Borate Ester Formation Research Articles

Mechanism and Reaction Energy Landscape for Apiose Cross-Linking by Boric Acid in Rhamnogalacturonan II

Rhamnogalacturonan II (RG-II)-the most complex polysaccharide known in nature-exists as a borate cross-linked dimer in the plant primary cell wall. Boric acid facilitates the formation of this cross-link on the apiosyl residues of RG-II's side chain A. Here, we detail the reaction mechanism for the cross-linking process with ab initio calculations coupled with transition state theory. We determine the formation of the first ester linkage to be the rate-limiting step of the mechanism. Our findings demonstrate that the regio- and stereospecific nature of subsequent steps in the reaction itinerary presents four distinct energetically plausible reaction pathways. This has significant implications for the overall structure of the cross-linked RG-II dimer assembly. Our transition state and reaction path analyses reveal key geometric insights that corroborate previous experimental hypotheses on borate ester formation reactions.

Monosaccharides Dehydration Assisted by Formation of Borate Esters of α-Hydroxyacids in Choline Chloride-Based Low Melting Mixtures.

The synthesis of 5-hydroxymethylfurfural (5-HMF) and 2-furfural (2-F) by hexoses and pentoses dehydration is considered as a promising path to produce materials from renewable resources. Low-transition-temperature mixtures (LTTMs) enable selective (>80%) dehydration of ketoses to furanic derivatives at moderate temperature (<100°C). However, aldoses dehydration generally requires higher temperatures and an isomerization catalyst. Chromium trichloride has been reported as one of the most efficient catalyst but its kinetic inertness could limit its performances below 100°C. Consequently, we investigate herein boric acid catalysis of aldoses dehydration in LTTMs based on choline halides and organic acids at 90°C. The limited activity of boric acid regarding furanic compounds synthesis (e.g., 5% 5-HMF yield and 23% glucose conversion after 1 h at 90°C with maleic acid) can be enhanced through tetrahydroxyborate esters (THBE) formation with α-hydroxyacids (e.g., 19% 5-HMF yield and 61% glucose conversion after 1 h at 90°C). THBE formation is however associated with H3O+ generation favoring the appearance of side products (humins). We demonstrate that boric acid catalysis is not straightforward and that the use of THBE under moderate acidity should be further investigated to limit humins formation and promote furanic derivatives synthesis.

Constructing boronate-bridged core-satellite gold nanoassembly and its application in high sensitive colorimetric detection of benzoyl peroxide residues in food matrices

Here, a new designed core/satellite gold nanoprobe was developed for detecting trace mount of benzoyl peroxide (BPO) based on its deboronation. This gold nanoassembly (the BE-AuNPs12/65) was constructed via borate ester formation between large 4-mercaptophenylboronic acid (MPBA) modified AuNPs (the MPBA-AuNPs65, as cores) and small dopamine modified AuNPs (the DPA-AuNPs12, as satellites). Particularly, upon addition of BPO, it would trigger the deboronation for the BE-AuNPs12/65 probes accompanying with distinct color changes from blue, purple to wine red, which implied the disassembly of the core/satellite nanostructure after the breakage of carbon to boron chemical bond. By measuring the absorbance ratio at 665 nm and 545 nm, quantification of BPO was achieved in the range of 10.0–100.0 nmol/L, which could also be easily observed by naked eyes. The nanoprobe utilized a boronate deprotection mechanism and the LSPR properties of AuNPs to provide high selectivity for detecting BPO over similar ROS/RNS with the limit of detection as low as 7.2 nmol/L. The practical applicability of this assay was verified through successful determining BPO in flour samples, which demonstrated its great potentials in food safety field.

PH-Assisted Gelation of Lactose-Modified Chitosan.

We report on a controlled process allowing for the gelation of a diol-rich chitosan-derivative named CTL (lactose-modified chitosan) in the presence of boric acid as the cross-linker. A two-step approach is described, namely (i) the mixing of CTL and boric acid at pH = 5, a condition where the inorganic component is mildly reactive; (ii) the addition of sodium bicarbonate (NaHCO3) as a trigger, allowing for the gradual and slow pH increase. The goal was to convert gradually the almost inert neutral boric acid into the much more reactive borate anion, the latter promoting the formation of borate esters with CTL diols. Gelling kinetics as well as mechanical behavior at small and large deformations was investigated by rheometry. CTL-boric acid gels behaved essentially as transient networks, hence continuously assembling and dissociating in a highly dynamic fashion. The present gelling mechanism preserves the strain-hardening behavior in the nonlinear region of stress-strain response, corroborating the already suggested potential applications of such gels as mimics of biological soft tissues.

A Solubility-based Separation of Group B Soyasaponins from the Whole Soybean Flour.

This note describes a simple and rapid method to separate specific soyasaponins, including the major and multi-functional bioactive species soyasaponin Bb, from the whole soybean flour. The method is based on the difference in solubility of the soyasaponins and other components in methanol, aqueous borax solution, and 1-octanol. First, the whole soybean flour was mixed with methanol to extract hydrophobic components, and the methanol solution was concentrated. Second, the methanol solution was mixed with a larger volume of aqueous borax solution, and the supernatant was acidified to obtain a precipitate. The precipitate was found to be containing mainly four group B soyasaponins. Two of those are non-conjugated molecules, soyasaponins Bb and Bc, and the other two are 2,3-dihydro-2,5-dihydroxy-6-methyl-4H-pyran-4-one conjugated ones, soyasaponins βg and βa. All of them have two cis-diol groups in their structure. Therefore, although they are fundamentally insoluble in water, they became soluble in the aqueous medium in the form of divalent anionic species by the borate ester formation, and that they were re-precipitated by the hydrolysis. Furthermore, the non-conjugated group B soyasaponins could be separated by washing with 1-octanol to remove highly hydrophobic soyasaponins βg and βa. The solubility-based technique would be useful as pretreatment in the purification of these group B soyasaponins.

Dispersion of Vesicles Composed of Industrially Produced Alkyl (Oligo) Glucoside Using Diol-Boron Complexation.

Alkyl (oligo)glucosides (AOG) are known to be environmentally compatible amphiphiles whose commercial applicability should be broadened. The present paper describes the preparation of molecular assemblies of industrially produced AOG, which is a mixture composed of different length of alkyl chains (C9-C12) with oligoglucoside moiety with a few (1-3) of glucose units. It was also described that regulation of the dispersibility of the molecular assemblies prepared by diol-boron complexation between the sugar moiety on AOG and boric acid in a dispersion medium. The molecular assembly of AOG was successfully formed by mixing AOG and cholesterols (CH). When using a suitable amount of CH (20-40 mol% with respect to AOG), the molecular assembly formed a vesicle structure. The dispersion ability of the resulting vesicle was dependent on both the boric acid concentration and pH of the dispersion medium. The light-scattering and ζ-potential measurements revealed that high concentrations (≥10 mM) of boric acid improved dispersibility the vesicles. In contrast, the vesicle agglomerated at low concentrations of boric acid (1-7.5 mM). In the absence of boric acid in dispersion medium, the vesicles were completely agglomerated. The optimum pH range for vesicle dispersion was found to be from neutral to basic (7.4-10.1). The (11)B NMR study revealed that borate ester formation occurred between boric acid and the diol of the sugar moiety on AOG vesicle. The present data suggest that borate ester formation that occurred on the surface of the vesicle provided negative charge to the vesicles, contributing to their dispersion via repulsive forces.

Controlling the transmembrane transport of nucleosides

Nucleoside analogues are used as drugs. Due to their hydrophilicity, nucleosides are poorly permeable to membranes and transporter proteins are required for efficient uptake. One approach towards improving membrane permeability of nucleosides is to use synthetic transporters. We describe ways to control transport of nucleosides across a liquid membrane. Hexanoylguanosine 1 selectively extracts and transports cytidines across a CHCl3 membrane. Transport catalysed by G 1 was influenced by the nucleoside's sugar, with a selectivity of dC 4 > rC 3 > araC 5. Selective transport could be modulated by adding compounds to the aqueous source phase or to the organic phase. Addition of K+2,6-DNP–8 to CHCl3 containing G 1 switched off transport of rC 3 and dC 4 due to formation of a G-quartet assembly. A lipophilic G 1·C 2 base pair could not transport dC 4, but did catalyse transport of dG 7 across the CHCl3 barrier. We propose that transport occurs because of formation of a base triple G 1·C 2·dG 7. Addition of Na2B4O79 to a source phase containing rC 3, dC 4 and araC 5 shuts down transport of rC 3 by G 1, due to formation of borate esters. These results indicate that one can control the selective transport of nucleosides.

Theoretical study on the mechanism for NH3BH3 reduction of ketones and imines

In spite of a potential hydrogen storage material, ammonia borane (AB) was recently found to be a good hydrogenation reagent. It can reduce certain ketones to alcohols or borate esters, and imines to amines. The mechanisms of these reactions are not fully understood yet, and have been systematically studied using high-level CCSD(T) calculations in this work. We have validated theoretically that the forming of alcohols and amines undergoes concerted double-hydrogen transfer (DHT) mechanism. Furthermore, we predicted that the DHT process is facile for more general ketones and imines. For the borate ester formation, we found a pretty high barrier for the experimentally derived stepwise mechanism. Alternatively, we propose that the reaction starts with the DHT process to form alcohol and NH2BH2, followed by alcoholysis of NH2BH2 to form the first B–O bond. This mechanism is in good agreement with the current experimental facts, and also explains why ketone reduction affords different products at different conditions. For these reaction systems, the performances of M06-2x and MP2 (underestimate the barrier by 5–7 kcal/mol, but with right trends) are better than B3LYP and BLYP methods (underestimate the barrier by 0–5 kcal/mol).

Functionalisation of free amino groups of lysine dendrigraft (DGL) polymers

Lysine dendrigraft polymers (DGL) are promising candidates as carriers for targeted drug/gene delivery and bio-imaging, as such deserving extensive chemical functionalisation studies. We describe here examples of complete grafting of DGL amino groups by various substituents: hydrophobic amino acids (Ala, Val, Leu), dicarboxylic acids (succinic, Asp), guanidyl and saccharides (Gal), by means of straightforward coupling reactions, thus opening to versatile tuning of DGL properties (hydrophobicity, nucleophilicity, electric charge…). DGL functionalisation by lactose (when carried out so as to avoid borate ester formation between sugars) however yields partially grafted DGL; besides, partial grafting can be achieved by tuning the reagent/DGL stoichiometric ratio.

Studies on the effects of arginine residues introduced to peptide ribonucleic acids (PRNA) on the complex stability with RNA

In this study, a series of novel alpha-Peptide ribonucleic acid (alpha-PRNA) oligomers, possessing alternative alpha-PRNA/arginine or serine sequences, were newly designed, synthesized, and evaluated as the third-generation PRNA. As expected, these alpha-PRNAs formed highly stable sequence-specific complexes with the complementary RNAs, for which both the conventional hydrogen-bonding interactions between the complementary nucleobase pairs and the electrostatic interactions between the arginine's guanidinium cation and the RNA's phosphate anion on the backbone are jointly responsible. Moreover, in the cases of alpha-PRNA and single point mismatched DNA mixing systems, appreciable T(m) could not be observed, thus alpha-PRNAs containing Arg were expected to have high nucleobase sequence discrimination abilities. It was demonstrated that the recognition behavior of alpha-PRNA with Arg/Ser backbone with complementary RNA can be controlled externally through the orientation change of pyrimidine nucleobase induced by borate ester formation of the ribose's 2',3'-cis-diol.

Synthesis of Peptide Ribonucleic Acid (PRNA)-DNA Chimera and Interaction with DNA and RNA

Recently, we have demonstrated that effective control of the recognition behavior of peptide ribonucleic acid (PRNA) with complementary DNA is possible through the anti-to-synorientational change of pyrimidine nucleobase induced by borate ester formation. In this study, DNA-PRNA chimera was prepared by the solidphase synthesis. In the DNA-PRNA chimeras, both PRNA and DNA domains work as recognition sites for the complementary DNA/RNAs to form stable complex, while DNA-RNA hybrids formed in the DNA domains of DNA-PRNA chimera should be substrates to the hydrolysis by RNase H and PRNA moieties work as recognition control/switching devices and as inhibitor for the hydrolysis by exonucleases. Interaction of the DNA-PRNA chimera with DNA and RNA has been discussed.

Ternary borate–nucleoside complex stabilization by ribonuclease A demonstrates phosphate mimicry

Phosphate esters play a central role in cellular energetics, biochemical activation, signal transduction and conformational switching. The structural homology of the borate anion with phosphate, combined with its ability to spontaneously esterify hydroxyl groups, suggested that phosphate ester recognition sites on proteins might exhibit significant affinity for nonenzymatically formed borate esters. (11)B NMR studies and activity measurements on ribonuclease A (RNase A) in the presence of borate and several cytidine analogs demonstrate the formation of a stable ternary RNase A.3'-deoxycytidine-2'-borate ternary complex that mimics the complex formed between RNase A and a 2'-cytidine monophosphate (2'-CMP) inhibitor. Alternatively, no slowly exchanging borate resonance is observed for a ternary RNase A, borate, 2'-deoxycytidine mixture, demonstrating the critical importance of the 2'-hydroxyl group for complex formation. Titration of the ternary complex with 2'-CMP shows that it can displace the bound borate ester with a binding constant that is close to the reported inhibition constant of RNase A by 2'-CMP. RNase A binding of a cyclic cytidine-2',3'-borate ester, which is a structural homolog of the cytidine-2',3'-cyclic phosphate substrate, could also be demonstrated. The apparent dissociation constant for the cytidine-2',3'-borate.RNase A complex is 0.8 mM, which compares with a Michaelis constant of 11 mM for cytidine-2',3'-cyclic phosphate at pH 7, indicating considerably stronger binding. However, the value is 1,000-fold larger than the reported dissociation constant of the RNase A complex with uridine-vanadate. These results are consistent with recent reports suggesting that in situ formation of borate esters that mimic the corresponding phosphate esters supports enzyme catalysis.

Boric acid binding studies with diol containing polyethylenimines as determined by 11B NMR spectroscopy

AbstractThree water‐soluble polymers incorporating increasing levels of 2,3‐dihydroxy propyl attached to polyethylenimine (PEI) backbone were synthesized, characterized by NMR, and investigated for their ability to bind boric acid (BA). 11B NMR spectroscopy showed that BA interacted with the polymeric 2,3‐dihydroxy propyls by forming borate monoester and borate diesters in the boron concentration range of 100–1000 ppm and at 0.0775M polymer. Borate monoester species predominated for low functionalization levels (33% of the PEI amines functionalized), whereas borate diester species dominated for the higher functionalized polymers (66–100% of the PEI amines functionalized). All three polymers showed that 100% of the BA was bound as a mixture of borate mono‐ and diesters at 100‐ppm boron. The overall best performer based on total borate ester formation was the 2/3‐PEI, with a binding Kd of 631 at 200 ppm boron. Borate ion concentration was measured from the 11B NMR chemical shift of the BA/borate peak and it decreased as 1/3‐PEI &gt; 2/3‐PEI &gt; 3/3‐PEI. Variable temperature 11B NMR showed drastic reduction of borate ester species at 65°C. Thus, PEI polymers, as the ones investigated in this work, are reasonable candidates for the selective recovery and recycle of BA. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 4411–4418, 2006

DNA Recognition and Recognition Control of α-Peptide Ribonucleic Acids by External Factors

Peptide ribonucleic acid (PRNA) is one of artificial nucleic acids, which enables us to control its nucleobase orientation and recognition behavior by external factors through the synergetic effects of reversible borate ester formation and intramolecular hydrogen bond formation. In this study, a series of novel alpha-PRNA oligomers, possessing alternative alpha-PRNA/basic amino acid sequences, were newly designed, synthesized, and evaluated as the second-generation PRNA. As expected, these alpha-PRNAs indeed formed highly stable sequence-specific complexes with the complementary DNAs, for which both the conventional hydrogen-bonding interactions between the complementary nucleobase pairs and the electrostatic interactions between the basic amino acid's ammonium cation and the DNA's phosphate anion on the backbone are jointly responsible.

Boric acid recovery using polymer filtration: Studies with alkyl monool, diol, and triol containing polyethylenimines

Three water-soluble polymers containing linear alkyl monool, 1,2-diol, and 1,2,3-triol groups, mostly on the primary amines of polyethylenimine, were synthesized, characterized, and tested for their ability to recover boric acid. The boron-binding capacities of these polymers and the backbone polyethylenimine were determined by titration, ultrafiltration, and inductively coupled plasma/atomic emission spectroscopy analysis. At low boron concentrations, the 1,2,3-triol polymer performed better than the 1,2-diol, whereas at high boron concentrations, the 1,2-diol outperformed the 1,2,3-triol. 11B-NMR spectroscopy and retention studies with various salt concentrations indicated that boron interacted with these two polymers by means of ion pairing with the protonated amines and by borate ester formation. For the monool and the polyethylenimine backbone, the mechanism for boron binding was ion pairing only. These polymers are under consideration for the selective recovery and recycling of enriched boric acid used in the primary coolant loop of pressurized water nuclear reactors. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1590–1604, 2005

Enhanced Anti-Fungal Activity of the Organo-Soluble Borate Ester, Tetra-n-butylammonium Bis(ortho-hydroxymethylphenolato)borate

The borate ester, tetra-n-butylammonium bis(ortho-hydroxymethylphenolato)borate, NBu4[B(o-hmp)2], was synthesized and characterized by NMR spectroscopy (1H, 13C, 11B), ES-MS, and X-ray crystallography. The anti-fungal activity of this compound, as well as its sodium salt, the parent phenol, tetra-n-butylammonium bromide, and boric acid were evaluated against two wood decay fungi. The tetraalkylammonium borate NBu4[B(o-hmp)2] shows the highest activity out of the compounds examined. This finding suggests that the formation of lipophilic borate esters is a promising approach for the development of leach-resistant, borate-based wood preservatives.

RNA-sensitive N-isopropylacrylamide/vinylphenylboronic acid random copolymer

Random copolymers of N-isopropylacrylamide (NIPA) and 4-vinylphenylboronic acid (VPBA) were obtained by solution polymerization using 2,2′-azobisizobutyronitrile as the initiator in ethanol at 65 °C. NIPA-co-VPBA copolymer exhibited both temperature- and pH-sensitivity. Thermally reversible phase transitions were observed both in the acidic and alkaline pH region for the copolymers produced with different VPBA/NIPA feed ratios. The pH dependency of the lower critical solution temperature (LCST) was stronger for the copolymers produced with higher VPBA feed concentrations. RNA was selected as a model biomolecule having vicinal-diol and amino groups that were potentially reactive with the boronic acid groups of NIPA-co-VPBA copolymer. The effect of RNA concentration on the LCST of NIPA-co-VPBA copolymer was investigated in aqueous media at different pHs. Although no significant effect was observed at pH 4, 7 or 10.5, the LCST decreased linearly with increasing RNA concentration at a pH approximately equal to the pKa of boronic acid. This behavior was explained by considering the binding of RNA onto the copolymer chains to occur via two types of complex formation. For the formation of these complexes, the amino and vicinal-diol groups of RNA should react with the boronic acid groups of the copolymer in the tetrahedral anionic form. The results indicated that NIPA-co-VPBA copolymer could be utilized as a new reagent for the determination of RNA concentration in aqueous media. The proposed method was valid for the RNA concentration range of 0–4 g · mL−1. The schematical representation of the possible interactions between NIPA-co-VPBA copolymer and RNA. (A) A typical structure of single-stranded RNA. (B) Tetrahedral anionic form of boronic acid groups. (C) The interaction between the amino groups of the unpaired bases of RNA and the boronic acid groups of the copolymer. (D) Cyclic borate ester formation by the interaction between vicinal diol groups located at the 3′-end of RNA and boronic acid groups of the copolymer.

Peptide Ribonucleic Acids (PRNA). 2. A Novel Strategy for Active Control of DNA Recognition through Borate Ester Formation

The effect of adding borax and boric acids on the nucleobase orientation and recognition behavior of novel mono- and oligomeric peptide ribonucleic acids (PRNAs) has been investigated. The base orientation of 5‘-amino-5‘-deoxyuridine and 5‘-amino-5‘-deoxycytidine was shown by CD and NOE difference spectral studies to switch from anti to syn in borate buffer or upon addition of borax. The origin of this phenomenon is elucidated to be the cooperative effect of the cyclic borate esterification of the sugar's cis-2‘,3‘-diol and the hydrogen-bonding interaction between the sugar's 5‘-amino proton and the base's 2-carbonyl oxygen. Because this new strategy for switching the base orientation through the addition of borate is potentially applicable to the recognition control of nucleic acids if the sugar's 5‘-proton and cis-2‘,3‘-diol remain unmodified, we synthesized a series of PRNAs, in which the 5‘-amino-5‘-deoxypyrimidine ribonucleoside moiety was appended to a mono- or oligo(γ-l-glutamic acid) backbone thro...

Recognition control of the nucleic acid model through conformational switching of nucleobase induced by borate ester formation of cis-2',3'-diol.

A novel nucleic acid model that possessed 5'-amino-5'-deoxyuridine at alpha- and gamma-position of L-glutamic acid through amide linkage using 5'-amino group was synthesized and the conformation and the hybridization properties were studied. The complex of alpha-PRNA with complementary DNA/RNA was more stable than the corresponding natural duplex in the absence of borate. Its recognition ability was however lost when borax was added to the solution.

Thermodynamics of borate ester formation by three readily grafted carbohydrates

Borate ester association constants for d- glycero- d- gulo-heptonamide ( 1), N-methyl- d-glucamine ( 2), and disorbitylamine ( 3) were derived from 11B NMR observations. Significant electrostatic effects were observed in the formation of borate esters with the ammonium derivative ( 2). 11B NMR studies of 3 were consistent with the simultaneous formation of intramolecular tetradentate borate diesters and oligomeric chains of 3 connected by intermolecular borate diesters. The process enthalpies for formation of borate monoesters and diesters with 1 are Δ H 1°=−37.2±4.9 kJ/mol and Δ H 2°=−19.6±2.5 kJ/mol, respectively. The corresponding process entropies are Δ S 1°=−51±17 J/mol/K and Δ S 2°=−28±8 J/mol/K.