Multilevel Changes in Protein Dynamics upon Complex Formation of the Calcium‐Loaded S100A4 with a Nonmuscle Myosin IIA Tail Fragment

Dysregulation of Ca²⁺‐binding S100 proteins plays important role in various diseases. The asymmetric complex of Ca²⁺‐bound S100A4 with nonmuscle myosin IIA has high stability and highly increased Ca²⁺ affinity. Here we investigated the possible causes of this allosteric effect by NMR spectroscopy. Chemical shift‐based secondary‐structure analysis did not show substantial changes for the complex. Backbone dynamics revealed slow‐timescale local motions in the H1 helices of homodimeric S100A4; these were less pronounced in the complex form and might be accompanied by an increase in dimer stability. Different mobilities in the Ca²⁺‐coordinating EF‐hand sites indicate that they communicate by an allosteric mechanism operating through changes in protein dynamics; this must be responsible for the elevated Ca²⁺ affinity. These multilevel changes in protein dynamics as conformational adaptation allow S100A4 fine‐tuning of its protein–protein interactions inside the cell during Ca²⁺ signaling.     

Complexation of poly(methyl methacrylate-g-propylene oxide) and alkali-metal ions

Summary Copolymers of poly(methyl methacrylate-g-propylene oxide) with grafted branches having different molecular weights were complexed with K⁺, Na⁺ and Ca²⁺. The efficiency of the graft copolymers in binding cations was evaluated from salt distribution equilibria in water-methylene chloride. Among the used cations, K⁺ was more easily complexed than Na⁺ and Ca²⁺.     

A Calcium(II)-Based <Emphasis Type="SmallCaps">l</Emphasis>-Arginine for ATP Binding and Hydrolysis

The supramolecular recognition of Ca(II) and N _α-4-tosyl-l-arginine methyl ester hydrochloride (TAME) with ATP were investigated using ¹H and ³¹P NMR spectra. In the Ca(II)–ATP–TAME ternary system, Ca²⁺ and TAME bind with ATP via the phosphate chain and adenine ring of ATP. The binding forces are mainly electrostatic and cation (Ca²⁺)–π and π–π stacking interaction. Furthermore, the hydrolysis of ATP catalyzed by Ca(II) and TAME was studied at pH 7.0 and 60 °C using ³¹P NMR spectra. Kinetics studies show that the ATP hydrolysis rate constant is 0.1035 h⁻¹ in the Ca(II)–TAME–ATP ternary system, whereas the value is 8.5 × 10⁻³ h⁻¹ under the same conditions without TAME and Ca²⁺. The Ca(II) ions and TAME accelerate the ATP hydrolysis process about 12-fold. The proposed mechanism of ATP hydrolysis catalyzed by Ca²⁺–TAME occurs through an addition–elimination reaction sequence. These results can help us get more useful information at the molecular level about the key amino acid residue(s) and metal ions that serve as cofactors in the ATPase effect on ATP hydrolysis/synthesis.     

Computational Analyses of the AtTPC1 (Arabidopsis Two-Pore Channel 1) Permeation Pathway

Two Pore Channels (TPCs) are cation-selective voltage- and ligand-gated ion channels in membranes of intracellular organelles of eukaryotic cells. In plants, the TPC1 subtype forms the slowly activating vacuolar (SV) channel, the most dominant ion channel in the vacuolar membrane. Controversial reports about the permeability properties of plant SV channels fueled speculations about the physiological roles of this channel type. TPC1 is thought to have high Ca²⁺ permeability, a conclusion derived from relative permeability analyses using the Goldman–Hodgkin–Katz (GHK) equation. Here, we investigated in computational analyses the properties of the permeation pathway of TPC1 from Arabidopsis thaliana. Using the crystal structure of AtTPC1, protein modeling, molecular dynamics (MD) simulations, and free energy calculations, we identified a free energy minimum for Ca²⁺, but not for K⁺, at the luminal side next to the selectivity filter. Residues D269 and E637 coordinate in particular Ca²⁺ as demonstrated in in silico mutagenesis experiments. Such a Ca²⁺-specific coordination site in the pore explains contradicting data for the relative Ca²⁺/K⁺ permeability and strongly suggests that the Ca²⁺ permeability of SV channels is largely overestimated from relative permeability analyses. This conclusion was further supported by in silico electrophysiological studies showing a remarkable permeation of K⁺ but not Ca²⁺ through the open channel.     

An Exceptionally Selective DNA Cooperatively Binding Two Ca2+ Ions

Ca²⁺ is a highly important metal ion in biology and in the environment, and thus there is extensive work in developing sensors for Ca²⁺ detection. Although many Ca²⁺‐binding proteins are known, few nucleic acids can selectively bind Ca²⁺. DNA‐based biosensors are attractive for their high stability and excellent programmability. We report a RNA‐cleaving DNAzyme, EtNa, cooperatively binding two Ca²⁺ ions but to only one Mg²⁺. Four DNAzymes with known Ca²⁺‐dependent activity were compared, and the EtNa had the best selectivity for Ca²⁺. The EtNa is 90 times more active in Ca²⁺ than in Mg²⁺. Phosphorothioate (PS) modification showed that both non‐bridging oxygen atoms at the scissile phosphate contribute equally to Ca²⁺ binding. The pH–rate profile suggests two concurrent deprotonation reactions. EtNa was further engineered for Ca²⁺ sensing, and found to have a detection limit of 17 μm Ca²⁺ and excellent selectivity. The detection of Ca²⁺ in tap water was performed, and the result was comparable with that by ICP‐MS. This study offers new fundamental insights into Ca²⁺ binding by nucleic acids and improved metal selectivity by having multiple cooperative metal binding sites.     

Calcium Regulates S100A12 Zinc Sequestration by Limiting Structural Variations

Antimicrobial proteins such as S100A12 and S100A8/A9 are highly expressed and secreted by neutrophils during infection and participate in human immune response by sequestering transition metals. At neutral pH, S100A12 sequesters Zn²⁺ with nanomolar affinity, which is further enhanced upon calcium binding. We investigated the pH dependence of human S100A12 zinc sequestration by using Co²⁺ as a surrogate. Apo-S100A12 exhibits strong Co²⁺ binding between pH 7.0 and 10.0 that progressively diminishes as the pH is decreased to 5.3. Ca²⁺-S100A12 can retain nanomolar Co²⁺ binding up to pH 5.7. NMR spectroscopic measurements revealed that calcium binding does not alter the side-chain protonation of the Co²⁺/Zn²⁺ binding histidine residues. Instead, the calcium-mediated modulation is achieved by restraining pH-dependent conformational changes to EF loop 1, which contains Co²⁺/Zn²⁺ binding Asp25. This calcium-induced enhancement of Co²⁺/Zn²⁺ binding might assist in the promotion of antimicrobial activities in humans by S100 proteins during neutrophil activation under subneutral pH conditions.     

Structural Insights into Retinal Guanylate Cyclase Activator Proteins (GCAPs)

Retinal guanylate cyclases (RetGCs) promote the Ca²⁺-dependent synthesis of cGMP that coordinates the recovery phase of visual phototransduction in retinal rods and cones. The Ca²⁺-sensitive activation of RetGCs is controlled by a family of photoreceptor Ca²⁺ binding proteins known as guanylate cyclase activator proteins (GCAPs). The Mg²⁺-bound/Ca²⁺-free GCAPs bind to RetGCs and activate cGMP synthesis (cyclase activity) at low cytosolic Ca²⁺ levels in light-activated photoreceptors. By contrast, Ca²⁺-bound GCAPs bind to RetGCs and inactivate cyclase activity at high cytosolic Ca²⁺ levels found in dark-adapted photoreceptors. Mutations in both RetGCs and GCAPs that disrupt the Ca²⁺-dependent cyclase activity are genetically linked to various retinal diseases known as cone-rod dystrophies. In this review, I will provide an overview of the known atomic-level structures of various GCAP proteins to understand how protein dimerization and Ca²⁺-dependent conformational changes in GCAPs control the cyclase activity of RetGCs. This review will also summarize recent structural studies on a GCAP homolog from zebrafish (GCAP5) that binds to Fe²⁺ and may serve as a Fe²⁺ sensor in photoreceptors. The GCAP structures reveal an exposed hydrophobic surface that controls both GCAP1 dimerization and RetGC binding. This exposed site could be targeted by therapeutics designed to inhibit the GCAP1 disease mutants, which may serve to mitigate the onset of retinal cone-rod dystrophies.     

A Novel GUCA1A Variant Associated with Cone Dystrophy Alters cGMP Signaling in Photoreceptors by Strongly Interacting with and Hyperactivating Retinal Guanylate Cyclase

Guanylate cyclase-activating protein 1 (GCAP1), encoded by the GUCA1A gene, is a neuronal calcium sensor protein involved in shaping the photoresponse kinetics in cones and rods. GCAP1 accelerates or slows the cGMP synthesis operated by retinal guanylate cyclase (GC) based on the light-dependent levels of intracellular Ca²⁺, thereby ensuring a timely regulation of the phototransduction cascade. We found a novel variant of GUCA1A in a patient affected by autosomal dominant cone dystrophy (adCOD), leading to the Asn104His (N104H) amino acid substitution at the protein level. While biochemical analysis of the recombinant protein showed impaired Ca²⁺ sensitivity of the variant, structural properties investigated by circular dichroism and limited proteolysis excluded major structural rearrangements induced by the mutation. Analytical gel filtration profiles and dynamic light scattering were compatible with a dimeric protein both in the presence of Mg²⁺ alone and Mg²⁺ and Ca²⁺. Enzymatic assays showed that N104H-GCAP1 strongly interacts with the GC, with an affinity that doubles that of the WT. The doubled IC₅₀ value of the novel variant (520 nM for N104H vs. 260 nM for the WT) is compatible with a constitutive activity of GC at physiological levels of Ca²⁺. The structural region at the interface with the GC may acquire enhanced flexibility under high Ca²⁺ conditions, as suggested by 2 μs molecular dynamics simulations. The altered interaction with GC would cause hyper-activity of the enzyme at both low and high Ca²⁺ levels, which would ultimately lead to toxic accumulation of cGMP and Ca²⁺ in the photoreceptor outer segment, thus triggering cell death.     

Strontium ion substituted alginate-based hydrogel fibers and its coordination binding model

Sr-alginate and Ca-alginate hydrogel fibers were fabricated via microfluidic spinning technology, and various analytical methods were adopted to characterize fibers and disclose the coordination model of Sr²⁺ binding with alginate molecule chain. For both fibers, the more crosslinking sites of Sr²⁺ with alginate molecule were illustrated in comparison with that of Ca²⁺. The more robust mechanical performance of Sr-alginate fibers than Ca-alginate counterpart was a strong indication of the more strong binding of Sr²⁺ with alginate molecular chain. FTIR and electric conductivity disclosed the chelation type of Sr²⁺ with alginate macromolecule being similar to that of Ca²⁺, which was core-shell of the analogous “egg-box” structure. Circular dichroism spectroscopy further certified the extra coordination sites for Sr²⁺ with alginate molecule than Ca²⁺. Research on the coordination model will be more beneficial to optimizing the physicochemical properties of alginate fibers. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48571.     

Competitive binding of divalent cations to poly(α-hydroxyacrylic acid)

The competitive binding of divalent cations (Mg²⁺, Ca²⁺, Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺) on poly(α-hydroxyacrylic acid) (PHA) and poly(acrylic acid) (PAA) was investigated by equilibrium dialysis. In the Mg/Ca mixed system, binding selectivity for Ca²⁺ over Mg²⁺ was significantly higher in PHA than in PAA; this was attributed to coordination of α-OH groups on PHA to Ca²⁺. The binding ability and selectivity for the transition metal cations were almost the same for PHA and PAA at neutral pH, while PHA maintained appreciably higher degrees of binding than PAA in acidic solutions (pH about 3). This cation binding ability of PHA was ascribed to the lower pK_a value in the relevant pH region. © 1998 SCI.     

Homology Modeling Study of Bovine μ-Calpain Inhibitor-Binding Domains

The activated mammalian CAPN-structures, the CAPN/CAST complex in particular, have become an invaluable target model using the structure-based virtual screening of drug candidates from the discovery phase to development for over-activated CAPN linked to several diseases, such as post-ischemic injury and cataract formation. The effect of Ca²⁺-binding to the enzyme is thought to include activation, as well as the dissociation, aggregation, and autolysis of small regular subunits. Unfortunately, the Ca²⁺-activated enzyme tends to aggregate when provided as a divalent ion at the high-concentration required for the protease crystallization. This is also makes it very difficult to crystallize the whole-length enzyme itself, as well as the enzyme-inhibitor complex. Several parameters that influence CAPN activity have been investigated to determine its roles in Ca²⁺-modulation, autoproteolysis, phosphorylation, and intracellular distribution and inhibition by its endogenous inhibitor CAST. CAST binds and inhibits CAPN via its CAPN-inhibitor domains (four repeating domains 1–4; CAST1–4) when CAPN is activated by Ca²⁺-binding. An important key to understanding CAPN1 inhibition by CAST is to determine how CAST interacts at the molecular level with CAPN1 to inhibit its protease activity. In this study, a 3D structure model of a CAPN1 bound bovine CAST4 complex was built by comparative modeling based on the only known template structure of a rat CAPN2/CAST4 complex. The complex model suggests certain residues of bovine CAST4, notably, the TIPPKYQ motif sequence, and the structural elements of these residues, which are important for CAPN1 inhibition. In particular, as CAST4 docks near the flexible active site of CAPN1, conformational changes at the interaction site after binding could be directly related to CAST4 inhibitory activity. These functional interfaces can serve as a guide to the site-mutagenesis in research on bovine CAPN1 structure-function relationships for the design of small molecules inhibitors to prevent uncontrolled and unspecific degradation in the proteolysis of key protease substrates.     

Molecular dynamics study of Ca2+ binding loop variants of parvalbumin with modifications at the `gateway' position

The helix–loop–helix (i.e. EF-hand) Ca²⁺ ion bindingmotif is characteristic of a large family of high-affinity Ca²⁺ion binding proteins, including the parvalbumins and calmodulins.In this paper we describe a set of molecular dynamics computationson the major parvalbumin from the silver hake (SHPV-B). In allvariants examined, both whole protein and fragments thereof,the ninth loop residue in the Ca²⁺ binding coordination sitein the CD helix–loop–helix region (the so-called`gateway' residue) has been mutated. The three gateway mutationsexamined are arginine, which has never been found at the gatewayposition of any EF-hand protein, cysteine, which is the residueobserved least in natural EF-hand sites, and serine, which isthe most common (by far) non-acidic residue substitution atthis position in EF-hand proteins in general, but never in parvalbumins.Results of the molecular dynamics simulations indicate thatall three modifications are disruptive to the integrity of themutated Ca²⁺ binding site in the whole parvalbumin protein.In contrast, only the arginine and cysteine mutations are disruptiveto the integrity of the mutated Ca²⁺ binding site in the CDfragment of the parvalbumin protein. Surprisingly, the serinevariant of the CD helix–loop–helix fragment exhibitedremarkable stability during the entire molecular dynamics simulation,with retention of the Ca²⁺ binding site. These results indicatethat there are no inherent problems (for Ca²⁺ ion binding) associatedwith the sequence of the CD helix–loop–helix fragmentthat precludes the incorporation of serine at the gateway position.Since the CD site is totally disrupted in the whole proteinserine variant, this indicates that the Ca²⁺ ion binding deficienciesare most likely related to the unique interaction that existsbetween the paired EF-hands in the whole protein. Our theoreticalresults correlate well with previous studies on engineered EF-handproteins and with all of our experimental evidence on the silverhake parvalbumin.     

Generation and Characterization of a New FRET-Based Ca2+ Sensor Targeted to the Nucleus

Calcium (Ca²⁺) exerts a pivotal role in controlling both physiological and detrimental cellular processes. This versatility is due to the existence of a cell-specific molecular Ca²⁺ toolkit and its fine subcellular compartmentalization. Study of the role of Ca²⁺ in cellular physiopathology greatly benefits from tools capable of quantitatively measuring its dynamic concentration ([Ca²⁺]) simultaneously within organelles and in the cytosol to correlate localized and global [Ca²⁺] changes. To this aim, as nucleoplasm Ca²⁺ changes mirror those of the cytosol, we generated a novel nuclear-targeted version of a Föster resonance energy transfer (FRET)-based Ca²⁺ probe. In particular, we modified the previously described nuclear Ca²⁺ sensor, H2BD3cpv, by substituting the donor ECFP with mCerulean3, a brighter and more photostable fluorescent protein. The thorough characterization of this sensor in HeLa cells demonstrated that it significantly improved the brightness and photostability compared to the original probe, thus obtaining a probe suitable for more accurate quantitative Ca²⁺ measurements. The affinity for Ca²⁺ was determined in situ. Finally, we successfully applied the new probe to confirm that cytoplasmic and nucleoplasmic Ca²⁺ levels were similar in both resting conditions and upon cell stimulation. Examples of simultaneous monitoring of Ca²⁺ signal dynamics in different subcellular compartments in the very same cells are also presented.     

Physicochemical characterization of ATP binding to human 5-lipoxygenase

Human 5-lipoxygenase requires ATP as a stimulatory factor. At the two preferred concentrations of the free Ca²⁺, 0.02 μM with a resting cell and 20 μM with a stimulated cell, Scatchard analysis revealed that 5-lipoxygenase has one affinity ATP binding site with aK _d of 4.6 μM at the low Ca²⁺ concentration but has two affinity ATP binding sites with a higherK _d of 4.4 μM and a lowerK _d of 14.5 μM at the high Ca²⁺ concentration. In contrast, in a Tween 20 reaction system, 5-lipoxygenase had similar activation coefficients for ATP at both Ca²⁺ concentrations; these were 12.7 μM at the low Ca²⁺ concentration and 12.0 μM at the high Ca²⁺ concentration. These results showed that 5-lipoxygenase has an ATP binding site and suggest that self-association of 5-lipoxygenase in 20 μM Ca²⁺ may affect ATP binding affinity as measured by Scatchard analysis.     

Skeletal Ryanodine Receptors Are Involved in Impaired Myogenic Differentiation in Duchenne Muscular Dystrophy Patients

Duchenne muscular dystrophy (DMD) is characterized by progressive muscle wasting following repeated muscle damage and inadequate regeneration. Impaired myogenesis and differentiation play a major role in DMD as well as intracellular calcium (Ca²⁺) mishandling. Ca²⁺ release from the sarcoplasmic reticulum is mostly mediated by the type 1 ryanodine receptor (RYR1) that is required for skeletal muscle differentiation in animals. The study objective was to determine whether altered RYR1-mediated Ca²⁺ release contributes to myogenic differentiation impairment in DMD patients. The comparison of primary cultured myoblasts from six boys with DMD and five healthy controls highlighted delayed myoblast differentiation in DMD. Silencing RYR1 expression using specific si-RNA in a healthy control induced a similar delayed differentiation. In DMD myotubes, resting intracellular Ca²⁺ concentration was increased, but RYR1-mediated Ca²⁺ release was not changed compared with control myotubes. Incubation with the RYR-calstabin interaction stabilizer S107 decreased resting Ca²⁺ concentration in DMD myotubes to control values and improved calstabin1 binding to the RYR1 complex. S107 also improved myogenic differentiation in DMD. Furthermore, intracellular Ca²⁺ concentration was correlated with endomysial fibrosis, which is the only myopathologic parameter associated with poor motor outcome in patients with DMD. This suggested a potential relationship between RYR1 dysfunction and motor impairment. Our study highlights RYR1-mediated Ca²⁺ leakage in human DMD myotubes and its key role in myogenic differentiation impairment. RYR1 stabilization may be an interesting adjunctive therapeutic strategy in DMD.     

IRAG2 Interacts with IP3-Receptor Types 1, 2, and 3 and Regulates Intracellular Ca2+ in Murine Pancreatic Acinar Cells

The inositol 1,4,5-triphosphate receptor-associated 2 (IRAG2) is also known as Jaw1 or lymphoid-restricted membrane protein (LRMP) and shares homology with the inositol 1,4,5-triphosphate receptor-associated cGMP kinase substrate 1 (IRAG1). IRAG1 interacts with inositol trisphosphate receptors (IP₃ receptors /IP₃R) via its coiled-coil domain and modulates Ca²⁺ release from intracellular stores. Due to the homology of IRAG1 and IRAG2, especially in its coiled-coil domain, it is possible that IRAG2 has similar interaction partners like IRAG1 and that IRAG2 also modulates intracellular Ca²⁺ signaling. In our study, we localized IRAG2 in pancreatic acinar cells of the exocrine pancreas, and we investigated the interaction of IRAG2 with IP₃ receptors and its impact on intracellular Ca²⁺ signaling and exocrine pancreatic function, like amylase secretion. We detected the interaction of IRAG2 with different subtypes of IP₃R and altered Ca²⁺ release in pancreatic acinar cells from mice lacking IRAG2. IRAG2 deficiency decreased basal levels of intracellular Ca²⁺, suggesting that IRAG2 leads to activation of IP₃R under unstimulated basal conditions. Moreover, we observed that loss of IRAG2 impacts the secretion of amylase. Our data, therefore, suggest that IRAG2 modulates intracellular Ca²⁺ signaling, which regulates exocrine pancreatic function.     

A study of the electrochemical oxidation of hydrogen peroxide on a platinum rotating disk electrode in the presence of calcium ions using Michaelis-Menten kinetics and binding isotherm analysis

The present work demonstrates a potential suppression in the electrochemical signal of H₂O₂ oxidation due to the presence of Ca²⁺ ions. A mechanistic scheme was proposed to include a reversible interaction of Ca²⁺ ions with either the electrode surface binding sites (competitive) or the complex sites (non-competitive). The degree of inhibition was inspected by evaluating the kinetic currents as a function of [Ca²⁺] applying Koutecky-Levich kinetics. These observations were further supported with models based on enzyme kinetics such as Michaelis-Menten model applying Lineweaver-Burk plot along with non-linear least-square fitting analysis. The experimental results suggests that the strength of the complex binding sites decreases considerably with increasing [Ca²⁺] and that a single H₂O₂ molecule is required to combine with one available active binding site.     

Synthesis and characterization of metal‐containing polyurethanes with antibacterial activity

Metal salts of mono(hydroxypentyl)phthalate [M(HPP)₂, where M is Ca²⁺, Cd²⁺, Pb²⁺, or Zn²⁺] were synthesized by the reaction of 1,5‐pentane diol, phthalic anhydride, and metal acetate. A new series of metal‐containing polyurethanes containing ionic links in the main chain were synthesized by the reaction of hexamethylene diisocyanate or toluylene 2,4‐diisocyanate with the M(HPP)₂ salts. The structures of the monomers and polymers were confirmed with infrared, ¹H‐NMR, and ¹³C‐NMR spectra and elemental analysis. The polymers were also characterized with thermogravimetric analysis, differential scanning calorimetry, and solubility and viscosity measurements. The antibacterial activity of these polyurethanes was investigated with the agar diffusion method. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1194–1206, 2002     

Anticancer Ruthenium(III) Complex KP1019 Interferes with ATP‐Dependent Ca2+ Translocation by Sarco‐Endoplasmic Reticulum Ca2+‐ATPase (SERCA)

Sarco‐endoplasmic reticulum Ca²⁺‐ATPase (SERCA), a P‐type ATPase that sustains Ca²⁺ transport and plays a major role in intracellular Ca²⁺ homeostasis, represents a therapeutic target for cancer therapy. Here, we investigated whether ruthenium‐based anticancer drugs, namely KP1019 (indazolium [trans‐tetrachlorobis(1H‐indazole)ruthenate(III)]), NAMI‐A (imidazolium [trans‐tetrachloro(1H‐imidazole)(S‐dimethylsulfoxide)ruthenate(III)]) and RAPTA‐C ([Ru(η⁶‐p‐cymene)dichloro(1,3,5‐triaza‐7‐phosphaadamantane)]), and cisplatin (cis‐diammineplatinum(II) dichloride) might act as inhibitors of SERCA. Charge displacement by SERCA adsorbed on a solid‐supported membrane was measured after ATP or Ca²⁺ concentration jumps. Our results show that KP1019, in contrast to the other metal compounds, is able to interfere with ATP‐dependent translocation of Ca²⁺ ions. An IC₅₀ value of 1 μM was determined for inhibition of calcium translocation by KP1019. Conversely, it appears that KP1019 does not significantly affect Ca²⁺ binding to the ATPase from the cytoplasmic side. Inhibition of SERCA at pharmacologically relevant concentrations may represent a crucial aspect in the overall pharmacological and toxicological profile of KP1019.     

Binding of calcium and magnesium by modified onion skins

The use of modified onion skins for binding of Ca²⁺ and Mg²⁺ from solutions has been investigated. The effect of time of equilibration, temperature, and pH on the sorption of the metal ions have been studied. Batch and column experiments have been performed and the adsorption isotherms have been plotted. The capacities with respect to Ca²⁺ and Mg²⁺ were found to be 4 and 16 meq, respectively, per gram of the substrate when separate column experiments were conducted using 1 L of solution containing 1000 meq of the respective metal ions at pH6. With a solution containing 10 meq each of Ca²⁺ and Mg²⁺ together, however, the substrate seems to exhibit greater preference for Ca²⁺ than Mg²⁺. The sorbed metal ions from the substrate can be leached into solution with a decinormal solution of HCl and the washed bed can be reused. In view of the complex organic nature of the onion skin and its considerable capacity to bind Ca²⁺ and Mg²⁺, the possibility of its use for preventing scale formation in boilers is indicated.