Cu-S Interaction Research Articles

Efficient Conjugation to Phosphorothioate Oligonucleotides by Cu-Catalyzed Huisgen 1,3-Dipolar Cycloaddition.

Improving oligonucleotide delivery is critical for the further development of oligonucleotide-based therapeutics. Covalent attachment of reporter molecules is one of the most promising approaches toward efficient oligonucleotide-based therapies. An efficient methods for the attachment of a variety of reporter groups is Cu(I)-catalyzed Huisgen azide-alkyne 1,3-dipolar cycloaddition. However, the majority of potential oligonucleotide (ON) therapeutics in clinical trials are carrying phosphorothioate (PS) linkages, and this robust conjugation method is not yet established for these ONs due to a general concern of Cu-S interaction. Here, we developed a method allowing for efficient conjugation of peptides to PS oligonucleotides. The method utilizes solid supported oligonucleotides that can be readily transformed into "clickable ONs" by simple linker conjugation and further reacted with an azido containing moiety (e.g., a peptide) using the CuBr × Me2S complex as a superior catalyst in that reaction. This study opens the way for further development of PS oligonucleotide-conjugates by means of efficient Cu(I)-catalyzed Huisgen azide-alkyne 1,3-dipolar cycloaddition.

The Magnesium-Sulfur Battery with High Rate and Improved Capacity Using Cu Nanoparticle Additives

The metallic Mg anode has a very high theoretical volumetric capacity of 3832 mAh/cm3, in comparison to 2062 mAh/cm3 for Li. Moreover, Mg has widespread natural abundance and is of lower cost. Sulfur is also a cheap and safe material which can be used as cathode for Mg batteries. However, the Mg/S battery has several disadvantages which block its practical application. One of these challenges is the polysulfide shuttle effect which is common in metal-sulfur batteries and results in decreased coulombic efficiency and battery lifespan. To solve this problem, many inorganic compounds have been used as additives to adsorb polysulfides in the Li/S battery cathode. The same method may also be effective in the Mg/S battery. It has been reported that the Mg/S battery with a Cu current collector is able to cycle for more 25 cycles while using other current collectors, the Mg/S cell has much poorer performance. This result suggests that the Cu-S interaction can influence the performance of Mg/S battery and improve the cycling stability. To study the interaction between Cu and S, Cu nanoparticles are synthesized and mixed with the sulfur cathode. It is found that Cu2S forms on the Cu surface and facilitates the reduction of sulfur in the Mg/S battery. The inclusion of Cu nanoparticles enables the battery to charge/discharge at higher current densities and also improves the cycling stability. The Mg/S battery with Cu additives can run at 0.5 C for more than 50 cycles and deliver a significant capacity.

Size-tunable copper nanocluster aggregates and their application in hydrogen sulfide sensing on paper-based devices.

Polystyrene sulfonate (PSS), a strong polyelectrolyte, was used to prepare red photoluminescent PSS-penicillamine (PA) copper (Cu) nanoclusters (NC) aggregates, which displayed high selectivity and sensitivity to the detection of hydrogen sulfide (H2S). The size of the PSS-PA-Cu NC aggregates could be readily controlled from 5.5 μm to 173 nm using different concentrations of PSS, which enabled better dispersity and higher sensitivity towards H2S. PSS-PA-Cu NC aggregates provided rapid H2S detection by using the strong Cu-S interaction to quench NC photoluminescence as a sensing mechanism. As a result, a detection limit of 650 nM, which is lower than the maximum level permitted in drinking water by the World Health Organization, was achieved for the analysis of H2S in spring-water samples. Moreover, highly dispersed PSS-PA-Cu NC aggregates could be incorporated into a plate-format paper-based analytical device which enables ultra-low sample volumes (5 μL) and feature shorter analysis times (30 min) compared to conventional solution-based methods. The advantages of low reagent consumption, rapid result readout, limited equipment, and long-term storage make this platform sensitive and simple enough to use without specialized training in resource constrained settings.

HHM Motif at the CuH-Site of Peptidylglycine Monooxygenase is a pH-Dependent Conformational Switch

Peptidylglycine monooxygenase is a copper-containing enzyme that catalyzes the amidation of neuropeptides hormones, the first step of which is the conversion of a glycine-extended pro-peptide to its α-hydroxyglcine intermediate. The enzyme contains two mononuclear Cu centers termed CuM (ligated to imidazole nitrogens of H242, H244 and the thioether S of M314) and CuH (ligated to imidazole nitrogens of H107, H108, and H172) with a Cu-Cu separation of 11 Å. During catalysis, the M site binds oxygen and substrate, and the H site donates the second electron required for hydroxylation. The WT enzyme shows maximum catalytic activity at pH 5.8 and undergoes loss of activity at lower pHs due to a protonation event with a pKA of 4.6. Low pH also causes a unique structural transition in which a new S ligand coordinates to copper with an identical pKA, manifest by a large increase in Cu-S intensity in the X- ray absorption spectroscopy. In previous work (Bauman, A. T., Broers, B. A., Kline, C. D., and Blackburn, N. J. (2011) Biochemistry 50, 10819-10828), we tentatively assigned the new Cu-S interaction to binding of M109 to the H-site (part of an HHM conserved motif common to all but one member of the family). Here we follow up on these findings via studies on the catalytic activity, pH-activity profiles, and spectroscopic (electron paramagnetic resonance, XAS, and Fourier transform infrared) properties of a number of H-site variants, including H107A, H108A, H172A, and M109I. Our results establish that M109 is indeed the coordinating ligand and confirm the prediction that the low pH structural transition with associated loss of activity is abrogated when the M109 thioether is absent. The histidine mutants show more complex behavior, but the almost complete lack of activity in all three variants coupled with only minor differences in their spectroscopic properties suggests that unique structural elements at H are critical for functionality. The data suggest a more general utility for the HHM motif as a copper- and pH-dependent conformational switch.

A Copper–Methionine Interaction Controls the pH-Dependent Activation of Peptidylglycine Monooxygenase

The pH dependence of native peptidylglycine monooxygenase (PHM) and its M314H variant has been studied in detail. For wild-type (WT) PHM, the intensity of the Cu-S interaction visible in the Cu(I) extended X-ray absorption fine structure (EXAFS) data is inversely proportional to catalytic activity over the pH range of 3-8. A previous model based on more limited data was interpreted in terms of two protein conformations involving an inactive Met-on form and an active flexible Met-off form [Bauman, A. T., et al. (2006) Biochemistry 45, 11140-11150] that derived its catalytic activity from the ability to couple into vibrational modes critical for proton tunneling. The new studies comparing the WT and M314H variant have led to the evolution of this model, in which the Met-on form has been found to be derived from coordination of an additional Met residue, rather than a more rigid conformer of M314 as previously proposed. The catalytic activity of the mutant decreased by 96% because of effects on both k(cat) and K(M), but it displayed the same activity-pH profile with a maximum around pH 6. At pH 8, the reduced Cu(I) form gave spectra that could be simulated by replacement of the Cu(M) Cu-S(Met) interaction with a Cu-N/O interaction, but the data did not unambiguously assign the ligand to the imidazole side chain of H314. At pH 3.5, the EXAFS still showed the presence of a strong Cu-S interaction, establishing that the Met-on form observed at low pH in WT cannot be due to a strengthening of the Cu(M)-methionine interaction but must arise from a different Cu-S interaction. Therefore, lowering the pH causes a conformational change at one of the Cu centers that brings a new S donor residue into a favorable orientation for coordination to copper and generates an inactive form. Cys coordination is unlikely because all Cys residues in PHM are engaged in disulfide cross-links. Sequence comparison with the PHM homologues tyramine β-monooxygenase and dopamine β-monooxygenase suggests that M109 (adjacent to H site ligands H107 and H108) is the most likely candidate. A model is presented in which H108 is protonated with a pK(a) of 4.6 to generate the inactive low-pH form with Cu(H) coordinated by M109, H107, and H172.

Temporal Evolution of Benzenethiolate SAMs on Cu(100)

The structure and thermal stability of self-assembled monolayers (SAMs) of benzenethiolate (BT) on Cu(100) have been studied by means of thermal desorption spectroscopy (TDS), scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), UV photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and near-edge X-ray adsorption fine structure spectroscopy (NEXAFS). Vapor deposition at room temperature yields a well-ordered, densely packed c(6 × 2) saturation structure. At room temperature, this film is, however, metastable and transforms via partial decomposition by cleavage of the S-C bond into a less densely packed layer that reveals a coexisting p(2 × 2) phase. Such a transition occurs on a time scale of several days and is accompanied by a reduction of the work function change with respect to the bare Cu(100) surface from Δϕ = -0.9 eV for a freshly prepared saturated layer to -0.5 eV for an aged film. TDS experiments exhibit the presence of two distinct desorption channels (dissociative and intact desorption) occurring at different temperatures that reflects a variation of the local Cu-S interaction strength of BT at differently coordinated adsorption sites. Heating to above room temperature causes a rapid degradation and continuous thinning of BT films whereas above 500 K all thiolate species have desorbed or dissociated, leaving a sulfide overlayer behind that is accompanied by a substrate reconstruction. Interestingly, the upright orientation of BT adopted in the saturated monolayer remains almost identical upon heating and demonstrates the absence of downward tilting upon thermally induced thinning of the film.

Metal island growth and dynamics on molybdenite surfaces

In order to understand the adsorption mechanism of metal atoms to semiconducting surfaces, we have studied, as a model system, the vapor phase adsorption of Ag, Au, and Cu on the (001) surface of molybdenite (MoS 2) and the subsequent surface diffusion of these adsorbates. Our scanning tunneling microscopy (STM) images show that, depending on the type of metal atom that is adsorbed, islands of a characteristic size (2 nm for Ag, 8 to 10 nm for Cu, two distinct sizes of 2 nm and 8 to 10 nm for Au), shape (well rounded in the lateral extension) and thickness (one monolayer for Ag, 1 to 1.5 nm for Cu) are formed during the initial stages of deposition. Whole islands are observed to surface diffuse without loss of size or shape. Despite the relatively large size of the copper islands on molybdenite, these islands surface diffuse extensively, suggesting that the Cu-S interaction is weak. Surface diffusion is only hindered once individual islands start to coalesce. As copper islands accumulate, the size and shape of the original islands can still be recognized, supporting the conclusion that these characteristics are constant and that monolayer growth occurs by the aggregation of islands across the surface. The strength and the nature of the Ag-S(MoS 2) bond were further investigated by using molecular orbital calculations, ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling spectroscopy (STS). By applying quantum mechanical approaches using a two-dimensional periodic molybdenite slab and hexagonal MoS 2 clusters of different sizes with metal atoms adsorbed to them, it is possible to calculate the electron transfer between the mineral surface and the metal atom as well as the adsorption energy as a function of surface coverage. In addition, we used the results from the quantum mechanical runs to derive empirical potentials that model the characteristics of the forces within the crystal, within the adsorbed islands, and the metal and mineral surface. The combination of quantum mechanical calculations and empirical force field calculations explain the electronic structure and the highest stability of Ag islands that have seven atoms in diameter, which exactly agrees with the size of experimentally observed islands. UPS results also suggest that a specific new state is formed (approximately 4.5 eV into the valence band) which may describe the Ag-S bond because it does not occur in pure silver or molybdenite. This study shows how the combination of microscopic (STM), spectroscopic (STS, UPS), compositional (X-ray photoelectron spectroscopy, XPS) and molecular modeling (quantum mechanical and empirical) techniques is a useful approach to understand the nature of the metal to sulfide bond. Further insights may be gained concerning the natural association of certain metals with sulfides.

Metal-metal bonding in biology: EXAFS evidence for a 2.5 A copper-copper bond in the CuA center of cytochrome oxidase.

Evidence for a direct Cu-Cu bond in the CuA center of cytochrome oxidase is reported. Simulation of the X-ray absorption spectrum of a recombinant CuA-binding domain of Bacillus subtilis cytochrome oxidase, and comparison with a structurally characterized directly-bonding Cu(1.5) ... Cu(1.5) inorganic complex, suggests that a Cu-Cu interaction of 2.5 +/- 0.1 A together with a short 2.2 A Cu-S interaction may be present in the CuA site. In light of these data, previous interpretations of the EXAFS of a number of cytochrome oxidase and nitrous oxide reductase enzymes which modeled the 2.6 A interaction as a long Cu-S(methionine) bond are possibly incorrect. A structural model based on the new data is presented which suggests that the CuA sites in cytochrome oxidase and N2O reductase are likely composed of a pair of modified type 1 copper centers with one histidine, one cysteine, and one weakly bound ligand (Met and/or Gln) joined by a Cu-Cu bond.