DOTASA revisited : 1H NMR and potentiometric studies of a highly asymmetrical ligand and its lanthanide(III) complexes

Poster apresentado no 8th FIGIPAS Meeting in Inorganic Chemistry, Atenas, Grécia, 6 - 9 Jul. 2005.DOTASA (1,4,7,10-tetraazacyclodo-decane-1-(R,S)-succinic acid-4,7,10-triacetic acid) is a DOTA-like macrocyclic ligand showing a carboxymethyl -CH2COOH substituent moiety at a C carbon of one of the four acetate pendant arms, present as a racemic mixture of R and S configurations.The protonation constants of the ligand were determined by potentiometry, giving values close to DOTA except for the extra pK3 value of 5.35 assigned to protonation of the extra carboxylate group in the succinyl arm. The 1H NMR spectra of DOTASA at different pH values are too complex to allow the determination of its microscopic protonation scheme, due to the presence of multiple isomeric structures in solution. The thermodynamic stability constant of its Gd3+ chelate was determined by a potentiometric method, and the value obtained, log KML = 27.2 (0.2), is higher than for the [Gd(DOTA)(H2O)]- complex. The solution structure of the asymmetrical Ln3+ chelates of DOTASA was studied by 1H NMR spectroscopy, indicating the presence of four isomers, corresponding to the combination of the antiprismatic (M) and twisted antiprismatic (m) helicities of the pendant arms and to the R and S configurations of the substituted pendant arm C atom. The m/M isomer ratio decreases along the lanthanide series, with the m isomer decreasing from 90% at La to about 50% from Eu-Lu. This shows that the expected m isomer population of the Gd3+ complex with DOTASA is higher than for the unsubstituted Gd3+-DOTA (~15%) but lower than for a Gd3+ chelate of a RRRR tetrasubstituted DOTA (~70%). Thus the stabilisation of the m isomer by C monosubstitution at the DOTA acetate pendant arms in [Gd(DOTASA)(H2O)]2- is responsible for its increased water exchange rate and higher relaxivity

New electrochemical and optical sensing techniques for the liquid phase

Three topics are summarized: advances in potentiometric sensors, in optical sensors, and in enzyme-based and enzyme-sensing biosensors. © Springer-Verlag 1999

Coupled Diffusion/Adsorption Model for Response of Precipitate-Based Iodide-Selective Electrodes to Primary-Ion Activity Steps

Switched wall-jet streams of Iodide solutions Impinging on Agl-based Ion-selective electrodes (ISEs) give potential responses after the first 20 ms. The responses to Iodide activity steps of constant ratio a (flnal)/a (Initial) are highly dependent on the starting activity. This Is contrary to most present theories of ISE response, particularly those describing reversible surface Ion exchange controlled by diffusion to a surface or first-order reaction kinetics. Experimentally, diffusion Is Involved over all concentration ranges but Is coupled with some surface adsorption Isotherm. At low activities, steps up or down are slower than diffusion because of the need to form or remove some of the adsorbed layer. At high activities, step responses approach pure diffusion control, since adsorption Is nearly saturated. Dependence on the starting activity requires a nonlinear adsorption Isotherm, which Is consistent with adsorption data showing logarithmic dependence of surface Ion concentration on bathing activity. Experimentally, rate-controlling dlssolutlon/preclpltatlon processes are not likely. © 1991, American Chemical Society. All rights reserved

Microfabricated potentiometric electrodes and their in vivo applications

New technologies that yield mass-produced, miniature, ion-selective electrodes for biology and medicine are described by Erno Lindner of the University of Memphis and Richard P. Buck of the University of North Carolina-Chapel Hill

Comparison of Proposed Response Mechanisms of Precipitate-Based Ion-Selective Electrodes in the Presence of Interfering Ions

New experimental results on nonmonotonic transients are reported here to confirm and extend published results. The new data support the notion that both adsorption/desorption and surface reactions of the crystal membrane are important processes determining transient selectivity coefficients. The former induces local changes in exterior surface concentrations of responsive ions. The latter change the composition of the membrane. Both affect the measured potential. Two proposed models for the response of a precipitate-based Ion-selective electrode (ISE) in the presence of primary and interfering ions are discussed. The mathematical expressions for the models are examined in light of ISE thermodynamics, finite ion-exchange kinetics, and non-steady-state diffusion. The adsorptlon/desorptlon model describes the overshoot/decay portion of the potential-time curves more accurately than the mixed-phase formation model, but a kinetic version of the latter applies at very long times. The combination of the expanded models was shown to describe all features of the experimental curves. © 1989, American Chemical Society. All rights reserved

Recomendations for nomenclature of ion-selective electrodes (IUPAC recommendations 1994)

The rapid growth of interest in the field of ion-selective electrodes (ISEs), within the larger field of electrochemical sensors based on potentiometric, amperometric, and conductometric principles, makes it highly desirable to achieve improved standardization of nomenclature in this area. The previous document, IUPAC Recommendations 1975 (1) has been corrected using recent experimental and theoretical findings. This report does not define or recommend activity standards or calibration procedures using activity standards. © 1994 IUPA

Calibration of a planar differential CO2 probe

The working mechanism of the differential CO2 sensor inherently contains several calibration problems. These difficulties are highlighted and some possibilities for eliminating them are discussed. Finally, a simple error analysis is shown for optimizing the composition of the standard solutions to be used for the calibration of the differential CO2 probe in blood electrolyte analyzers. © Springer-Verlag 1999

Tailored transport through ion-selective membranes for improved detection limits and selectivity coefficients

The analytical performances of ionophore based potassium and calcium sensitive membranes were tested under current control or following a DC current pretreatment. The effects of the direction and magnitude of the applied current were evaluated on the response slopes, the detection limits and selectivity coefficients. The results are discussed in the light of expectations and the theoretical and practical difficulties

Responses of H\u3csup\u3e+\u3c/sup\u3e selective solvent polymeric membrane electrodes fabricated from modified PVC membranes

Potentiometric responses of a novel class of pH sensitive ionophores, namely several phenoxazine derivatives, were tested in different modified PVC matrices. The ionophores were compounded into liquid membranes as usual or were covalently coupled to the polymeric matrix. The general analytical performance of the membranes and other membrane characteristics (i.e., resistance and response time, as measures of membrane decomposition or structural changes) were followed in time. The transient responses of membranes with mobile ionophores in high molecular weight (HMW) and carboxylated PVC (PVC-COOH) were compared to those with immobilized ionophores. The response time of membranes with immobilized ionophores was found to be between those with mobile ionophores in HMW (fast response) and PVC-COOH (sluggish response). Accordingly, the rate of response was correlated primarily to the -COOH content of the membranes. © 1993

Piezoelectric chemical sensors (IUPAC Technical Report)

Piezoelectric chemical and electrochemical sensors are classified. Basic terms and definitions related to piezoelectric materials and sensing devices, electric transducers, and analytical measurements with piezoelectric chemical sensors are provided. In addition, the more essential response equations are presented