Solvent and Temperature Dependencies of the Rates of Thermal -to- Isomerization of Tetra-()substituted 4-Aminoazobenzenes Containing 2,6-Dimethoxy Groups.

The kinetics of thermal -to- isomerization of two tetra-()substituted 4-aminoazobenzene derivatives containing 2,6-dimethoxy groups in the 4-aminobenzene ring and either 2',6'-dimethyl () or 2',6'-dichloro groups in another ring was studied in 16 and 9 solvents, respectively, at room temperature. In addition, the kinetics of isomerization of was studied at variable temperatures in 6 solvents. The solvent effects were analyzed in terms of multiparameter correlations using the Kamlet-Taft, Catalan, and Laurence scales.

Exploring the Influence of Spacers in EDTA-β-Cyclodextrin Dendrimers: Physicochemical Properties and In Vitro Biological Behavior.

The synthesis of a new family of ethylenediaminetetraacetic acid (EDTA) core dimers and G0 dendrimers end-capped with two and four β-cyclodextrin (βCD) moieties was performed by click-chemistry conjugation, varying the spacers attached to the core. The structure analyses were achieved in DMSO- and the self-inclusion process was studied in DO by H-NMR spectroscopy for all platforms. It was demonstrated that the interaction with adamantane carboxylic acid (AdCOOH) results in a guest-induced shift of the self-inclusion effect, demonstrating the full host ability of the βCD units in these new platforms without any influence of the spacer.

Zwitterion-neutral form equilibria and binding selectivity of pyridineboronic acids.

A B NMR study of 3-pyridineboronic acid at variable pH in water and 50 vol% aqueous dioxane confirms that the tautomeric equilibrium of the acid is shifted to the zwitterionic form in water, but to the molecular form in the mixed organic solvent. Interactions of 3- and 4-pyridineboronic acids with sialic acid, fructose and several other diols were studied by potentiometric titrations in a wide range of pH in water and water-organic mixtures. In all reaction media the stability of boronate complexes increases upon an increase in pH for neutral low acidic diols such as fructose and glucose but has the opposite trend for highly acidic sialic and lactic acids occurring as anionic species.

Probing the Role of the Bridging Nitrogen in the Signaling Mechanism of an Anthracene-Boronic Acid Sugar Sensor and a Different Version of the PET-Based Mechanism.

The N-quaternized derivative of the James-Shinkai anthracene-boronic acid fluorescence sugar sensor was prepared to probe the role of the bridging nitrogen in the signaling mechanism of . Both and contain positively charged bridging groups NMe or NH, respectively, but lacks the ability to form the intramolecular ammonium-boronate doubly ionic hydrogen bond present in . Receptors and display opposite fluorescence vs pH profiles with a small turn-on effect of the sugar binding to the zwitterion of in contrast to a large effect observed with .

Anion Recognition by Benzoxaborole.

The binding types (H-bonding or coordinate) and stability constants for complexes of 11 mono- and di-anions with benzoxaborole () were determined by H and B NMR titrations in DMSO or MeCN. Compared to phenylboronic acid (PBA), 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, HPO, and PhPO anions, which are coordinately bonded to , and partially with SO, which forms only the H-bonded complex with PBA, but both H-bonded and coordinate complexes with .

Examination of pinanediol-boronic acid ester formation in aqueous media: relevance to the relative stability of trigonal and tetrahedral boronate esters.

The interaction of pinanediol with 2-fluorophenylboronic acid and several other substituted phenylboronic acids was studied in 40% vol. aqueous acetonitrile by 1H and 11B NMR, potentiometric and spectrophotometric titrations at variable pH values. The experimental results reveal the formation of a very stable trigonal ester (Ktrig ≈ 2 × 104 M-1) and a significantly less stable tetrahedral hydroxocomplex (Ktet ≈ 5 × 103 M-1) in contrast to the traditionally observed inverted order of stabilities Ktrig < Ktet.

Mechanistic study of carboxylic acid and phosphate ester cleavage by oximate metal complexes surpassing the limiting reactivity of highly basic free oximate anions.

Two tridentate and one tetradentate new ligands containing the terminal oxime group separated from secondary amino and pyridine groups as additional binding sites by two or three methylene groups have been prepared. Their acid-base properties, as well as the composition and stability of their complexes with Zn(ii) and Cd(ii) ions, were determined by potentiometric and spectrophotometric titrations. The X-ray structure of a Cd(ii) complex of a related tridentate oxime ligand previously studied in solution was determined.

Synthesis and anion recognition studies of new oligomethylene bis(nitrophenylureylbenzamide) receptors.

A new series of oligomethylene bis(nitrophenylureylbenzamide) receptors were synthesized varying the relative position of the urea and amide groups (4 and 8) and the length of the oligomethylene chain (C to C). An anion recognition study was performed with TBAX salts (X = AcO, BzO, F, HPO , and HPO ) by UV-vis and H NMR. The flexibility of these receptors allows a cooperative effect of both ureylbenzamide units in the receptors.

Stability of doubly and triply H-bonded complexes governed by acidity-basicity relationships.

We used our recently proposed acidity-basicity interplay (ABI) model (Chem. Sci., 2018, 9, 4402) and the Jorgensen secondary interactions hypothesis (JSIH) to rationalise the experimentally observed trends in the formation constants of doubly and triply H-bonded systems with -NHO[double bond, length as m-dash]C- and -NHN- interactions.

Substrate Specificity and Leaving Group Effect in Ester Cleavage by Metal Complexes of an Oximate Nucleophile.

Deprotonated zinc(II) and cadmium(II) complexes of a tridentate oxime nucleophile (1, OxH) show a very high reactivity, breaking by 2-3 orders of magnitude the previously established limiting reactivity of oximate nucleophiles in the cleavage of substituted phenyl acetates and phosphate triesters, but are unreactive with p-nitrophenyl phosphate di- and monoesters. With reactive substrates, these complexes operate as true catalysts through an acylation-deacylation mechanism. Detailed speciation and kinetic studies in a wide pH interval allowed us to establish as catalytically active forms [Cd(Ox)], [Zn(Ox)(OH)], and [Zn(Ox)(OH)] complexes.

Brønsted versus Lewis Acid Type Anion Recognition by Arylboronic Acids.

Interactions between arylboronic acids and a series of anions as tetrabutylammonium salts in DMSO and MeCN were studied by (1)H and (11)B NMR as well as spectrophotometrically. Boronic acids act as Brønsted acid type receptors through hydrogen bonding with B(OH)2 hydroxyl groups toward Cl(-), Br(-), HSO4(-), and AcO(-), but they act as Lewis acid type receptors toward F(-) and H2PO4(-), which form tetrahedral adducts with the B(III) center of boronic acids, although there is also evidence for some contribution of hydrogen bonding with these anions. The Hammett plot for the binding constants of AcO(-) with 3- and 4-substituted phenylboronic acids in DMSO is nonlinear, with a small negative slope for electron-donating and weakly electron-accepting substituents and a large positive slope for strongly electron-accepting substituents.

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.

Electrophilic assistance to the cleavage of an RNA model phopshodiester via specific and general base-catalyzed mechanisms.

Kinetics of transesterification of the RNA model substrate 2-hydroxypropyl 4-nitrophenyl phosphate promoted by Mg(2+) and Ca(2+), the most common biological metals acting as cofactors for nuclease enzymes and ribozymes, as well as by Co(NH(3))(6)(3+), Co(en)(3)(3+), Li(+), and Na(+) cations, often employed as mechanistic probes, was studied in 80% v/v (50 mol %) aqueous DMSO, a medium that allows one to discriminate easily specific base (OH(-)-catalyzed) and general base (buffer-catalyzed) reaction paths. All cations assist the specific base reaction, but only Mg(2+) and Na(+) assist the general base reaction. For Mg(2+)-assisted reactions, the solvent deuterium isotope effects are 1.

Schiff base formation and recognition of amino sugars, aminoglycosides and biological polyamines by 2-formyl phenylboronic acid in aqueous solution.

Interactions of 2-, 3- and 4-formyl phenylboronic acids (FPBAs) with sugars, amino sugars, aminoglycosides and various poly- and monoamines have been studied by UV-vis, (1)H and (11)B NMR titrations in water at variable pH. Behavior of 2-FPBA was anomalous in several aspects. Transformation of the acid into its conjugate base was slow in NMR time scale and was accompanied by intramolecular cyclization affording the respective benzoboroxole.

Host-guest chemistry of alkaloids.

Binding of alkaloids by different hosts (native and modified cyclodextrins, cucurbiturils, calixarenes, and metal complexes of porphyrin and Salphen-type ligands), as well as receptor properties of alkaloid based hosts are reviewed. With alkaloids as guests, the largest binding constants and most significant spectral changes, in particular strong fluorescence enhancements induced by complexation with isoquinoline alkaloids, are observed with cucurbituril hosts. Cyclodextrins are successfully employed for improvement of solubility and for chiral separation of alkaloids of different types.

Protonation of kanamycin A: detailing of thermodynamics and protonation sites assignment.

Protonation of an aminoglycoside antibiotic kanamycin A sulfate was studied by potentiometric titrations at variable ionic strength, sulfate concentration and temperature. From these results the association constants of differently protonated forms of kanamycin A with sulfate and enthalpy changes for protonation of each amino group were determined. The protonation of all amino groups of kanamycin A is exothermic, but the protonation enthalpy does not correlate with basicity as in a case of simple polyamines.