Divalent Metal Cations Research Articles

Metal Adsorption Controls Stability of Layered Manganese Oxides.

Hexagonal birnessite, a typical layered Mn oxide (LMO), can adsorb and oxidize Mn(II) and thereby transform to Mn(III)-rich hexagonal birnessite, triclinic birnessite, or tunneled Mn oxides (TMOs), remarkably changing the environmental behavior of Mn oxides. We have determined the effects of coexisting cations on the transformation by incubating Mn(II)-bearing δ-MnO2 at pH 8 under anoxic conditions for 25 d (dissolved Mn < 11 μM). In the Li+, Na+, and K+ chloride solutions, the Mn(II)-bearing δ-MnO2 first transforms to Mn(III)-rich δ-MnO2 or triclinic birnessite (T-bir) due to the Mn(II)-Mn(IV) comproportionation, most of which eventually transform to a 4 × 4 TMO. In contrast, Mn(III)-rich δ-MnO2 and T-bir form and persist in the Mg2+ and Ca2+ chloride solutions. However, in the presence of surface adsorbed Cu(II), Mn(II)-bearing δ-MnO2 turns into Mn(III)-rich δ-MnO2 without forming T-bir or TMOs. The stabilizing power of the cations on the δ-MnO2 structure positively correlates with their binding strength to δ-MnO2 (Li+, Na+, and K+ < Mg2+ and Ca2+ < Cu(II)). Since metal adsorption decreases the surface energy of minerals, our finding suggests that the surface energy largely controls the thermodynamic stability of LMOs. Our study indicates that the adsorption of divalent metal cations, particularly transition metals, can be an important cause of the high abundance of LMOs, rather than the more stable TMO phases, in the environment.

Disulfonated azo dyes: metal coordination and ion-pair separation in twelve MII compounds of Ponceau Xylidine and Crystal Scarlet.

The structures of seven divalent metal cation compounds of Ponceau Xylidine {PX; systematic name of dication: 4-[2-(3,4-dimethylphenyl)hydrazin-1-ylidene]-3-oxo-3,4-dihydronaphthalene-2,7-disulfonate}, also known as Acid Red 26, CI 16150, and of five divalent metal cation compounds of Crystal Scarlet {CS; systematic name of dication: 8-[2-(naphthalen-1-yl)hydrazin-1-ylidene]-7-oxo-7,8-dihydronaphthalene-1,3-disulfonate}, also known as Acid Red 44, CI 16250, are presented. These are hexaaquamagnesium(II) PX dimethylformamide (DMF) monosolvate, [Mg(H2O)6](C18H14N2O7S2)·C3H7NO, (I); heptaaquacalcium(II) PX 2.5-hydrate, [Ca(H2O)7](C18H14N2O7S2)·2.5H2O, (II); catena-poly[aqua(μ-DMF)tris(DMF)bis(μ3-PX)distrontium(II)], [Sr(C18H14N2O7S2)(C3H7NO)2(H2O)0.5]n, (III); the transition-metal series hexaaquametal(II) PX DMF monosolvate, [M(H2O)6](C18H14N2O7S2)·C3H7NO, where M (metal) = Co, (IV), Ni, (V), Cu, (VI), and Zn, (VII); heptaaquacalcium(II) CS monohydrate, [Ca(H2O)7](C20H13N2O7S2)·H2O, (VIII); octaaquastrontium(II) CS monohydrate, [Sr(H2O)8](C20H13N2O7S2)·H2O, (IX); catena-poly[[triaqua(DMF)barium(II)]-μ-CS], [Ba(C20H13N2O7S2)(C3H7NO)(H2O)3]n, (X); tetrakis(DMF)(CS)copper(II) monohydrate, [Cu(C20H13N2O7S2)(C3H7NO)4]·H2O, (XI); and catena-poly[[[aquatris(DMF)zinc(III)]-μ-CS] diethyl ether hemisolvate], {[Zn(C20H13N2O7S2)(C3H7NO)3(H2O)]·0.5C4H10O}n, (XII). In all cases, the structures obtained were solvates with dimethylformamide (DMF) and/or water present. The disulfonated naphthalene-based azo anions adopt hydrazone tautomeric forms. The structures of the Mg salt and of four transition-metal forms (M = Co, Ni, Cu and Zn) of PX are found to form an isostructural series. All have solvent-separated ion-pair (SSIP) type structures and the formula [M(H2O)6][PX]·DMF. The Ca salt of PX also has an SSIP structure, but has a higher hydration state, [Ca(H2O)7][PX]·2.5H2O. In contrast, the Sr salt of PX, [Sr(PX)(DMF)2(H2O)0.5]n forms a one-dimensional coordination polymer. Both the Ca and the Sr salt of CS have an SSIP structure, namely [Ca(H2O)7][CS]·H2O and [Sr(H2O)8][CS]·H2O, whilst the heavier Ba analogue, [Ba(CS)(DMF)(H2O)3]n, forms a one-dimensional coordination polymer. Unlike PX, two CS structures containing transition metals are found to be coordination complexes, [Cu(CS)(DMF)4]·H2O and {[Zn(CS)(DMF)3(H2O)]·0.5Et2O}n. This suggests that CS is a better ligand than PX for transition metals. The Cu complex forms discrete molecules with Cu in a square-pyramidal environment, whilst the Zn species is a one-dimensional coordination polymer based on octahedral Zn centres.

Colorimetric Detection of Mercury Ions in Water with Capped Silver Nanoprisms.

The emission of mercury (II) from coal combustion and other industrial processes may have impacts on water resources, and the detection with sensitive but rapid testing methods is desirable for environmental screening. Towards this end, silver nanoprisms were chemically synthesized resulting in a blue reagent solution that transitioned towards red and yellow solutions when exposed to Hg2+ ions at concentrations from 0.5 to 100 µM. A galvanic reduction of Hg2+ onto the surfaces is apparently responsible for a change in nanoprism shape towards spherical nanoparticles, leading to the change in solution color. There were no interferences by other tested mono- and divalent metal cations in solution and pH had minimal influence in the range of 6.5 to 9.8. The silver nanoprism reagent provided a detection limit of approximately 1.5 µM (300 µg/L) for mercury (II), which compared reasonably well with other reported nanoparticle-based techniques. Further optimization may reduce this detection limit, but matrix effects in realistic water samples require further investigation and amelioration.

Copper-Cobalt Spinel Nanostructures As Multifunctional Electrocatalysts

In recent years, the spinels, AB2X4 (A = divalent alkaline-earth and/or transition metal cations, B = trivalent group 13 elements and/or transition metals cations, and X = O, S, Se, Te, N, etc.) have attracted a great deal of research interest owing to their wide range of applications in conversion reactions, energy conversion, energy storage, data storage, biotechnology (magnetic sensing), electronics (spintronic devices) and laser devices. Their controllable composition, electronic structure, valence states or redox characteristics, morphology, and high catalytic property have made them suitable as catalysts in various conversion reactions such as oxygen evolution reaction, oxygen reduction reaction, hydrogen evolution reaction, alcohols oxidation, CO2 reduction, NH3 oxidation, H2O2 decomposition, urea oxidation, and methylene blue degradation. In addition, the high intrinsic electrical and excellent redox characteristics of spinels extended their applications into energy storage fields, such as supercapacitors and metal−ion batteries (such as Li-ion, Na-ion, Mg-ion Zn-ion batteries). Among the numerous varieties of spinels, CuCo2O4 is one of the most intriguing composite oxides, because of its high electrical conductivity, electrocatalytic activity, prominent Jahn–Teller effect and low toxicity. Herein, we report the multifunctional activities of CuCo2O4 toward water splitting, alcohol oxidation, H2O2 decomposition, urea oxidation, and energy-storage.

Regulating Ca2+ Electrolyte Transport and Stability for Calcium Metal Deposition

Beyond Li-ion electrochemical energy storage systems based on divalent metal cations occupy a new frontier of research and development efforts related to materials chemistry and design. High energy density batteries based on the promise of high capacity Mg or Ca metal anodes are yet to be realized due to problems associated with the stability and ion transport properties of organic electrolytes and insertion cathodes. Controlling the stability of Ca2+ electrolytes at calcium metal anodes has been a particularly challenging task, although a recent demonstration suggests a possible path forward for reversible calcium electrodeposition in ethereal electrolytes.1 The calcium system, then, represents an intriguing exemplar for understanding divalent metal electrodeposition under conditions that push the boundaries of organic electrolyte stability. In this presentation we will discuss how interactions among species within Ca2+ electrolytes such as Ca(BH4)2/THF and other recently developed systems facilitate transport and delivery of the working cation to the interface for deposition and how surface interactions influence the stability of supporting electrolyte species and direct the system toward inefficiency and byproduct formation as a function of species susceptibility. These studies utilize a combination of experimental spectroscopy, microscopy and electrochemistry along with computational simulation of Ca2+ coordination cluster structures and stabilities. We will discuss the impact of these factors on calcium metal morphology and composition providing new insight into the rules connecting Ca2+ electrolyte properties to interfaces for Ca metal deposition. These design principles will serve as a guide to the development of advanced electrolyte concepts capable of pushing the boundaries of reductive stability in divalent metal electrolytes toward higher voltage beyond Li-ion batteries. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. References Wang, D.; Gao, X.; Chen, Y.; Jin, L.; Kuss, C.; Bruce, P. G., Plating and stripping calcium in an organic electrolyte. Nature Materials 2017, 17, 16.

Interaction of divalent cations with carboxylate group in TEMPO-oxidized microfibrillated cellulose systems

Cellulose-based films can potentially replace non-biodegradable plastic films in various applications such as food packaging. In this work we produced and studied films made of mixtures of chemical pulps and catalytically oxidized microfibrillated cellulose. The films were prepared on a support which was then soaked in solutions of CaCl2 and MgCl2 to exchange the sodium ions originally present in the film to divalent metal cations. We assumed that the electrostatic interaction of the anionic pulp fibers and the fibrils with Ca2+ and Mg2+ would promote internal bonding of the fiber-fibril network that would then reflect positively on the film properties. The immersion of the wet film into aqueous CaCl2 or MgCl2 solidified the film with time. When the solidified films were dried with an excess of the salt, elastic, skin-like materials were formed. Rewetting in water and redrying the materials produced paper-like films with improved mechanical properties in comparison with films prepared without the divalent cation salts. SEM imaging of the fracture surfaces provided support for the increased internal film strength by the divalent cations. The new knowledge on their role could be utilized in tailoring cellulosic film properties for specific uses.

Molecular Dynamics Investigation into the Effect of Zinc(II) on the Structure and Membrane Interactions of the Antimicrobial Peptide Clavanin A.

Clavanin A (ClavA) is an antimicrobial peptide (AMP) whose antimicrobial activity is enhanced in the presence of Zn(II) ions. The antimicrobial activity of ClavA has been shown to increase 16-fold in the presence of Zn(II) ions. In this study, we investigate the potential sources of this enhancement, namely, the effect of Zn(II) binding on the helical conformation of ClavA and on the ClavA interaction with a model for gram-negative bacterial membranes. In addition, we investigate the effect of Zn(II) on the membrane mechanical properties. We employed all-atom equilibrium molecular dynamics simulations initiated from both fully helical and random coil structures of ClavA. We observe that Zn(II) can stabilize an existing helical conformation in the Zn(II)-binding region, but we do not observe induction of helical conformations in systems initiated in random coil configurations. Zn(II) binding to ClavA provides more favorable electrostatics for membrane association in the C-terminal region. This is evidenced by longer and stronger C-terminal-lipid interactions. Zn(II) is also capable of modulating the membrane properties in a manner which favors ClavA insertion and the potential for enhanced translocation into the cell. This work provides insights into the role of divalent metal cations in the antimicrobial activity of ClavA. This information can be used for the development of synthetic AMPs containing motifs that can bind metals (metalloAMPs) for therapeutic and medical purposes.

Metal binding to the dynamic cytoplasmic domain of the cation diffusion facilitator (CDF) protein MamM induces a 'locked-in' configuration.

Cation diffusion facilitator (CDF) proteins are a conserved family of transmembrane transporters that ensure cellular homeostasis of divalent transition metal cations. Metal cations bind to CDF protein's cytoplasmic C-terminal domain (CTD), leading to closure from its apo open V-shaped dimer to a tighter packed structure, followed by a conformational change of the transmembrane domain, thus enabling transport of the metal cation. By implementing a comprehensive range of biochemical and biophysical methods, we studied the molecular mechanism of metal binding to the magnetotactic bacterial CDF protein MamM CTD. Our results reveal that the CTD is rather dynamic in its apo form, and that two dependent metal-binding sites, a single central binding site and two symmetrical, peripheral sites, are available for metal binding. However, only cation binding to the peripheral sites leads to conformational changes that lock the protein in a compact state. Thus, this work reveals how metal binding is regulating the sequential uptakes of metal cations by MamM, and extends our understanding of the complex regulation mechanism of CDF proteins. DATABASE: Structural data are available in RCSB Protein Data Bank under the accession numbers: 6G64, 6G55, 6G5E and 6G6I (for CS, C267S, CS-C267S and W247A, respectively).

An acidic loop within the human soluble CD23 protein may direct the interaction between sCD23 and the αXβ2 integrin

CD23 is involved in a myriad of immune reactions. It is not only a receptor for IgE, but also functions in the regulation of IgE synthesis, isotype switching in B cells, and induction of the inflammatory response. These effector functions of CD23 arise through its interaction with another leukocyte-specific cell surface receptor – the β2 integrin subfamily. It has been shown that CD23 is also capable of interacting with the β3 and β5 integrin β-subunit of integrins via a basic RKC motif in a metal cation-independent fashion. In this study the interaction was probed for whether or not the RKC motif governs the interaction between CD23 and the αXβ2 integrin as well. This was done by performing bioinformatic docking predictions between CD23 and αXβ2 integrin αI domain and SPR spectroscopy analysis of the interaction. This revealed that in the absence of cations, the RKC motif is involved in interaction with the integrin αI domain. However, in the presence of divalent metal cations the interaction showed the involvement of a novel acidic motif within the CD23 protein. This same pattern of interaction was seen in docking predictions between CD23 and the β3I-like domain. This study thus presents an alternative site as a possible contributor to the CD23-integrin interaction exhibiting cation-dependence.

The invariant glutamate of human PrimPol DxE motif is critical for its Mn2+-dependent distinctive activities

PrimPol is a human primase/polymerase specialized in downstream repriming of stalled forks during both nuclear and mitochondrial DNA replication. Like most primases and polymerases, PrimPol requires divalent metal cations, as Mg2+ or Mn2+, used as cofactors for catalysis. However, little is known about the consequences of using these two metal cofactors in combination, which would be the most physiological scenario during PrimPol-mediated reactions, and the individual contribution of the putative carboxylate residues (Asp114, Glu116 and Asp280) acting as metal ligands. By site-directed mutagenesis in human PrimPol, we confirmed the catalytic relevance of these three carboxylates, and identified Glu116 as a relevant enhancer of distinctive PrimPol reactions, which are highly dependent on Mn2+. Herein, we evidenced that PrimPol Glu116 contributes to error-prone tolerance of 8oxodG more markedly when both Mg2+ and Mn2+ ions are present. Moreover, Glu116 was important for TLS events mediated by primer/template realignments, and crucial to achieving an optimal primase activity, processes in which Mn2+ is largely preferred. EMSA analysis of PrimPol:ssDNA:dNTP pre-ternary complex indicated a critical role of each metal ligand, and a significant impairment when Glu116 was changed to a more conventional aspartate. These data suggest that PrimPol active site requires a specific motif A (DxE) to favor the use of Mn2+ ions in order to achieve optimal incoming nucleotide stabilization, especially required during primer synthesis.

RHEOLOGICAL PROPERTIES OF MICELLAR SOLUTIONS OF A MIXTURE OF COCAMIDOPROPYLBETAINE AND SULFOSUCCINATES OF ETOXYLATED FATTY ALCOHOLS

Restoring and improving the filtration characteristics of the bottomhole zone of carbonate reservoirs using acidic compositions is the most com- monly used geological and technical measure. One of the main recent trends to increase the effectiveness of acidic compositions is the modifi- cation of their compositions, which is associated with the addition of various organic and inorganic components in order to give the reagent the required technological properties. An example of the acidic composi- tions modification is the use of viscoelastic viscoelastic surfactants, which give acidic reagents «self-diverting» properties. When hydrochlo- ric acid interacts with carbonate rock, a change in the pH of the medium changes and divalent metal cations concentration increase, which leads to a thickening of the system and the deviation of a fresh acid portion, moreover viscoelastic properties are not manifested in the presence of hydrocarbons. Thus, it is a continuous diverting process during the acid composition injection. The change in the rheological properties of the composition is explained by the aggregation of surfactants with the for- mation of a three-dimensional micellar structure with viscoelastic char- acteristics. This paper presents the results of laboratory experiments of the rheo- logical properties of aqueous and hydrochloric acid solutions of the mixture of cocamidopropylbetaine and sulfosuccinates etoxylated fatty alcohol sulfosuccinates. The rheological properties study results of mixed micellar aqueous and hydrochloric acid solutions based on the optimal ratio of surfactant, the dependence of the viscosity of mixed zwitterionic-anionic surfactant solutions on the total concentration and the viscosity dependence of the composition on reactions with carbonate rock, also the temperature and hydrocarbons effect on the rheological properties of a viscoelastic surfactant system indicate that the proposed surfactants combination enables to develop acid composition which has viscoelastic gel properties.

Microfluidic Production of Pyrophosphate Catalyzed by Mineral Membranes with Steep pH Gradients.

Pyrophosphate might have functioned as an energy storage/currency molecule on early Earth, essential for the emergence of life. Here we synthesized mineral membranes involving iron(II), iron(III), and other divalent metal cations (calcium, manganese, cobalt, copper, zinc, and nickel) and tested their ability to catalyze the formation of pyrophosphate from phosphate and acetyl phosphate across steep pH gradients in microfluidic devices. We studied the chemical compositions of the precipitate membranes (which included vivianite, goethite, and green rust) using in situ and ex situ micro-Raman spectroscopy. The yields of pyrophosphate were determined by aqueous 31 P NMR spectroscopy. We found that Fe2+ and Ca2+ were the best catalysts for pyrophosphate synthesis among investigated ions; Fe3+ and mixed-valence iron membranes were also able to promote pyrophosphate formation. In addition, the pH gradients across the membranes affected the pyrophosphate yields and the smallest pH gradient resulted in the highest yield. These results suggest a possible route of substrate phosphorylation in prebiotic hydrothermal systems.

Structural basis of small RNA hydrolysis by oligoribonuclease (CpsORN) from Colwellia psychrerythraea strain 34H

Cells regulate their intracellular mRNA levels by using specific ribonucleases. Oligoribonuclease (ORN) is a 3′–5′ exoribonuclease for small RNA molecules, important in RNA degradation and re-utilisation. However, there is no structural information on the ligand-binding form of ORNs. In this study, the crystal structures of oligoribonuclease from Colwellia psychrerythraea strain 34H (CpsORN) were determined in four different forms: unliganded-structure, thymidine 5′-monophosphate p-nitrophenyl ester (pNP-TMP)-bound, two separated uridine-bound, and two linked uridine (U-U)-bound forms. The crystal structures show that CpsORN is a tight dimer, with two separated active sites and one divalent metal cation ion in each active site. These structures represent several snapshots of the enzymatic reaction process, which allowed us to suggest a possible one-metal-dependent reaction mechanism for CpsORN. Moreover, the biochemical data support our suggested mechanism and identified the key residues responsible for enzymatic catalysis of CpsORN.

Catalytic properties of the metal ion variants of mandelate racemase reveal alterations in the apparent electrophilicity of the metal cofactor.

Mandalate racemase (MR) from Pseudomonas putida requires a divalent metal cation, usually Mg2+, to catalyse the interconversion of the enantiomers of mandelate. Although the active site Mg2+ may be replaced by Mn2+, Co2+, or Ni2+, substitution by these metal ions does not markedly (<10-fold) alter the kinetic parameters Kappm, kappcat, and (kcat/Km)app for the substrates (R)- and (S)-mandelate, and the alternative substrate (S)-trifluorolactate. Viscosity variation experiments with Mn2+-MR showed that the metal ion plays a role in the uniform binding of the transition states for enzyme-substrate association, the chemical step, and enzyme-product dissociation. Surprisingly, the competitive inhibition constants (Ki) for inhibition of each metalloenzyme variant by benzohydroxamate did not vary significantly with the identity of the metal ion unlike the marked variation of the stability constants (K1) observed for M2+·BzH complex formation in solution. A similar trend was observed for the inhibition of the metalloenzyme variants by F-, except for Mg2+-MR, which bound F- tighter than would be predicted based on the stability constants for formation of M2+·F- complexes in solution. Thus, the enzyme modifies the enatic state of the bound metal ion cofactor so that the apparent electrophilicity of Mg2+ is enhanced, while that of Ni2+ is attenuated, resulting in a levelling effect relative to the trends observed for the free metals in solution.

Preparation and Properties of F-Substituted Layered Double Hydroxides (LDHs):Anion-Intercalation Material

Introduction: There have been increasing attention to anion-intercalation behavior and materials. In lithium ion battery field, intercalation property of PF6 − into graphitic carbon was studied for a hybrid capacitor. In ion-exchange field, selective removal of toxic anionic species by anion-exchangeable layered compounds has been focused. Layered double hydroxides (LDHs) was representative of anion-exchangeable layered compounds, represented by a general formula of [M2+ 1−x M3+ x (OH)2] x +[A nx /n ] x − mH2O consisting of octahedral brucite-like host-layers (M2+/M3+: divalent and trivalent metal cations), charge-balancing anions (A n −) and interlayer water molecules. Their ion-exchange properties, selectivity and capacity, have been widely studied. Miyata previously reported the order of affinity of various anions to Mg-Al LDHs; OH− > F− > Cl− > Br− > NO3 − > I−. Although other researchers have tried to control the affinity by changing composition of metal cations (M2+/M3+) to control charge density of host layers, significant change in the order was hardly achieved. Looking back to intrinsic crystalline structure of LDHs, host-layers, optimal positions of interlayer anions, and water molecules is supposed to be determined with hydrogen-bonding interaction among them. In other words, the affinity of anion to LDHs should be controlled by tuning strength of hydrogen-bonding in addition to charge density of host layers. In this presentation, we tried to prepare F-substituted Mg-Al LDHs (F-LDHs), OH groups belonging to host layer of LDHs was substituted by F atoms, and anion-exchange selectivity was investigated. Experiment: Based on traditional co-precipitation method with solution pH = 10, F-LDHs was prepared by using AlCl3, NaAlF6, and MgCl2·6H2O as a precursor. Mg/Al ratio was controlled as 2.5，3.0，3.5，and 4.0 by changing ([AlCl3]+[NaAlF6])/[MgCl2] ratio and F-substitution ratio was controlled by changing [AlCl3]/[NaAlF6] ratio. Crystal structure, morphology, chemical composition, local structure of Al and F atoms of obtained samples were analyzed by XRD, FE-SEM (EDS), XPS, solid-state NMR. Next, anion-exchanging properties of obtained F-LDHs was tested. Firstly, as a pre-treatment, LDHs samples was immersed in acidic NaCl aqueous solution for 24 hours at room temperature and interlayer “contaminated” carbonate was exchanged by chloride ions as much as possible. Seconds, by using chloride-exchanged samples, anion-exchange test was carried out with 1.2~1.5 mM NaNO3, Na2SO4, Na2HPO4, NaF, and NaI aqueous solutions for 24 h, where solid to liquid ratio was fixed to 1.0 gL-1. After 24 h, exchanged amount was analyzed by ion-chromatography. Results and discussion: XRD patterns of all samples exhibit successful formation of LDHs without byproducts even after chloride-exchange treatment. Successful inclusion of F atoms into host-layer of LDHs was confirmed by solid-state NMR and, in all samples, 80% of interlayer anions was exchanged by chloride ions, which is confirmed by compositional analysis with EDS and XPS. XRD patterns also revealed that the basal spacing, d 003, of F-LDHs tend to decrease comparing to original LDHs, which implies that host-layers strongly interacts with interlayer anions and water molecules. In addition, F-substitution amount was found out to be limited by about 10%, LDHs can be obtained without byproducts. Next, distribution coefficient, useful indicator of the affinity of respective anions to LDHs, was calculated from the result of ion-exchange test. It clarified that the order of affinity was NO3 − > HPO4 2− > Br− > F− > SO4 2− > I−, which is totally different from previous reports. Furthermore, the affinity of NO3 − ions comparing to SO4 2− ion and F− ions was increased depending on F-substitution amount. In conference, detailed formation mechanism of F-LDHs and anion-exchanging mechanism was presented.

Effects of metal cations on coupled birnessite structural transformation and natural organic matter adsorption and oxidation

Abstract Birnessite, a layered manganese (Mn) oxide, possesses extraordinary metal adsorption and oxidation activity, and thus imposes impacts on many biogeochemical processes. The reactivity of birnessite strongly relies on its Mn oxidation state composition (the proportions of Mn II, III and IV), particularly by the Mn(III) proportion. Partial reduction of birnessite transforms birnessite to be Mn(II, III)-rich or to MnOOH and Mn3O4, and thus strongly affects birnessite reactivity. As a metal scavenger, naturally occurring birnessite contains abundant transition and alkali and alkaline earth metal cations in its structure; however, the effects of these metal cations on the partial reduction-induced transformation of birnessite remain unknown. We examined the effects of Zn2+, Mg2+, Ca2+ and ionic strength (controlled by NaCl) on transformation of birnessite (δ-MnO2) and adsorption and oxidation of natural organic matter during partial reduction by fulvic acid (FA) at pH 8 and FA/MnO2 mass ratios (R) of 0.1 or 1 over 600 h under anoxic conditions. Results showed that low ionic strength (0 versus 50 mM NaCl) disfavored FA adsorption, fractionation and oxidation, and thus disfavored formation of Mn(III) in the reacted birnessite. Compared to the 50 mM NaCl system, all divalent cations (Mg2+, Ca2+ and Zn2+) favored FA adsorption and fractionation. Both Mg2+ and Ca2+ significantly enhanced FA oxidation at the early stage but barely at the late stage, whereas Zn2+ strongly suppressed FA oxidation during the entire experimental period. Due to adsorption competition, the presence of the divalent cations resulted in low concentration of Mn(II) adsorbed on vacancies of birnessite. Both Ca2+ and Mg2+ favored Mn(III) production in MnO6 layers, while Zn2+ inhibited it. A small portion of birnessite also transformed to feitknechtite and hausmannite, and the transformation seemed faster in the presence of Ca2+ or Mg2+ than in NaCl solution. In the presence of Zn2+ at the high FA/MnO2 ratio (R = 1), Zn-substituted hansmannite formed extenstively. The formation of Mn(III) in the reacted birnessite can be ascribed to comproportionation between Mn(IV) and Mn(II) adsorbed on either vacancies or edge sites of birnessite. The low-valence Mn oxide phases likely formed via the comproportionation on the edges. The divalent cations affected Mn(III) concentrations of birnessite and formation of the low-valence Mn oxides by competing with Mn(II) for adsorption on edge/vacancy sites or stabilizing Mn(III) in the layers. This work indicates that divalent metal cations strongly influence reactivity and transformation of birnessite in the coupled Mn and carbon redox cycles, and that birnessite containing divalent cations can be an important adsorbent for natural organic carbon in Mn-rich environments. Overall, this study provides insights into the coupled cycles of Mn, trace metals and organic carbon in alkaline and saline environments.

DNAzymes at Work: A DFT Computational Investigation on the Mechanism of 9DB1.

The 9DB1 DNAzyme follows an addition-elimination (AN+DN) two-step mechanism, involving a phosphorane intermediate, where the 3'-hydroxyl group (nucleophile) of one RNA fragment attacks the 5'-triphosphate of another RNA fragment. This mechanism does not involve a divalent metal cation in agreement with the experimental evidence. The process is assisted by two proton transfers that activate the nucleophile (first step) and the leaving group (second step). The dA13 nucleotide is not directly involved in the reaction. However, it plays an important role in determining the regioselectivity of the process: since the dA13 phosphate forms a strong hydrogen bond with the 2'-hydroxyl, only the 3'-hydroxyl can behave as a nucleophile and form the new 3'-5' bond. In silico mutagenesis, where the dA13 phosphate oxygen involved in the hydrogen contact was replaced by a sulfur atom, causes a significant rearrangement of the A50 ribose position with an increase in the activation barrier and a consequent lower enzymatic activity in agreement with the experimental evidence. A similar effect is determined by the replacement of the 2'-hydroxyl with different groups such as F, H, and OMe.

Solubility Difference between Pectic Fractions from Creeping Fig Seeds.

Crude water-extracted pectin (WEP) isolated from creeping fig seeds were mainly fractionated into WEP-0.3 and WEP-0.4 fractions. Fractions were confirmed to be nonstarch, nonreducing sugars, nonpolyphenols and protein-unbounded acidic polysaccharides. Interestingly, a significant difference in solubility was found between WEP-0.3 (higher solubility than WEP) and WEP-0.4 (remarkably insoluble), which was consistent with the amorphous and porous sponge-like structure of WEP-0.3 as well as the crystalline and dense rod-like state of WEP-0.4. However, the result of the FT-IR spectra was contradicted by the solubility of WEP-0.4, which possessed the lowest degree of methoxylation and ought to possess the highest solubility. Through mineral analysis, a considerably high content of Ca2+ was found in WEP-0.4, suggesting that the low solubility of WEP-0.4 was probably attributable to the formation of microgels during dialysis. Therefore, metal divalent cations in the dialysate were suggested to be depleted for the dialysis of low methoxyl pectin.

Metal ion adsorption from wastewater by g-C3N4 modified with hydroxyapatite: a case study from Sarcheshmeh Acid Mine Drainage

Hydroxyapatite (Hap) modified graphitic carbon nitride (g-C3N4) powders (Hap/g-C3N4) were prepared and characterized. The sorption activities of g-C3N4 and Hap/g-C3N4 were evaluated for the removal of Cu2+, Mn2+, Zn2+, Pb2+, Fe3+ and Cd2+ metal ions from simulated wastewater (synthetic solutions containing metal ions) and acid mine drainage from the sSarcheshmeh Copper Mine. The results indicated that the sorption capacities (SC) of g-C3N4 remarkably increased after the modification by Hap. Hap/g-C3N4 possessed higher SC than unmodified g-C3N4. So, the sorption mechanisms appear mainly attributable to the chemical interactions between the metal ions and functional groups on the Hap/g-C3N4 surface. More adsorption sites were formed on Hap/g-C3N4, which possessed abundant surface hydroxyl, phosphate and –NH2/–NH–/=N groups causing the higher efficiency of metal ion uptakes. Divalent metal cations are first adsorbed on the Hap/g-C3N4 surface and then substitution with Ca2+ ion can occur. Effect of pH evaluations confirmed that SC increased steeply with the pH increasing on both g-C3N4 and Hap/g-C3N4 adsorbents. The kinetic study demonstrated that the adsorption data were correlated well with the Langmuir isotherm model, and the adsorption process successfully followed the pseudo-second-order model.

Synthesis and structural characterization of transition metal dithiolene derivatives containing divalent metals as counter-cations

Complexes containing [Fe2(SC6H2Cl2S)4]2− or [Ni(SC6H2Cl2S)2]− entities and divalent metal cations as well as an 1D-coordination polymer K/Zn have been obtained and their magnetic behaviour studied.