Anion Recognition by Benzoxaborole.

The binding types (H-bonding or coordinate) and stability constants for complexes of 11 mono- and di-anions with benzoxaborole (1) were determined by 1H and 11B NMR titrations in DMSO or MeCN. Compared to phenylboronic acid (PBA), 1 is a stronger Lewis acid and a poorer H-bond donor with only one B-OH group, which is expected therefore to recognize anions mostly through the coordinate bonding. This is the case however only with F-, HPO42-, and PhPO32- anions, which are coordinately bonded to 1, and partially with SO42-, which forms only the H-bonded complex with PBA, but both H-bonded and coordinate complexes with 1. The majority of tested anions (AcO-, PhPO3H-, (PhO)2PO2-, Cl-, and Br-) form H-bonded complexes with both 1 and PBA, whereas H2PO4- changes the binding mode from coordinate for PBA to H-bonded for 1. The preferable binding type for each anion is confirmed by calculations of DFT-optimized structures of the anion complexes of 1. The preferable binding type can be rationalized considering the effects of the steric hindrance, more significant for the coordinate bonding, and of increased anion basicity, which is favorable for both binding types, but enhances the strength of coordinate bonding more significantly than the strength of H-bonding.

Substituent effects and pH profiles for stability constants of arylboronic acid diol esters.

Stability constants of boronic acid diol esters in aqueous solution have been determined potentiometrically for a series of meta-, para-substituted phenylboronic acids and diols of variable acidity. The constants ß(11-1) for reactions between neutral forms of reactants producing the anionic ester plus proton follow the Hammett equation with ρ depending on pKa of diol and varying from 2.0 for glucose to 1.29 for 4-nitrocatechol. Observed stability constants (K(obs)) measured by UV-vis and fluorometric titrations at variable pH for esters of 4,5-dihydroxy-1,3-benzenedisulfonate (Tiron) generally agree with those expected on the basis of ß(11-1) values, but the direct fitting of K(obs) vs pH profiles gives shifted pKa values both for boronic acids and diol as a result of significant interdependence of fitting parameters. The subsituent effects on absorption and fluorescence spectra of Tiron arylboronate esters are characterized. The K(obs) for Tiron determined by (11)B NMR titrations are approximately 1 order of magnitude smaller than those determined by UV-vis titrations under identical conditions. A general equation, which makes possible an estimate of ß(11-1) for any pair of boronic acid and diol from their pKa values, is proposed on the basis of established Brönsted-type correlation of Hammett parameters for ß(11-1) with acidity of diols. The equation allows one to calculate stability constants expected only on basis of acid-base properties of the components, thus permitting more strict evaluation of contributions of additional factors such as steric or charge effects to the ester stability.

Water-soluble fluorescent chemosensor for sorbitol based on a dicationic diboronic receptor. Crystal structure and spectroscopic studies.

Selective recognition of saccharides by phenylboronic dyes capable of functioning in aqueous conditions is a central topic of modern supramolecular chemistry that impacts analytical sciences and biological chemistry. Herein, a new dicationic diboronic acid structure 11 was synthesized, structurally described by single-crystal X-ray diffraction, and studied in-depth as fluorescent receptor for six saccharides in pure water at pH = 7.4. This dicationic receptor 11 has been designed particularly to respond to sorbitol and involves two convergent and strongly acidified phenyl boronic acids, with a pKa of 6.6, that operate as binding sites. The addition of sorbitol in the micromolar concentration range to receptor 11 induces strong fluorescence change, but in the presence of fructose, mannitol, glucose, lactose and sucrose, only moderate optical changes are observed. This change in emission is attributed to a static complexation photoinduced electron transfer mechanism as evidenced by lifetime experiments and different spectroscopic tools. The diboronic receptor has a high affinity/selectivity to sorbitol (K = 31 800 M-1) over other saccharides including common interfering species such as mannitol and fructose. The results based on 1H, 11B NMR spectroscopy, high-resolution mass spectrometry and density functional theory calculations, support that sorbitol is efficiently bound to 11 in a 1 : 1 mode involving a chelating diboronate-sorbitol complexation. Since the experimental B⋯B distance (5.3 Å) in 11 is very close to the calculated distance from the DFT-optimized complex with sorbitol, the efficient binding is attributed to strong acidification and preorganization of boronic acids. These results highlight the usefulness of a new diboronic acid receptor with a strong ability for fluorescent recognition of sorbitol in physiological conditions.

Selective fluorometric detection of pyrophosphate by 3-hydroxyflavone-diphenyltin(IV) complex in aqueous micellar medium.

3-Hydroxyflavonate-diphenyltin(IV) chloride (Ph(2)Sn(Ofl)Cl) was prepared and characterized structurally by single crystal X-ray diffraction. The compound possesses high fluorescence (λ(max) 464 nm with excitation at 400 nm) in neutral aqueous solutions containing 5 mM cetyltrimethylammonium chloride and does not lose the flavonol ligand even at high dilution. The fluorescence is selectively quenched by pyrophosphate (PPi) (linear range 0-5 µM, detection limit 0.1 µM). Determination of 1 µM PPi tolerates the presence of 100-fold excess of AMP, ADP, inorganic phosphate and acetate, and 10-fold excess of ATP. From concentration profiles of the quenching effect it is concluded that quenching occurs due to formation of the complex (Ph(2)Sn(Ofl))(3)PPi, which is transformed at higher PPi concentrations to a simple 1 : 1 complex Ph(2)Sn(Ofl)(PPi). No flavonol ligand exchange with PPi is observed. A series of related organotin(IV) complexes were prepared in situ by reacting 3-hydroxyflavone with Me(2)SnCl(2), PhSnCl(3) and n-BuSnCl(3). The derivatives of Me(2)SnCl(2) and PhSnCl(3) behave similarly to Ph(2)Sn(Ofl)Cl but with lower sensitivity to PPi. However, the n-butyltin(IV) derivative tentatively of the composition n-BuSn(Ofl)(2) undergoes fluorescence enhancement rather than quenching in the presence of PPi in the µM concentration range. The results of this study demonstrate that the use of metal flavonol complexes as optical sensors for anions is a promising approach.

Spectrophotometric, fluorimetric and electrochemical selective pyrophosphate/ATP sensing based on the dimethyltin(IV)-tiron system.

Sensing of pyrophosphate anion (PPi) in the presence of nucleotide triphosphates allows the real time monitoring of the polymerase chain reaction. To get a deeper understanding of the factors involved in PPi/nucleotide triphosphate discrimination, a detailed study on the performance of a dimethyltin (IV)-catecholate complex capable of both separate fluorimetric or electrochemical detection of PPi in the presence of adenosine triphosphate (ATP) has been undertaken. Dimethyltin (IV) tightly binds PPi or ATP, and forms a stable 1:1 complex with tiron (4,5-dihydroxy-1,3-benzenedisulfonic acid) in water. The complexation equilibria with all components are characterized quantitatively by potentiometric and spectroscopic titrations. Pyrophosphate anion can be detected owing to its ability to release free tiron from the complex by measuring either a fluorimetric or an electrochemical signal. On the contrary, ATP does not displace tiron but causes an interference with PPi in the fluorimetric detection method due to the formation of a ternary Me2Sn(IV)-tiron-ATP complex with optical properties intermediate between those of free and bound tiron. In the electrochemical (square wave voltammetry) method, the ternary ATP complex shows a separate peak which does not coincide with the peaks of neither free nor bound tiron, thus making possible the simultaneous detection of ATP in addition to PPi.

Selective fluorometric detection of pyrophosphate by interaction with alizarin red S-dimethyltin(IV) complex.

Highly selective detection of pyrophosphate in the presence of inorganic phosphate, halides, acetate, ATP, other nucleotides and phosphate esters in neutral aqueous solutions is achieved by using fluorescent dimethyltin(iv)-alizarin red S complex.