Endeavor toward Redox-Responsive Transition Metal Contrast Agents Based on the Cross-Bridge Cyclam Platform.

We present the synthesis and characterization of a series of Mn(III), Co(III), and Ni(II) complexes with cross-bridge cyclam derivatives (CB-cyclam = 1,4,8,11-tetraazabicyclo[6.6.2]hexadecane) containing acetamide or acetic acid pendant arms. The X-ray structures of [Ni(CB-TE2AM)]Cl2·2H2O and [Mn(CB-TE1AM)(OH)](PF6)2 evidence the octahedral coordination of the ligands around the Ni(II) and Mn(III) metal ions, with a terminal hydroxide ligand being coordinated to Mn(III). Cyclic voltammetry studies on solutions of the [Mn(CB-TE1AM)(OH)]2+ and [Mn(CB-TE1A)(OH)]+ complexes (0.15 M NaCl) show an intricate redox behavior with waves due to the MnIII/MnIV and MnII/MnIII pairs. The Co(III) and Ni(II) complexes with CB-TE2A and CB-TE2AM show quasi-reversible features due to the CoIII/CoII or NiII/NiIII pairs. The [Co(CB-TE2AM)]3+ complex is readily reduced by dithionite in aqueous solution, as evidenced by 1H NMR studies, but does not react with ascorbate. The [Mn(CB-TE1A)(OH)]+ complex is however reduced very quickly by ascorbate following a simple kinetic scheme (k0 = k1[AH-], where [AH-] is the ascorbate concentration and k1 = 628 ± 7 M-1 s-1). The reduction of the Mn(III) complex to Mn(II) by ascorbate provokes complex dissociation, as demonstrated by 1H nuclear magnetic relaxation dispersion studies. The [Ni(CB-TE2AM)]2+ complex shows significant chemical exchange saturation transfer effects upon saturation of the amide proton signals at 71 and 3 ppm with respect to the bulk water signal.

On the dissociation pathways of copper complexes relevant as PET imaging agents.

Several bifunctional chelators have been synthesized in the last years for the development of new 64Cu-based PET agents for in vivo imaging. When designing a metal-based PET probe, it is important to achieve high stability and kinetic inertness once the radioisotope is coordinated. Different competitive assays are commonly used to evaluate the possible dissociation mechanisms that may induce Cu(II) release in the body. Among them, acid-assisted dissociation tests or transchelation challenges employing EDTA or SOD are frequently used to evaluate both solution thermodynamics and the kinetic behavior of potential metal-based systems. Despite of this, the Cu(II)/Cu(I) bioreduction pathway that could be promoted by the presence of bioreductants still remains little explored. To fill this gap we present here a detailed spectroscopic study of the kinetic behavior of different macrocyclic Cu(II) complexes. The complexes investigated include the cross-bridge cyclam derivative [Cu(CB-TE1A)]+, whose structure was determined using single-crystal X-ray diffraction. The acid-assisted dissociation mechanism was investigated using HClO4 and HCl to analyse the effect of the counterion on the rate constants. The complexes were selected so that the effects of complex charge and coordination polyhedron could be assessed. Cyclic voltammetry experiments were conducted to investigate whether the reduction to Cu(I) falls within the window of common bioreducing agents. The most striking behavior concerns the [Cu(NO2Th)]2+ complex, a 1,4,7-triazacyclononane derivative containing two methylthiazolyl pendant arms. This complex is extremely inert with respect to dissociation following the acid-catalyzed mechanism, but dissociates rather quickly in the presence of a bioreductant like ascorbic acid.

Gold Nanoparticles as Colorimetric Sensors for the Detection of DNA Bases and Related Compounds.

Results regarding interaction of colloidal gold solutions with nucleobases, including uracil (U), as well as its sulfur derivatives, 2-thiouracil (2TU) and 4-thiouracil (4TU), cytosine (C), adenine (A), and guanine (G), as well as urea and thiourea (TU), are reported. Anionic stabilized citrate gold nanoparticles (AuNPs) were synthesized by reducing the tetrachloroaurate (III) trihydrate with trisodium citrate. The surface plasmon resonance (SPR) band was used in the characterization of synthesized AuNPs, as well as transmission electron microscope (TEM) imaging, which was used in the characterization of dispersed and aggregated gold nanoparticles. Interactions of nucleobases with the gold surface was analyzed by following the plasmon absorbance band red shift of the AuNPs. The sulfur-containing compounds adsorbed to the nanoparticle surfaces by chemisorption-type interactions; with TU and 4TU, the process is accompanied by a sudden change in color; in contrast, 2TU forms stable functionalized gold nanoparticles. Urea and U do not adsorb to nanoparticle surfaces, but the other heterocyclic bases containing nitrogen interact effectively with the gold surface, causing the assembly of nanoparticles, even though the interparticle self-aggregation process was slower than that mediated by either TU or 4TU. The method is efficient in the colorimetric detection of nucleobases and derivatives at concentration levels on the order of 1 µM.

A pentadentate member of the picolinate family for Mn(ii) complexation and an amphiphilic derivative.

We report a pentadentate ligand containing a 2,2'-azanediyldiacetic acid moiety functionalized with a picolinate group at the nitrogen atom (H3paada), as well as a lipophylic derivative functionalized with a dodecyloxy group at position 4 of the pyridyl ring (H3C12Opaada). The protonation constants of the paada3- ligand and the stability constant of the Mn(ii) complex were determined using a combination of potentiometric and spectrophotometric titrations (25 °C, 0.15 M NaCl). A detailed relaxometric characterisation was accomplished by recording 1H Nuclear Magnetic Relaxation Dispersion (NMRD) profiles and 17O chemical shifts and relaxation rates. These studies provide detailed information on the microscopic parameters that control their efficiency as relaxation agents in vitro. For the sake of completeness and to facilitate comparison, we also characterised the related [Mn(nta)]- complex (nta = nitrilotriacetate). Both the [Mn(paada)]- and [Mn(nta)]- complexes turned out to contain two inner-sphere water molecules in aqueous solution. The exchange rate of these coordinated water molecules was slower in [Mn(paada)]- (k298ex = 90 × 107 s-1) than in [Mn(nta)]- (k298ex = 280 × 107 s-1). The complexes were also characterised using both DFT (TPSSh/def2-TZVP) and ab initio CAS(5,5) calculations. The lipophylic [Mn(C12Opaada)]- complex forms micelles in solution characterised by a critical micellar concentration (cmc) of 0.31 ± 0.01 mM. This complex also forms a rather strong adduct with Bovine Serum Albumin (BSA) with an association constant of 5.5 × 104 M-1 at 25 °C. The enthalpy and entropy changes obtained for the formation of the adduct indicate that the binding event is driven by hydrophobic interactions.

Interaction of gold nanoparticles mediated by captopril and S-nitrosocaptopril: the effect of manganese ions in mild acid medium.

We report herein results regarding reactivity and assembly of citrate-capped gold nanoparticles (AuNPs) mediated by captopril (cap) and S-nitrosocaptopril (NOcap), two angiotensin converting enzyme inhibitors and antihypertensive agents. The results were compared with that of cysteine (Cys), a thiol-containing amino acid found in plasma. The interparticle interactions were characterized by monitoring the evolution of the surface plasmon resonance band using the spectrophotometric method. The original gold nanoparticles were efficiently modified by small amounts of Mn(+2) ions, which are adsorbed onto the surface of 15.4 nm citrate-capped gold nanoparticles, giving rise to manganese-gold nanoparticles (Mn-AuNPs) that, in mild acid medium, have proved to be highly sensitive and a rapid colorimetric detection method for thiols. Depending on the concentration of the Mn(+2) ions the aggregation of AuNPs can be rapidly induced. The kinetics of the assembly process has been studied. Good first-order kinetics has been observed, with the exception of captopril-mediated nanoparticle aggregation at low concentration of either cap or acid. The rate of Cys-mediated assembly of gold nanoparticles in aqueous 10 mM acetic acid is more than 20-times faster than pure AuNPs and concentrations of Cys as low as 34 nM can be detected in less than 40 min under conditions of stable Mn-AuNPs. Similar effects were observed with cap or NOcap. The assembly-disassembly reversibility is shown with cap and NOcap and depends highly on pH.

A further study of acetylacetone nitrosation.

The nitrosation of acetylacetone (AcAc) has been revised in an aqueous acid medium of perchloric acid and buffers of mono-, di-, or tri-chloroacetic acid. The results show that in the presence of buffers, under conditions of [nit] ≪ [AcAc] (nit = sodium nitrite) the reaction cannot be studied by UV-Vis spectroscopy, contrary to the recently published paper by García-Rio et al. (J. Org. Chem., 2008, 73, 8198). The present study also corroborates the previously published mechanism of AcAc nitrosation, where no base-catalysis was observed. Contrarily, the low effect of buffers was attributed to the formation of nitrosyl chloro-, dichloro- or trichloro-acetate salts that are new nitrosating agents.

S-nitrosocaptopril formation in aqueous acid and basic medium. A vasodilator and angiotensin converting enzyme inhibitor.

The reaction of S-nitrosocaptopril (NOcap) formation was studied in both aqueous acid and basic medium. Captopril (cap) reacts rapidly with nitrous acid in strong acid medium to give the stable--in the timescale of the experiments--NOcap. The kinetic study of the reaction involving the use of stopped-flow, shows that at low sodium nitrite (nit) concentration, the reaction is first-order in both [nit], [H(+)], and is strongly catalysed by Cl(-) or Br(-) (= X(-)): rate = (k(3) + k(4)[X(-)])[H(+)][nit][cap]. In aqueous buffered solution of acetic acid-acetate the reaction rate is much slower and the decomposition of NOcap was observed; however, the rate of NOcap decay is more than 30-fold slower than its formation. In aqueous basic medium of carbonate-hydrogen carbonate buffer, as well as in alkaline medium, the kinetics of the nitroso group (NO) transfer from tert-butyl nitrite (tBN) to cap was studied using either conventional or stopped flow methods. In mild basic medium, the NOcap decomposes. The NOcap formation is first-order in both tBN and cap concentrations, and the reaction rate increases with pH until to, approximately, pH 11.5, above which value it becomes pH independent or even invariable with the [OH(-)]. Kinetic results show that the thiolate ion of cap is the reactive species. In fact, the presence of anionic micelles of sodium dodecyl sulfate (SDS) inhibits the reaction due to the separation of the reagents; whereas, cationic micelles of tetradecyltrimethylammonium bromide (TTABr) catalyse the reaction at low surfactant concentration due to reagents concentration in the small volume of the micelle. The rate equation is: rate = k(f) K(SH)[cap][tBN]/(K(SH) + [H(+)]). The rate of NOcap decomposition in mild basic medium is first-order in both [cap] and [NOcap], and decreases on increasing pH; but, in alkaline medium the NOcap is stable within the timescale of the experiments. Based on the results, the NOcap decomposition yields the disulfide compound that is formed in the nucleophilic attack of the -SH group of cap to the sulfur electrophilic center of NOcap, -S-N=O. The resulting rate equation is: rate = k(d)[H(+)][cap][NOcap]/(K(SH) + [H(+)]).

Iron(III) complexation by Vanchrobactin, a siderophore of the bacterial fish pathogen Vibrio anguillarum.

The bacterial fish pathogen Vibrio anguillarum serotype O2 strain RV22 produces the mono catecholate siderophore Vanchrobactin (Vb) under conditions of iron deficiency. Vb contains two potential bidentate coordination sites: catecholate and salicylate groups. The iron(III) coordination properties of Vb is investigated in aqueous solutions using spectrophotometric and potentiometric methods. The stepwise equilibrium constants (log K) for successive addition of Vb dianion to a ferric ion are 19.9; 13.3, and 9.5, respectively, for an overall association constant of 42.7. Based on the previous results, we estimated the equilibrium concentration of free iron(III) under physiological conditions for pH 7.4 solution containing 10(-6) M total iron and 10(-5) M total Vb as pFe = 20 (=-log[Fe(3+)]). The Vb model compounds catechol (Cat) and 2,4-dihydroxy-N-(2-hydroxyethyl)benzamide (Dhb) have also been examined, and the obtained results show that the interaction of the whole system of Vb that contains the ferric-chelating groups of both Dhb and Cat, is synergically greater than the separate parts; i.e. Vb is the best chelating agent either in acid or basic media. In summary, bacteria employing Vb-mediated iron transport thus are able to compete effectively for iron with other microorganisms within which they live.

Exploring the effect of supramolecular structures of micelles and cyclodextrins on fluorescence emission of local anesthetics.

Benzocaine (ethyl 4-aminobenzoate, 4) and its derivatives ethyl 2-aminobenzoate, 2, and ethyl 3-aminobenzoate, 3, were found to form association complexes with supramolecular structures of micelles and cyclodextrins (CDs). The fluorescence emission of 2, 3 or 4 dissolved in the pseudo-micellar phase or included into α-, ß-, or γ-CD cavity increases dramatically with respect to that observed in only water. High percentages of organic solvents like dioxane, acetonitrile, DMSO in the aqueous solution lead to a similar effect. The stability constants of the complexes formed between these drugs and cyclodextrins have been determined. In neutral or acid medium, a 1:1 stoichiometry for drug:CD complexes have been found, whereas in alkaline medium 1:2 stoichiometry was also detected in some cases. Kinetic studies of both the nitrosation of the amine group and the alkaline hydrolysis of the ester function was employed to infer the conformation of the complexes as well as to evaluate their stability constants. Theoretical calculations to optimize the molecular structure of 2, 3 and 4 allow us to propose possible geometries of the complexes that are in agreement with the experimental data.

Inclusion complexation of novocaine by beta-cyclodextrin in aqueous solutions.

The formation of inclusion complexes between beta-cyclodextrin (beta-CD) and the local anesthetic 2-(diethylamino)ethyl-p-amino-benzoate (novocaine) in aqueous solutions under different acidity conditions, using steady-state fluorescence or UV-vis spectroscopies, electrical conductivity, or the kinetic study of both the nitrosation reaction of the primary amine group in a mild acid medium and the hydrolysis of the ester function under an alkaline medium, has been studied. The inclusion complex formation between neutral or protonated novocaine and beta-CD of 1:1 stoichiometry was observed; however, the magnitude of the binding constants depends on the nature of both the guest and the host, and the higher-affinity guest-host was found under conditions when both the novocaine and the beta-CD were neutral molecules.

Tautomerization of 2-acetylcyclohexanone. 1. Characterization of keto-enol/enolate equilibria and reaction rates in water.

The keto-enol tautomerism of 2-acetylcyclohexanone (ACHE) was studied in water under different experimental conditions. By contrast with other previously studied beta-diketones, the keto-enol interconversion in the ACHE system is a slow process. Under equilibrium conditions, the analysis of the absorbance readings of ACHE aqueous solutions yielded more than 40% of enol content at 25 degrees C; nevertheless, in aprotic solvents such as dioxane, ACHE is almost completely enolized. In alkaline medium, the enolate ion is the only existing species; the study of the effect of pH on the UV-absorption spectrum of ACHE yielded a value of 9.85 for the overall pK(a) of ACHE. Under nonequilibrium conditions, the keto-enol tautomerization was studied in water. Several factors affecting the reaction have been investigated, which include H(+)-catalysis, ionic strength effect, buffer catalysis, deuterium isotope effects, temperature effect, or solvent effects.

Nitrosation of 2-acetylcyclohexanone. 2. Reaction in water in the absence and presence of cyclodextrins.

The kinetic study of the nitrosation of the enol of 2-acetylcyclohexanone (ACHE) has been performed in aqueous acid media in the absence and presence of alpha- and beta-cyclodextrin. The reaction is first-order with respect to both reactants concentration: [nitrite] and [ACHE]; but, unexpectly, the dependence of both [H(+)] or [X(-)] (X(-) = Cl(-), Br(-), or SCN(-)) is not simple first-order. The experimental findings have been explained on the basis of a reaction mechanism that considers the formation of a chelate-nitrosyl complex intermediate in steady-state. Addition of both alpha-cyclodextrin (alpha-CD) or beta-cyclodextrin (beta-CD) diminishs strongly the observed rate constant, k(o), measured either for enol-nitrosation or for enol-ketonization reactions. In the case of beta-CD, the inhibition effect is explained through the formation of nonproductive inclusion complexes between the enol (EH) and beta-CD of 1:1 stoichiometry. Nevertheless, the quantitative interpretation of k(o)-[alpha-CD] profiles requires the assumption that the inclusion complexes formation of both 1:1 (EH/alpha-CD) and 1:2 (EH/alpha-CD(2)) stoichiometries. In the case of enol-ketonization, the EH/alpha-CD complex is nearly as reactive as the uncomplexed enol.