorescence (TADF) with a high photoluminescence quantum yield of up to 0.67 and longer excited-state lifetimes on the microsecond time scale in the solid state. Green and yellow light-emitting diode (LED) devices based on Cu(I) complexes have successfully achieved good color rendering indices. Moreover, the anti-counterfeiting patterns and QR codes made of Cu(I) complexes have been applied to clothing, banknotes, books and glass plates with excellent anti-counterfeiting effects.

Identification of a copper ion recognition peptide sequence in the subunit II of cytochrome c oxidase: a combined theoretical and experimental study

The role of the pentapeptide, NHSFM, derived from the surface exposed part of the metal ion binding loop of the subunit II of cytochrome c oxidase on the maturation of the binuclear purple CuA center of the enzyme has been investigated using several experimental and computational methods. The copper ion was found to form 1:1 complex of the pentapeptide with a binding constant ~ 10⁴ M^-1 to 10⁵ M^-1, where a 4 ligand coordination from the peptide in a type 2 copper center was revealed. The pH dependence of the metal-peptide was associated with a [Formula: see text] of ~ 10 suggesting deprotonation of the N-terminal amine. EXAFS studies as well as DFT calculations of the metal-peptide complexes revealed pH dependent changes in the metal-ligand bond distances. Spectroscopic properties of the metal peptides calculated from TDDFT studies agreed with the experimental results. Restrained molecular dynamics (RMD) simulations indicated coordination of a carbonyl oxygen from the asparagine (N) side chain and of water molecules apart from histidine (H), methionine (M) and terminal amine of asparagine (N) in a distorted square planar geometry of Cu-NHSFM. Analyses of the backbone distances as well as B-factors for the metal peptide suggested that the peptide backbone becomes more compact and rigid on binding of the metal ion. This indicated that binding of copper ion to this pentapeptide in the protein possibly cause movement of the protein backbone bringing other coordinating residues closer to the copper ion, and thus helping in sequential uptake of copper ions to the protein.

A Binuclear CuA Center Designed in an All α-Helical Protein Scaffold

The primary and secondary coordination spheres of metal binding sites in metalloproteins have been investigated extensively, leading to the creation of high-performing functional metalloproteins; however, the impact of the overall structure of the protein scaffold on the unique properties of metalloproteins has rarely been studied. A primary example is the binuclear Cu_A center, an electron transfer cupredoxin domain of photosynthetic and respiratory complexes and, recently, a protein coregulated with particulate methane and ammonia monooxygenases. The redox potential, Cu-Cu spectroscopic features, and a valence delocalized state of Cu_A are difficult to reproduce in synthetic models, and every artificial protein Cu_A center to-date has used a modified cupredoxin. Here, we present a fully functional Cu_A center designed in a structurally nonhomologous protein, cytochrome c peroxidase (CcP), by only two mutations (Cu_ACcP). We demonstrate with UV-visible absorption, resonance Raman, and magnetic circular dichroism spectroscopy that Cu_ACcP is valence delocalized. Continuous wave and pulsed (HYSCORE) X-band EPR show it has a highly compact g_z area and small A_z hyperfine principal value with g and A tensors that resemble axially perturbed Cu_A. Stopped-flow kinetics found that Cu_A formation proceeds through a single T2Cu intermediate. The reduction potential of Cu_ACcP is comparable to native Cu_A and can transfer electrons to a physiological redox partner. We built a structural model of the designed Cu binding site from extended X-ray absorption fine structure spectroscopy and validated it by mutation of coordinating Cys and His residues, revealing that a triad of residues (R48C, W51C, and His52) rigidly arranged on one α-helix is responsible for chelating the first Cu(II) and that His175 stabilizes the binuclear complex by rearrangement of the CcP heme-coordinating helix. This design is a demonstration that a highly conserved protein fold is not uniquely necessary to induce certain characteristic physical and chemical properties in a metal redox center.

Structural Basis for a Quadratic Relationship between Electronic Absorption and Electronic Paramagnetic Resonance Parameters of Type 1 Copper Proteins

Type 1 copper (T1Cu) proteins play important roles in electron transfer in biology, largely due to the unique structure of the T1Cu center, which is reflected by its spectroscopic properties. Previous reports have suggested a correlation between a high ratio of electronic absorbance at ∼450 nm to that at ∼600 nm (R = A₄₅₀/A₆₀₀) and a large copper(II) hyperfine coupling in the z direction (A_z) in electron paramagnetic resonance (EPR). However, this correlation does not have a clear physical meaning, nor does it hold for many proteins with a perturbed T1Cu center. To address this issue, a new parameter of R' [A₄₅₀/(A₄₅₀ + A₆₀₀)] with a better physical meaning of a fractional S_Cys pseudo-σ to Cu(II) charge transfer transition intensity is defined and a quadratic relationship between R' and A_z is found on the basis of a comprehensive analysis of ultraviolet-visible absorption, EPR, and structural parameters of T1Cu proteins. We are able to find good correlations between R' and the displacement of copper from the trigonal plane defined by the His₂Cys ligands and the angle between the N_His1-Cu-N_His2 plane and the S_Cys-Cu-axial ligand plane, providing a structural basis for the observed correlation. These findings and analyses provide a new framework for a deeper understanding of the spectroscopic and electronic properties of T1Cu proteins, which may allow better design and applications of this important class of proteins for redox and electron transfer functions.

A comparative study on the nickel binding ability of peptides containing separate cysteinyl residues

Nickel(ii) complexes of peptides CSSACS-NH2, ACSSACS-NH2, SSCSSACS-NH2 and GACAAH-NH2 have been studied by potentiometric and various spectroscopic (UV-vis, CD, NMR, and ESI-MS) techniques. All peptides have high nickel(ii) binding ability in the form of square planar complexes and the stability order of the peptides is: CSSACS-NH2 > ACSSACS-NH2 > SSCSSACS-NH2 ∼ GACAAH-NH2. The different metal binding affinities of these peptides are related to the differences in the speciation and in the binding modes of the major species. An almost exclusive formation of bis(ligand) complexes via an (NH2,S-) 5-membered chelate from the amino terminus is characteristic of CSSACS-NH2. The (NH2,N-,S-) tridentate chelate is the major coordination mode of ACSSACS-NH2 but the distant cysteine can also contribute to metal binding. The higher nickel(ii) binding ability of AC[combining low line]SSAC[combining low line]S-NH2 relative to the peptides containing an N-terminal XY-Cys motif may have important biological consequences. For example, the occurrence of the (NH2,N-,S-,S-) donor set is a common feature of both the ACSSACS-NH2 ligand and the nickel(ii) binding loop of the NiSOD enzyme (HC[combining low line]DLPC[combining low line]G…..,). In the case of SSCSSACS-NH2 and GACAAH-NH2 the amino terminus of one peptide can completely saturate the coordination sphere of the nickel(ii) ion via the formation of the (NH2,N-,N-,S-) binding mode. This rules out the formation of bis(ligand) complexes and any contribution of the distant cysteine or histidine to nickel(ii) binding in the 1 : 1 complexes. On the other hand the distant cysteine of SSCSSACS-NH2 and histidine of GACAAH-NH2 can behave as independent metal binding sites for the formation of dinuclear complexes in the presence of excess metal ions.