Absence Of Metal Ions Research Articles

A Cu(II) Indicator Platform Based on Cu(II) Induced Swelling that Changes the Extent of Fluorescein Self-Quenching.

In this study, we established a new fluorescent indicator platform. The responsive element consists of poly(N-isopropylacrylamide) nanospheres that include small percentages of fluorescein and a ligand, anilinodiacetate (phenylIDA). Nanosphere diameters were determined to be in the range from 50 to 90 nm by scanning electron microscopy. They were entrapped in a polyacrylamide gel to prevent nanosphere aggregation. At pH 6, the ligand is negatively charged in the absence of metal ions. Charge-charge repulsion causes the nanosphere to swell. Dynamic light scattering measurements show that these nanospheres do not shrink and aggregate at high temperature. Cu(II) binding neutralizes the charge causing the particles to shrink. This brings fluoresceins closer together, increasing the degree of self-quenching. The intensity decreases by 30% as Cu(II) concentration increases. To rule out the possibility that the observed decrease in intensity was due to Cu(II) quenching of fluorescence, we also added Zn(II) and observed a decrease in intensity. This approach can be adapted to sense different metal ions and different concentrations of Cu(II) by changing the ligand.

Silver(I)-Ion-Mediated Cytosine-Containing Base Pairs: Metal Ion Specificity for Duplex Stabilization and Susceptibility toward DNA Polymerases.

Spectroscopic characterization of AgI -ion-mediated C-AgI -A and C-AgI -T base pairs found in primer extension reactions catalyzed by DNA polymerases was conducted. UV melting experiments revealed that C-A and C-T mismatched base pairs in oligodeoxynucleotide duplexes are specifically stabilized by AgI ions in 1:1 stoichiometry in the same manner as a C-C mismatched base pair. Although the stability of the mismatched base pairs in the absence of AgI ions is in the order C-A≈C-T>C-C, the stabilizing effect of AgI ions follows the order C-C>C-A≈C-T. However, the comparative susceptibility of dNTPs to AgI -mediated enzymatic incorporation into the site opposite templating C is dATP>dTTP≫dCTP, as reported. The net charge, as well as the size and/or shape complementarity of the metal-mediated base pairs, or the stabilities of mismatched base pairs in the absence of metal ions, would be more important than the stability of the metallo-base pairs in the replicating reaction catalyzed by DNA polymerases.

Turn-off and -on fluorescence switching of a self-assembled sensor for mercury(II) induced by anionic micelles

The self-assembling of the 10-methylacridinium perchlorate fluorophore and azathiocrown ether derivative receptor through sodium dodecyl sulfate (SDS) or sodium 4-octylbenzenesulfonate (SOBS) aggregates in water has allowed the realization of a new effective sensing system for Hg2+ ions. In the absence of metal ions, occurrence of the fluorophore-receptor interaction was signaled through the fluorescence quenching of acridinium. Upon addition of increasing quantities of Hg2+, fluorescence experiments pointed out an unusual further quenching of the acridinium fluorescence, followed by an emission enhancement with a switching point around 1 × 10−7 and 1 × 10−8 M in SDS and SOBS, respectively. Such a behavior was attributed to the formation of mercury-receptor complexes with different coordination number and accordingly with distinct redox activity. The stoichiometry of the mercury complexes was assessed by spectrophotometric titration, while insights into their structures were obtained with density functional theory (DFT) calculation. Hg2+ ions can be selectively detected in SDS solutions, and its concentration in the nanomolar range can be linearly determined. Altogether, this modular approach allows quick variation of the components, which makes the optimal tuning of the process up to its maximum efficiency level very easy.

Metal-Free Transamidation of Secondary Amides by N-C Cleavage.

Transamidation reactions represent a fundamental chemical process involving conversion of one amide functional group into another. Herein, we report a facile, highly chemoselective method for transamidation of N-tert-butoxycarbonylation (N-Boc) activated secondary amides that proceeds under exceedingly mild conditions in the absence of any additives. Because this reaction is performed in the absence of metals, oxidants, or reductants, the reaction tolerates a large number of useful functionalities. The reaction is compatible with diverse amides and nucleophilic amines, affording the transamidation products in excellent yields through direct nucleophilic addition to the amide bond. The utility of this methodology is highlighted in the synthesis of Tigan, a commercial antiemetic, directly from the amide bond. We expect that this new metal-free transamidation will have broad implications for the development of new transformations involving direct nucleophilic addition to the amide bond as a key step.

Protein profiling of metal-resistant Bacillus cereus VITSH1.

The aim of the study was to investigate the proteomic changes and antioxidant enzyme activity in chromium-resistant Bacillus cereus VITSH1 in response to heavy metal toxicity. The variation in protein expression and antioxidant enzyme activity in the presence and absence of metal was studied in B. cereus VITSH1. The differentially expressed proteins in chromium-treated conditions were determined by SDS PAGE. The proteins involved in metal binding, protein biosynthesis, protein folding, energy metabolism and motility were identified by mass spectrometry coupled with bioinformatics search tools. The in gel assays for antioxidant enzymes indicated a change in their activity under metal stress conditions. The findings of this study suggest that the organism combats metal stress probably by restricting the entry of metal inside the cell. The role of the differentially expressed proteins clearly indicates that the first line of defence is to avoid the entry of metal into the cell either by possessing a modified outer membrane or by moving away from the toxicant. Further work on the identification of other proteins playing a role in resistance would help in integrating the available knowledge on the resistance mechanisms the organisms employ to combat toxicity. The proteomic changes in the metal-exposed bacteria would give an insight into the proteins involved in metal resistance mechanisms and thereby aid in the development of biosensor-based technology for heavy metal detection.

Study of complexation of dopamine analogues with bioactive metals by cyclic voltammetry

Dopamine analogues are reported to have remarkable biological activities. Levodopa and carbidopa are the active ingredients of drugs used for the treatment of Parkinson and Alzheimer diseases. Methyldopa is another molecule belonging to the same class of compounds and is used as hypertensive drug. Concurrent ingestion of the metal ions can cause marked changes in the bioavailability of these drug molecules. In the present study, interaction of dopamine analogues with the bioessential metals like iron, copper, and aluminum is investigated. These bioessential metals have a significant role in the normal functioning of the brain and nerves. The work is aimed to establish a convenient, simple, and fast method through which the complexation of dopamine analogues with different bioactive metals can be explored. For this purpose, cyclic voltammetric probe is used and electrochemical parameters of electroactive drug in the presence and absence of metal ion are served as an analytical tool. It was found that the drug molecule undergoes coupled chemical reaction with the metal ions, and this interaction is pH dependent. Electrochemical behavior of the drug molecule was found to be changed in the presence of the metal ion.

Microporous 3D Covalent Organic Frameworks for Liquid Chromatographic Separation of Xylene Isomers and Ethylbenzene.

Microporous covalent organic frameworks (COFs) hold great potential for small molecule separation but are yet challenging to design and synthesize. Here we report a framework interpenetration strategy to make microporous COFs for efficient separations of C8 alkyl-aromatic isomers. Two pairs of microporous three-dimensional (3D) salen- and Zn(salen)-based COFs are prepared by Schiff-base condensation of ethanediamine with tetrahedral tetra(salicylaldehyde)-silane or -methane derivatives in the presence or absence of metal ions. The four 3D COFs are isostructural and have a 7-fold interpenetrated diamondoid open framework with less than 8.0 Å wide tubular channels. They exhibit permanent porosity, high thermal stability, and good chemical resistance. The two COFs functionalized with uncoordinated salen groups can serve as stationary phases for high-performance liquid chromatography to provide baseline separation of xylene isomers and ethylbenzene with excellent column efficiency and precision, whereas the COFs with Zn(salen) motifs cannot achieve high-resolution separation. The salen-COFs showed high affinity to the o-xylene, allowing fast and selective separation of the o-isomer from the other isomers within 7 min. This is the first report utilizing COFs to separate the practically important aromatic isomers. This work highlights new opportunities in designing microporous COFs and paves the way to expand the potential applications of COF materials.

Tetraphenylethene decorated hyperbranched poly(amido amine)s as metal/organic-solvent-free turn-on AIE probe for specific pyrophosphate detection

“Turn-on” fluorescent materials in the absence of metal and organic solvent are highly desirable as bioprobes to selectively and sensitively detect pyrophosphate (PPi) anion. In this work, we embedded tetraphenylethene (TPE) inside the relatively rigid hyperbranched poly(amido amine) (HPA) to afford highly water soluble TPE-HPA materials, where the TPE fluorescence was pre-quenched in water by the hyperbranched structure of HPA without participation of any metal or organic solvents. The obtained TPE-HPA could be directly used as fluorescence “turn-on” bioprobe under physiological conditions to selectively detect PPi. There was a good linearity between the photoluminescence enhancement and the PPi concentration in the range of 0–1.0 μM with a limit of detection (LOD) of 6.5 nM. Compared with the other reported AIE probes for PPi detection, TPE-HPA with the optimal TPE content showed the best comprehensive performance, that is metal-free, organic-solvent free and nanomolar level LOD. The extreme selectivity and sensitivity towards PPi allowed TPE-HPA to accurately detect the PPi concentration in human urine.

Design of High-Affinity Metal-Controlled Protein Dimers.

The ability to precisely control protein complex formation has high utility in the expanding field of biomaterials. Driving protein-protein binding through metal-ligand bridging interactions is a promising method of achieving this goal. Furthermore, the capacity to precisely regulate both complex formation and dissociation enables additional control not available with constitutive protein complexes. Here we describe the design of three metal-controlled protein dimers that are completely monomeric in the absence of metal yet form high-affinity symmetric homodimers in the presence of zinc sulfate. The scaffold used for the designed dimers is the β1 domain of streptococcal protein G. In addition to forming high-affinity dimers in the presence of metal, the complexes also dissociate upon addition of EDTA. Biophysical characterization revealed that the proteins maintain relatively high thermal stability, bind with high affinity, and are completely monodisperse in the monomeric and dimeric states. High-resolution crystal structures revealed that the dimers adopt the target structure and that the designed metal-binding histidine residues successfully bind zinc and function to drive dimer formation.

Studies on the interaction between ciprofloxacin hydrochloride and diclofenac sodium

Purpose: To study the interaction between ciprofloxacin hydrochloride (Cipro) and diclofenac sodium (DS) in the presence and absence of metal ions. Methods: Complexes were prepared in the aqueous phase at different molar ratios (r) of Cipro:DS (ranged from 0.2 – 2.0). The complexes were characterized by Fourier transform-infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), and high pressure liquid chromatography (HPLC). Their properties, i.e., solubility, dissolution and partition coefficient (log P), were studied along with their permeability across Caco-2 cells. Furthermore, the antimicrobial activity of Cipro and its complexes was determined using standard broth dilution method and expressed as minimum inhibitory concentration (MIC). Results: Cipro formed an ion pair with DS. The product was confirmed to be a combination of the two drugs, DS and Cipro, but in a ratio that is dependent on the added amounts of each component (r = 1:1 or 1:2). The 1:1 product was more lipophilic than the individual components leading to a lower aqueous solubility and a higher octanol/water partition coefficient log P (6.7 vs. 0.77). The presence of DS within the dissolution medium appeared to modify the dissolution of Cipro depending on the concentration. Moreover, ternary complexes involving Cipro, DS and metal ions (iron and/or calcium) exhibited improved antimicrobial effect (MIC, 0.016 μg/ml compared to 0.258 μg/ml for Cipro). Caco-2 cell permeation data indicate that the presence of DS significantly improved the apparent permeability coefficient (Papp) of Cipro (20.6 × 10 -6 cm/s) which was three times higher than that of free Cipro (p < 0.05). DS also appeared to counteract the well-known negative effect of metal ions on the bioavailability of Cipro. Conclusion: There is a clinically relevant interaction between DS and Cipro at the absorption level as a result of ion pair formation, which might even counteract the negative effect of metals on the absorption of Cipro. These findings should aid the design of new Cipro ion pairs that provide higher bioavailability than free Cipro. Keywords: Ciprofloxacin, Diclofenac, Interaction, Ion pair, Permeability coefficient, Bioavailability, Absorption

Multicolor Raman Beads for Multiplexed Tumor Cell and Tissue Imaging and in Vivo Tumor Spectral Detection

Developing new nanomaterials with strong and distinctive Raman vibrations in the biological Raman-silent region (1800-2800 cm-1) were highly desirable for Raman hyperspectral detection and imaging in living cells and animals. Herein, polymeric nanoparticles with monomers containing alkyne, cyanide, azide, and carbon-deuterate were prepared as Raman-active nanomaterials (Raman beads) for bioimaging applications. Intense Raman signals were obtained due to the high density of alkyne, cyanide, azide, and carbon-deuterate in single nanoparticles, in absence of metal (such as Au or Ag) as Raman enhancers. We have developed a library of Raman beads for frequency multiplexing through the end-capping substitutions of monomers and demonstrated five-color SRS imaging of mixed nanoparticles with distinct Raman frequencies. In addition, with further surface functionalization of targeting moieties (such as nucleic acid aptamers and targeting peptides), targetable Raman beads were successfully used as probes for tumor targeting and Raman spectroscopic detection, including multicolor SRS imaging in living tumor cells and tissues with high specificity. Further in vivo studies indicated that Raman beads anchored with targeting moieties were successfully employed to target tumors in living mice after tail intravenous injection, and Raman spectral detection of tumor in live mice was achieved only through spontaneous Raman signal at the biological Raman-silent region without any signal enhancement due to a high density of Raman reporters in Raman beads. With further copolymerization of these monomers, Raman beads with supermultiplex barcoding could be readily achieved.

Acid Environment-improved fluorescence sensing performance: A quinoline Schiff base-containing sensor for Cd2+ with high sensitivity and selectivity

The development of acid environment-applicable fluorescence sensor is challenging but attractive topic, which can achieve the rapid and comprehensive evaluation of total soluble heavy metal content in natural water. In this work, a quinoline-containing Schiff base, AMQD, was utilized as fluorescence probe for Cd2+. Interestingly, the obtained chemosensor exhibited much better fluorescence detection sensitivity and selectivity toward Cd2+ in acidic 10% methanol aqueous solution (pH 4) comparing to those in neutral environment. Initially, the fluorescence emission of AMQD was almost invisible with the absence of metal ions, while a significant turn-on fluorescence response (~425 nm) can be observed with the addition of Cd2+. The fluorescence detection possesses excellent selectivity without the interference of any other metal cation. The recognition ratio between the fluorescence sensor AMQD and Cd2+ was confirmed to be 1:1, and the detection limit was calculated to be 2.4 nM.

Selective coordination of three transition metal ions within a coiled-coil peptide scaffold.

Designing peptides that fold and assemble in response to metal ions tests our understanding of how peptide folding and metal binding influence one another. Here, histidine residues are introduced into the hydrophobic core of a coiled-coil trimer, generating a peptide that self-assembles upon the addition of metal ions. HisAD, the resulting peptide, is unstructured in the absence of metal and folds selectively to form an α-helical construct upon complexation with Cu(ii) and Ni(ii) but not Co(ii) or Zn(ii). The structure, and metal-binding ability, of HisAD is probed using a combination of circular dichroism (CD) spectroscopy, analytical ultracentrifugation (AUC), nuclear magnetic resonance (NMR) spectroscopy and X-ray crystallography. These show the peptide is trimeric and binds to both Cu(ii) and Ni(ii) in a 1 : 1 ratio with the histidine residues involved in the metal coordination, as designed. The X-ray crystal structure of the HisAD-Cu(ii) complex reveals the trimeric HisAD peptide coordinates three Cu(ii) ions; this is the first example of such a structure. Additionally, HisAD demonstrates an unprecedented discrimination between transition metal ions, the basis of which is likely to be related to the stability of the peptide-metal complexes formed.

Ligand-modulated ring-expansion.

An unstrained palladium-containing macrocycle was expanded by a ring-opening metathesis strategy. This reaction was driven by coordination of a bulky ligand, 2,6-lutidine, to the palladium center, enabling a monometallic macrocycle to transform into a bimetallic dimeric macrocycle.

Cleavage of lignin model compounds and ligninox using aqueous oxalic acid.

Aqueous oxalic acid cleaves oxidised β-O-4 lignin model compounds by two distinct mechanisms that are dependent on the presence of the hydroxymethyl substituent. Various β-O-4 phenoxyacetophenones that do not contain the hydroxymethyl substituent undergo oxidative cleavage upon exposure to aqueous oxalic acid in the presence of air, likely through concerted ring opening of a dioxetane intermediate to give the corresponding benzoic acid and phenyl formate. Importantly, detrimental side reactions arising from singlet oxygen and hydroperoxy radicals (from both O2 and oxalic acid) are minimal when the cleavage is run under air compared to neat oxygen. When oxidised β-O-4 lignin model compounds bearing the hydroxymethyl group are cleaved by aqueous oxalic acid, the resulting diketone and phenol products arise from a redox neutral cleavage that is analogous to the formic acid-sodium formate mediated lignin cleavage process reported by Stahl. Aqueous oxalic acid also cleaves lignin itself, with oxidised milled wood lignin (MWLox) from Pinus radiata giving a 14% yield of ethyl acetate soluble aromatics with good selectivity for vanillin. Aqueous oxalic acid appears to be a promising lignin cleavage system given the benign, bio-based reagents, absence of metals and organic solvents and a simple extraction procedure that enables oxalic acid recycling.

Ni(II), Hg(II), and Pb(II) Coordination in the Prokaryotic Zinc-Finger Ros87.

Zinc ion binding is a principal event in the achievement of the correct fold in classical zinc finger domains since the motif is largely unfolded in the absence of metal. In the case of a prokaryotic zinc finger, the larger βββαα domain contributes to the folding mechanism with a larger hydrophobic core. For these reasons, following the great amount of attention devoted to unveiling the effect of xenobiotic metal ion replacement in zinc fingers and in zinc-containing proteins in general, the prokaryotic zinc finger domain appears to be an interesting model for studying metal ion interaction with metalloproteins. Here, we explore the binding of Ni(II), Hg(II), and Pb(II) to Ros87, the DNA binding domain of the prokaryotic zinc finger protein Ros. We measured Ros87-metal ion dissociation constants and monitored the effects on the structure and function of the domain. Interestingly, we found that the protein folds in the presence of Ni(II) with important structural perturbations, while in the presence of Pb(II) and Hg(II) it does not appear to be significantly folded. Accordingly, an overall strong reduction in the DNA binding capability is observed for all of the examined proteins. Our data integrate and complement the information collected in the past few years concerning the functional and structural effects of metal ion substitution in classical zinc fingers in order to contribute to a better comprehension of the toxicity of these metals in biological systems.

A sulfide perspective on iron isotope fractionation during ocean island basalt petrogenesis

Iron isotopic compositions are demonstrably powerful tracers of foundational planetary processes, including crust and core formation. In many volcanic environments, however, geochemical vestiges of these processes are obscured by the effects of magmatic differentiation on Fe isotopic compositions. Recent decades have witnessed continued refinement of observational and experimental approaches to Fe isotope fractionation during silicate differentiation. In contrast, the influence of sulfide fractionation on Fe isotopic compositions in terrestrial environments is known only from theoretical approaches and limited experimental data for relatively siliceous magmatic systems. One reason for this may be that sulfide fractionation is difficult to definitively trace using traditional major and minor element variation patterns. We utilize well-characterized lavas and cumulate xenoliths from Piton de la Fournaise and Piton des Neiges, Réunion Island, that have previously been examined for their highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) contents to investigate the effect of sulfide fractionation on Fe isotopes. The Fe isotopic compositions of the basalts range from δ56Fe values of 0.04 to 0.15‰ (average: 0.10‰) and the compositions of the cumulate xenoliths range from δ56Fe values of −0.07 to 0.08‰ (average: 0‰). In the absence of metal, HSE preferentially partition into sulfide phases, making them important tracers of sulfide segregation during magmatic differentiation. We find that commonly-observed co-variations between Fe isotopic compositions and major element oxide abundances are relatively underdeveloped for Réunion lavas. The correlation between Fe isotopic composition and MgO, for example, has a similar statistical significance to the correlation between Fe isotopic composition and Pd/Ir ratios, suggesting an important role of sulfides during Fe isotopic fractionation. After accounting for sulfide segregation, we determine that the parental magma Fe isotopic composition calculated for Piton de la Fournaise would be overestimated by 0.04‰ (within propagated error, 0.01–0.06‰) when considering silicate differentiation alone. An analogous calculation for Kilauea Iki basalts, for which there is available Fe isotopic and HSE data, yields a somewhat smaller difference of 0.02‰ (0–0.03‰). These differences may partially explain Fe isotopic compositions in other settings that could not previously be reconciled with a dominantly peridotitic and/or chondritic mantle source. This discovery may warrant discussion of the apparent decoupling between Fe and radiogenic isotopes in ocean island basalts, where the latter shows significant global variations and the former may show little or none. Our work highlights the need for additional constraints on the behavior of Fe isotopes during crustal recycling processes and reinforces the notion that consideration must be given to the effect of magmatic differentiation on Fe isotopic compositions.

Amino-Acid-Based Metallo-Hydrogel That Acts Like an Esterase.

A histidine-based amphiphile containing a C14 fatty acyl chain, N- histidyl N'-myristry ethyl amine (AM1, 14.7 mM) forms hydrogels in the presence of Fe3+ (within the range 1.47 to 4.41 mM) and Hg2+ (within the range 3.67 to 11.02 mM) ions in aqueous dispersions at pH 6.6 (27 °C). The imidazole ring of the histidine residue plays a vital role to interact with these metal-ions. The thermal and mechanical stability of these metallo-hydrogels can be tuned by changing the proportion of amphiphile to metal ion ratio (1:0.1 to 1:0.3 for Fe3+-containing gel and 1:0.25 to 1:0.75 for Hg2+-containing gel). The metallo-hydrogels were characterized by different spectroscopic and microscopic techniques, low- and wide-angle powder X-ray diffraction, and small-angle X-ray scattering studies. FT-IR and NMR spectroscopic studies indicate the participation of the imidazole ring in metal-ion binding. Low- and wide-angle powder X-ray diffraction and small-angle X-ray scattering data are in favor of a layered structure of the supramolecular assembly of the AM1 in the presence of metal-ions. Both, the amphiphiles and the metal ion induced hydrogels reveal catalytic activity of p-nitrophenyl esters hydrolysis for the acetyl, n-butyl and n-octyl esters . Ferric ion containing metallo-hydrogel exhibits higher catalytic activity than the corresponding AM1 aggregate in the absence of metal ions.

Spectroscopic and metal binding properties of a de novo metalloprotein binding a tetrazinc cluster.

De novo design provides an attractive approach, which allows one to test and refine the principles guiding metalloproteins in defining the geometry and reactivity of their metal ion cofactors. Although impressive progress has been made in designing proteins that bind transition metal ions including iron-sulfur clusters, the design of tetranuclear clusters with oxygen-rich environments remains in its infancy. In previous work, we described the design of homotetrameric four-helix bundles that bind tetra-Zn2+ clusters. The crystal structures of the helical proteins were in good agreement with the overall design, and the metal-binding and conformational properties of the helical bundles in solution were consistent with the crystal structures. However, the corresponding apo-proteins were not fully folded in solution. In this work, we design three peptides, based on the crystal structure of the original bundles. One of the peptides forms tetramers in aqueous solution in the absence of metal ions as assessed by CD and NMR. It also binds Zn2+ in the intended stoichiometry. These studies strongly suggest that the desired structure has been achieved in the apo state, providing evidence that the peptide is able to actively impart the designed geometry to the metal cluster.

Influence of Alkali and Alkaline-Earth Metals on the Cleavage of Glycosidic Bond in Biomass Pyrolysis: A DFT Study Using Cellobiose as a Model Compound.

Fast pyrolysis is a promising technology for the production of renewable fuels and chemicals from lignocellulosic biomass. The product distribution (bio-oil, char) and the composition of bio-oil are significantly influenced by the presence of naturally occurring alkali and alkaline-earth metals (AAEMs). In this paper, we investigate, at the molecular level, the influence of Na(I), K(I), Ca(II), and Mg(II) ions on glycosidic bond breaking reactions using density functional theory. Glycosidic bond breaking reactions are categorized as direct C-O breaking mechanisms, namely, transglycosylation, glycosylation, and ring contraction and the two-step pathways, which include the mannose pathway, dehydration, and ring opening. Our calculations show that in the absence of metal, transglycosylation and dehydration pathways (activation barriers ∼55 kcal.mol-1) are kinetically most facile. The linkage type (α- or β-1,4) has an insignificant effect on kinetics of glycosidic bond cleavage. Mg(II) ions have a pronounced effect on lowering the activation barriers of glycosylation, ring contraction, and the mannose pathway, requiring activation enthalpies of 32-52 kcal.mol-1. Conversely, Mg(II) and Ca(II) ions inhibit the dehydration pathway. Na(I) and K(I) ions do not significantly influence the activation barriers of glycosidic bond cleavage reactions, as the reduction is only about 5-10 kcal.mol-1. Thus, AAEM ions exhibit different catalytic effects on glycosidic bond breaking reactions.