Boric Acid Complexes Research Articles

Chiral separation by nonaqueous capillary electrophoresis using l-sorbose–boric acid complexes as chiral ion-pair selectors

A chiral nonaqueous capillary electrophoresis (NACE) method using l-sorbose–boric acid complexes as the chiral ion-pair selectors was developed for enantioseparation of nineteen chiral analytes.

Enantioseparation of fourteen amino alcohols by nonaqueous capillary electrophoresis using lactobionic acid/d-(+)-xylose–boric acid complexes as chiral selectors

An interesting study on chiral nonaqueous capillary electrophoresis (NACE) was developed in this paper.

폴리올과 붕산의 착화합물 형성원리를 이용한 수용액 중의 보론 제거에 관한 라만 분광학 연구

해수 중에 보론은 주로 붕산(boric acid, <TEX>$B(OH)_3$</TEX>)으로 존재하며 그 크기가 역삼투압(RO, reverse osmosis) 공정의 막의 공칭공경(nominal pore size)만큼 작아 이를 제거하기가 쉽지 않다. 이에 본 연구에서는 수용액 중 보론 화합물의 크기를 역삼투압 공정에서 제거가 용이하도록 키우기 위한 방법으로 <TEX>$B(OH)_3$</TEX>가 폴리올과 착화합물을 형성하는 원리를 이용하고자하며 착화합물의 형성 여부를 라만 분광학 연구를 통해 확인하고자 하였다. 먼저 <TEX>$B(OH)_3$</TEX>와 붕산염(borate ion, <TEX>${B(OH)_4}^-$</TEX>) 이온의 대칭(symmetric) B-O 신축운동 모드(stretching vibrational mode)의 진동수를 확인하기 위하여 pH를 증가시키며 라만 스펙트럼을 측정한 결과 <TEX>$877cm^{-1}$</TEX>에서 <TEX>$730cm^{-1}$</TEX>으로 피크의 이동이 관찰되었다. 이는 수용액 중의 <TEX>$B(OH)_3$</TEX>가 pH가 증가하면서 <TEX>${B(OH)_4}^-$</TEX>로 전환됨을 나타내고 이러한 피크의 이동은 진동수 계산(frequency calculation)에 의한 예측과 정확하게 일치하였다. 반면 <TEX>$B(OH)_3$</TEX> 수용액에 폴리올을 첨가한 경우 pH가 증가하여도 <TEX>${B(OH)_4}^-$</TEX>의 특성 진동수인 <TEX>$730cm^{-1}$</TEX>가 나타나지 않았으며 이는 <TEX>$B(OH)_3$</TEX>가 <TEX>${B(OH)_4}^-$</TEX>로 전환되는 것이 아니라 폴리올과 착화합물을 형성함을 간접적으로 보여주고 있다. Boron is difficult to be removed from seawater by simple RO (reverse osmosis) membrane process, because the size of boric acid (<TEX>$B(OH)_3$</TEX>), the major form of aqueous boron, is as small as the nominal pore size of RO membrane. Thus, the complexation of boric acid with polyols was suggested as an alternative way to increase the size of aqueous boron compounds and the complexation behavior was investigated with Raman spectroscopy. As a reference, the Raman peak for symmetric B-O stretching vibrational mode both in boric acid and borate ion (<TEX>${B(OH)_4}^-$</TEX>) was selected. A Raman peak shift (<TEX>$877cm^{-1}{\rightarrow}730cm^{-1}$</TEX>) was observed to confirm that boric acid in water is converted to borate ion as the pH increases, which is also correctly predicted by frequency calculation. Meanwhile, the Raman peak of borate ion (<TEX>$730cm^{-1}$</TEX>) did not appear as the pH increased when polyols were applied into aqueous solution of boric acid, suggesting that the boric acid forms complexing compounds by combining with polyols.

Simple boron removal from seawater by using polyols as complexing agents: A computational mechanistic study

The complexation of boric acid (B(OH)3), the primary form of aqueous boron at moderate pH, with polyols is proposed and mechanistically studied as an efficient way to improve membrane processes such as reverse osmosis (RO) for removing boron in seawater by increasing the size of aqueous boron compounds. Computational chemistry based on the density functional theory (DFT) was used to manifest the reaction pathways of the complexation of B(OH)3 with various polyols such as glycerol, xylitol, and mannitol. The reaction energies were calculated as −80.6, −98.1, and −87.2 kcal/mol for glycerol, xylitol, and mannitol, respectively, indicating that xylitol is the most thermodynamically favorable for the complexation with B(OH)3. Moreover, the 1: 2 molar ratio of B(OH)3 to polyol was found to be more favorable than the ratio of 1: 1 for the complexation. Meanwhile, latest lab-scale actual RO experiments successfully supported our computational prediction that 2 moles of xylitol are the most effective as the complexing agent for 1 mole of B(OH)3 in aqueous solution.

Enantioseparations of 11 Amino Alcohols Using Di-n-amyl l-Tartrate–Boric Acid Complex as Chiral Mobile Phase Additive by RP-HPLC

An effective method for the enantioseparations of 11 amino alcohols was developed by reversed-phase high-performance liquid chromatography (RP-HPLC) using a self-prepared di-n-amyl l-tartrate–boric acid complex as the chiral mobile phase additive. Mass spectrometry (MS) directly confirmed that the negatively charged di-n-amyl l-tartrate–boric acid complex counter ion with a complex ratio of 2:1 was the real chiral selector in the RP-HPLC chiral separation system. In order to achieve good enantioseparations, the effects of di-n-amyl l-tartrate and boric acid concentrations, the type, concentration and pH of buffers, methanol content as well as analyte molecular structures were extensively investigated. Under the optimized conditions, baseline enantioseparations were achieved for 9 amino alcohols including cycloclenbuterol, clenbuterol, bambuterol, tulobuterol, terbutaline, metoprolol, esmolol, bisoprolol, and propranolol; partial enantioseparations were obtained for sotalol and atenolol. The effect of the molecular structure of amino alcohols on enantioseparation was discussed in terms of molecular interactions. The ion-pair interactions between the negatively charged chiral selector and the basic enantiomers were thought to be the key factor giving the complex chiral selector a superior chiral recognition capability.

Boric acid: a simple molecule of physiologic, therapeutic and prebiotic significance

Abstract Boric acid, H3BO3, is a weak acid and at physiological pH is in the form of an uncharged small molecule. Behaving as a Lewis acid, it forms complexes with amino- and hydroxy acids, carbohydrates, nucleotides and vitamins through electron donor-acceptor interactions. These interactions are believed to be beneficial for human health. Synthetic bis-chelate complexes of boric acid with organic biomolecules are therefore considered for nutritional and/or pharmaceutical applications. The use of boric acid for BNCT has gained attention due to the short biological half-life, solubility, plasma circulation and the non-selective soft tissue accumulation properties of this simple molecule. Complexation of boric acid with sugars is of particular importance in understanding the role of boron as a carrier for nucleotides and carbohydrates. A potential and catalytic role of boric acid in peptide and nucleic acid synthesis and in the stabilization of sugar molecules by acting as a complexing agent have been demonstrated. Its possible role as a phosphorylation chaperone in a prebiotic world has been recently suggested. This contribution reviews the highlights in the physiologic, therapeutic and prebiotic significance of boric acid in the last decade.

Boric acid complexes with thiamine (vitamin B1) and pyridoxine (vitamin B6)

Complexation of boric acid with vitamins B1 (thiamine) and B6 (pyridoxine) was investigated. The products were isolated from aqueous solutions and characterized by elemental analysis, FT-IR, 13C MAS NMR and thermal methods. The results of chemical and spectral analyses suggested that the reaction between thiaminechloride hydrochloride and H3BO3 in water yielded a salt-like productwith no direct bonding of the thiamine cation to boron while mono- and bis-chelate complexes were obtained with pyridoxine. The bis-chelate pyridoxine complex containing two equivalent pyridoxine ligands was obtained in crystal form. X-ray crystallography verified that complexation with boric acid occurs through the deprotonated phenolic and deprotonated adjacent alcoholic oxygen atoms forming six membered chelate rings. The negative charge on tetrahedral boron is balanced by the sodium ion. The crystal structure contains discernible tunnels parallel to the [010] direction constructed by hydrogen bonded complex molecules.

Surface Functionalization of Microporous Polypropylene Membrane with Polyols for Removal of Boron Acid from Aqueous Solution

Affinity membranes are fabricated for boric acid removal by the surface functionalization of microporous polypropylene membrane (MPPM) with lactose-based polyols. The affinity is based on specific complexation between boric acid and saccharide polyols. A photoinduced grafting-chemical reaction sequence was used to prepare these affinity membranes. Poly(2-aminoethyl methacrylate hydrochloride) [poly(AEMA)] was grafted on the surfaces of MPPM by UV-induced graft polymerization. Grafting in the membrane pores was visualized by dying the cross-section of poly(AEMA)-grafted MPPM with fluorescein disodium and imaging with confocal laser scanning microscopy. It is concluded that lactose ligands can be covalently immobilized on the external surface and in the pores by the subsequent coupling of poly(AEMA) with lactobionic acid (LA). Physical and chemical properties of the affinity membranes were characterized by field emission scanning electron microscopy and Fourier Transform Infrared/Attenuated Total Refraction spectroscopy (FT-IR/ATR). 3-Aminophenyl boric acid (3-APBA) was removed from aqueous solution by a single piece of lactose-functionalized MPPM in a dynamic filtration system. The results show that the 3-APBA removal reaches an optimal efficiency (39.5%) under the alkaline condition (pH 9.1), which can be improved by increasing the immobilization density of LA. Regeneration of these affinity membranes can be easily realized through acid-base washing because the complexation of boric acid and saccharide polyol is reversible.

Enantioseparations of three amino alcohols using a di-n-butyl-l-tartrate–boric acid complex as the mobile phase additive by reversed-phase high performance liquid chromatography

Three amino alcohols, propranolol, clenbuterol and cycloclenbuterol, were enantioseparated on an achiral column by reversed-phase high performance liquid chromatography (RP-HPLC). A chiral chromatographic system was established based on C18-bonded silica gel as the support and a self-prepared di-n-butyl-L-tartrate–boric acid complex as the chiral mobile phase additive. In order to obtain better enantioseparations, the influences of di-n-butyl-L-tartrate and boric acid concentrations, the type, concentration and pH of the buffer, as well as methanol content were extensively investigated. It was found that the mobile phase composition played an important role in improving the enantioseparations and all of the enantiomers could be baseline resolved under the optimized experimental conditions. Three pairs of enantiomers, which could not be separated with only di-n-butyl-L-tartrate, obtained good chiral separations using the complex chiral selector. The primary driving forces responsible for the chiral recognition were assumed to be the ion-pair interaction between the enantiomers and the chiral selector; and the interactions between the ion-pairs above and the stationary phase also played important roles for the chiral separations in RP-HPLC.

Selective determination of trace boron based on resonance Rayleigh scattering energy transfer from nanogold aggregate to complex of boric acid–azomethine-H

The nanogold particles were aggregated to stable nanogold aggregates as nanoprobes that exhibited a resonance Rayleigh scattering (RRS) peak at 400 nm in the pH 5.6 NH4Ac–HAc buffer solutions and in the presence of azomethine-H (AMH). Upon addition of boric acid, it reacts with AMH to form AMH–boric acid (AMH–B) complexes. When the complexes (as receptors) are close to the nanogold aggregates (as donors), the RRS-energy transfer (ET) takes place, which results in the RRS signal quenching at 400 nm. The quenching intensity responds linearly with the concentration of boron over 5–500 ng mL−1 B.

Removal of boric acid, monoborate and boron complexes with polyols by reverse osmosis membranes

The effects of reverse osmosis (RO) membrane type on the rejection efficiency of boric acid, monoborate and boron complexes with d-mannitol, sodium d-gluconate and N-methyl d-glucamine was revealed. The membranes examined included: XLE, TW-30, BW-30 and SW-30, supplied by DOW™ FILMTEC™. The mass transport coefficients: permeability and reflection coefficient were determined for each species in boric acid–polyol aqueous system. The influence of the membrane type upon these coefficients was evaluated and quantitative, comparative analysis of the efficacy of boron rejection at varying permeate flux, the feedwater boron content, the alcohol/boron molar ratio and the pH was conducted. It was found that boron rejection in the above systems was determined by the extent of boric acid transport, even when boric acid constituted only a minor component of the feedwater. At high permeate flux the effectiveness in boric acid rejection decreased in the following descending membrane order: SW-30>BW-30>TW-30>XLE. The results presented here enable the selection of the best membrane, the most suitable operating conditions for boron separation by RO in the presence or absence of polyols, and for quantitative prediction of the efficiency of boron removal with various RO membranes.

Introducing CEC′ mechanism: Electrochemical oxidation of 4-methylesculetin–boric acid complex in the presence of glutathione

The electrochemical behavior of 4-methylesculetin–boric acid complex (4MS–B) in aqueous solution has been studied at a glassy carbon electrode using cyclic voltammetry. The results indicate that 4MS–B is involved in the CE mechanism. In the next step the electrochemical oxidation of 4-methylesculetin (4MS) was studied in the presence of glutathione using cyclic voltammetry. The results indicate a complicated catalytic system. Finally the electrochemical behavior of 4MS–B in the presence of glutathione has been studied using cyclic voltammetry as a diagnostic technique. The results indicate that this system is involved in the CEC′ process as a new electrochemical mechanism. In this work, the impact of empirical parameters on the shape of the voltammograms is examined based on a CEC′ mechanism.

An ion-pair principle for enantioseparations of basic analytes by nonaqueous capillary electrophoresis using the di-n-butyl l-tartrate–boric acid complex as chiral selector

A chiral recognition mechanism of ion-pair principle has been proposed in this study. It rationalized the enantioseparations of some basic analytes using the complex of di-n-butyl l-tartrate and boric acid as the chiral selector in methanolic background electrolytes (BGEs) by nonaqueous capillary electrophoresis (NACE). An approach of mass spectrometer (MS) directly confirmed that triethylamine promoted the formation of negatively charged di-n-butyl l-tartrate–boric acid complex chiral counter ion with a complex ratio of 2:1. And the negatively charged counter ion was the real chiral selector in the ion-pair principle enantioseparations. It was assumed that triethylamine should play its role by adjusting the apparent acidity (pH*) of the running buffer to a higher value. Consequently, the effects of various basic electrolytes including inorganic and organic ones on the enantioseparations in NACE were investigated. The results showed that most of the basic electrolytes tested were favorable for the enantioseparations of basic analytes using di-n-butyl l-tartrate–boric acid complex as the chiral ion-pair selector.

Transport model for boric acid, monoborate and borate complexes across thin-film composite reverse osmosis membrane

The transport mechanism of boric acid, monoborate and borate complexes with polyols across thin-film composite reverse osmosis (RO) membrane was investigated. The polyols applied were d-mannitol, sodium d-gluconate and N-methyl d-glucamine. A mathematical model for boric acid dissociation and complex formation equilibrium was combined with the Keedem–Katchalsky or Spiegler–Keedem model for mass transport in RO membranes. The mass transport coefficients: permeabilities and reflection coefficients for individual species were estimated. The proposed model was shown to predict boron flux with sufficient accuracy. The estimated mass transport coefficients were found to strongly depend on the species type. The highest permeability and the lowest reflection coefficient were observed for boric acid. Borate complexes with polyols were characterised by the lowest permeabilities and the highest reflection coefficients. The results of numerical calculations with the proposed model allowed for the assessment of the effect of process parameters on the effectiveness of boron rejection. The effect of retentate pH on boron rejection and boron flux is discussed. It is shown that complex formation reaction decreases the retentate pH required to achieve a given rejection of boron. The effects of permeate flux, retentate boron content and alcohol/boron molar ratio are also presented.

Study on Novel Structure of Potassium Borate Hydrate: K(H&lt;sub&gt;4&lt;/sub&gt;B&lt;sub&gt;5&lt;/sub&gt;O&lt;sub&gt;10&lt;/sub&gt;)·2(H&lt;sub&gt;2&lt;/sub&gt;O)

Numerous stable complexes of boric acid with polyhydroxy compounds, including tartaric, salicylic, citric, malic, and other acids, are known. The structure of some compounds contains polyanion. In this paper, a novel potassium borate hydrate [K(H4B5O10) •2(H2O)] has been synthesized from a solution reaction and the crystal structure has been determined by means of single-crystal X-ray diffraction. Orthorhombic, Aba2. a = 11.0781(14) Å b = 11.1780(15) Å c = 9.0508(11) Å α=β=γ=90°. V= 1120.8(2) Å3. Z=4. Rgt = 0.0244, wRref = 0.0623. T= 298 K. The crystal packing is stabilized by O-H...O hydrogen bonds interaction and three dimensional framwork structure is formed. The work is originality and has a new crystallographic structure shape.

In situ synthesis of twelve dialkyltartrate–boric acid complexes and two polyols–boric acid complexes and their applications as chiral ion-pair selectors in nonaqueous capillary electrophoresis

In this paper, twelve dialkyltartrate–boric acid complexes and two polyols–boric acid complexes were in situ synthesized by the reaction of different dialkyltartrates or polyols with boric acid in methanol containing triethylamine. All of the twelve dialkyltartrate–boric acid complexes were found to have relatively good chiral separation performance in nonaqueous capillary electrophoresis (NACE). Their chiral recognition effects in terms of both enantioselectivity (α) and resolution (Rs) were similar when the number of carbon atoms was below six in the alkyl group of alcohol moiety. The dialkyltartrates containing alkyl groups of different structures but the same number of carbon atoms, i.e. one of straight chain and one of branched chain, also provided similar chiral recognition effects. Furthermore, it was demonstrated for the first time that two methanol insoluble polyols, d-mannitol and d-sorbitol, could react with boric acid to prepare chiral ion-pair selectors using methanol as the solvent medium.

Is boron a prebiotic element? A mini-review of the essentiality of boron for the appearance of life on earth.

Boron is probably a prebiotic element with special importance in the so-called "sugars world". Boron is not present on Earth in its elemental form. It is found only in compounds, e.g., borax, boric acid, kernite, ulexite, colemanite and other borates. Volcanic spring waters sometimes contain boron-based acids (e.g., boric, metaboric, tetraboric and pyroboric acid). Borates influence the formation of ribofuranose from formaldehyde that feeds the "prebiotic metabolic cycle". The importance of boron in the living world is strongly related to its implications in the prebiotic origins of genetic material; consequently, we believe that throughout the evolution of life, the primary role of boron has been to provide thermal and chemical stability in hostile environments. The complexation of boric acid and borates with organic cis-diols remains the most probable chemical mechanism for the role of this element in the evolution of the living world. Because borates can stabilize ribose and form borate ester nucleotides, boron may have provided an essential contribution to the "pre-RNA world".

Boron removal and its concentration by reverse osmosis in the presence of polyol compounds

Reverse osmosis membrane (TW-30 and XLE, Filmtec™) boron removal and its concentration in the presence of chelating polyols containing a 1,2-diol group were examined in the pH range from 7 to 11 and pressure range from 5 to 20atm at an initial boron feedwater concentration of 70mg/L. The polyols applied were: d-mannitol, N-methyl d-glucamine and sodium d-gluconate, and all of them were subjected to create a dichelate ionic complexes at high pH. It was found that apparent boron rejection as well as permeate flux are strongly dependent on the above conditions. The effect of the amount of the alcohol and feedwater pH on the boron rejection was explained based on the alcohol–boric acid complex formation equilibrium reaction. The apparent boron rejection in the boric acid–alcohol mixtures was found to decrease in the following order: boric acid–N-methyl-d-glucamine mixture>boric acid–d-mannitol mixture≈boric acid–sodium d-gluconate>plain boric acid solution. On the other hand, the permeate flux values in the mixtures decreased in an alternate order: plain boric acid>boric acid–d-mannitol>boric acid–N-methyl-d-glucamine>boric acid–sodium d-gluconate. Also, the effect of other operating conditions, such as: excess pressure, feedwater recovery and concentration factor on the effectiveness of boron removal was evaluated. Finally, the possibility of reducing the boron content to less than 1mg/L even at a high feedwater recovery level of 90% has been demonstrated.

Mixed-ligand complexes of boric acid with organic biomolecules

AbstractBoric acid forms meta-stable complexes with biomolecules like amino and hydroxy acids and stable complexes with the diol group containing carbohydrates, vitamins, and nucleotides, yielding mono-chelate (1: 1 complex) or bis-chelate (1: 2 complex) structures with negatively charged tetrahedral borate anions. Here we report water-soluble, bio-available mixed-ligand boron adducts for potential nutritional and/or pharmaceutical applications. The complexes were prepared by complete esterification of boric acid with a number of acyclic- and cyclic hydroxy-functionalized biomolecules employing sodium as the counter ion. Structural and thermal properties of the complexes were investigated using chemical analysis, 11B NMR, FTIR, and TGA-DTA techniques. Complexes containing salicylic acid as one of the ligands displayed higher thermal and hydrolytic stabilities.

CEC mechanism in electrochemical oxidation of nitrocatechol–boric acid complexes

The electrochemical behavior of nitrocatechols–boric acid complexes in aqueous solution has been studied using cyclic voltammetry. The results indicate that nitrocatechol–boric acid complex derivatives are involved in the CEC mechanism. In this work, the impact of empirical parameters on the shape of the voltammograms is examined based on a CEC mechanism. In addition, homogeneous rate constants of both the preceding and the following reactions were estimated by comparing the experimental cyclic voltammograms with the digitally simulated results. The calculated dissociation constants for the complexes (Kd) and for ring cleavage of nitroquinone (kf2) were found to vary in the following order: 4-nitrocatechol>3-methylnitrocatechol>3-metoxynitrocatechol.