Reinhard Stösser

Non-destructive and localized assessment of acidic microenvironments inside biodegradable polyanhydrides by spectral spatial electron paramagnetic resonance imaging

Spectral spatial electron paramagnetic resonance imaging (e.p.r.i.) was applied to characterize the microenvironment inside 2 mm thick poly(1,3-bis-p-carboxyphenoxypropane-co-sebacic anhydride) (14) polymer discs. Incorporated nitroxides are highly immobilized in the dry polymer. Exposure to 0.1 M phosphate buffer (pH 7.4) resulted in the formation of a front of degraded polymer from outside to inside. The localized measurement of pH values inside the polymer matrix was performed, using a pH sensitive nitroxide (pKa 6.1). A pH gradient was found between the degrading polymer and the buffer, but also inside the polymer. The pH inside the delivery matrix rises with time for 4.7 to 7.4. The results demonstrate that difficulties in the characterization of the pH, one of the most important parameters for drug delivery and polymer degradation, can be overcome by spectral spatial e.p.r.i.

New directions in the preparation and redox chemistry of fluoride-templated tetranuclear vanadium phosphonate cage compounds, M(n+)[(V2O3)2(RPO3)4<F]n.

A new and simple preparation method for fluoride-templated tetranuclear vanadium phosphonate cage compounds, M(n+)[(V2O3)2(RPO3)4<F]n is outlined. The crystalline products were characterized by X-ray diffraction, elemental analysis, and thermogravimetric analysis. Using the acceptable solubility of the products, multinuclear NMR could be performed on the corresponding solutions. Some insight into the process of formation of the cage compounds in solution could be reached by monitoring the corresponding reaction mixture by multinuclear NMR. The template function of the F(-) ion could be demonstrated together with the fact that the non-transition-metal ions (M(n+)) used here (phosphonium ions) have no direct effect on the formation of the cage. In contrast, the redox behavior of these compounds in the solid state distinctly depends on the cations. This could be easily investigated by electron paramagnetic resonance because the mixed-valence species (3V(V)/V(IV)) can be produced chemically or thermally induced in solution as well as in the solid state. In the latter case, the reaction of the cage with H2 activated on the platinum powder can be regarded as a key experiment for understanding the redox process of the title compounds in the solid state. The role of specific interactions in solution at the tumbling rate and the localization of spin density in the cage could be demonstrated by the reduction performed with 1-methylimidazole and quinoline. While the substituents R = Me and Ph have only a small influence on the cage formation in solution, they have a significant influence on the redox reaction and structural relaxation in the solid state.

Detection of free radicals in living mice after inhalation of DTBN by X-band ESR

A new application of X-band electron spin resonance (ESR) is demonstrated for in vivo investigations on living mice. It is the first report on in vivo detection, identification, and localization of a free radical after uptake in the body of a living mammal by inhalation. The volatile di-tert-butyl-nitroxyl (DTBN) has been used as a suitable spin probe. After inhalation of DTBN, a superimposed ESR spectrum consisting of a polar (g = 2.0057; a = 1.65 mT) and a nonpolar paramagnetic species (g = 2.0061; a = 1.51 mT) was detected in mice tails. Collected blood exhibits only the polar spectrum. The decay of signals after removing of the DTBN-soaked swab follows first-order kinetics with k = 0.08 min-1.

Oxovanadium(IV) silsesquioxane complexes.

In the context of a potential modeling of reduced oxovanadium species occurring on the surfaces of silica-supported vanadia catalysts in the course of its turnover, the incompletely condensed silsesquioxane H(3)(c-pentyl)T(7) was reacted with Cl(4)V(THF)(2) (where THF = tetrahydrofuran) in the presence of triethylamine. Precipitation of 3 equiv of HNEt(3)Cl seemed to point to the clean formation of [((c-pentyl)T(7))(V(IV)Cl)] (1), which was supported by electron paramagnetic resonance studies performed for the resulting solutions, but further analytical and spectroscopic investigations showed that the processes occurring at that stage are more complex than that and even include the formation of [((c-pentyl)T(7))(V(V)O)](2) as a side product. Storage of a red-brown hexane solution of this product mixture reproducibly led to the precipitation of blue crystals belonging to the chloride-free compound [((c-pentyl)T(7))(2)(V(IV)=O)(3)(THF)(2)] (2), as revealed by single-crystal X-ray diffraction. Performing the same reaction in the presence of 2 equiv of pyridine leads to an analogous product, where the THF ligands are replaced by pyridine. Subsequent investigations showed that the terminal oxo ligands at the vanadium centers are, on the one hand, due to the presence of adventitious water; on the other hand, the [(c-pentyl)T(7)](3-) ligand also acted as a source of O(2-). The results of SQUID measurements performed for 2 can be interpreted in terms of a ferromagnetic coupling between the vanadyl units. Exposing 2 to a dioxygen atmosphere resulted in its immediate oxidation to yield the V(V) complex [((c-pentyl)T(7))(V(V)O)](2), which may model a fast reoxidation reaction of oxovanadium(IV) trimers on silica surfaces.

Pharmaceutical aspects of ESR investigations on drug delivery systems, tissues and living systems

Examples are given for the application of S- and X-band ESR in the fields of pharmacy and medicine respectively. Drug delivery systems, frozen tissue inclusive subsystems and in vivo ESR on fungi and mice are included. Endogeneous paramagnetic species like Fe(III), Mn(II), Cu(II) in various complex environments located in the samples are used to characterize the drug delivery as well as the in vitro systems like liver and kidney tissues, blood and others. By means of spin labeling techniques it was possible to determine microviscosity, micropolarity, pH-values and partial pressure of oxygen. Pharmaceutical aspects of ESR imaging as well as in vivo ESR are discussed, especially the liberation, distribution and biotransformation of the applied paramagnetic model drugs.

The function of paramagnetic iron species in amorphous materials formed by sol-gel method and conventional melting techniques

Abstract A 9 and 35 GHz electron paramagnetic resonance investigation of conventional Na 2 Oue5f8Al 2 O 3 ue5f8SiO 2 and CaOue5f8Al 2 O 3 ue5f8SiO 2 glasses has shown that: (a) a fraction of Fe 3+ acts as a network former, (b) precursors of non-stoichiometric FeO x phases are formed, (c) the ratio of the concentration of Fe 2+ ions to the concentration of Fe 3+ ions controls the formation and stabilization of radiation-induced paramagnetic hole centres. 9 GHz EPR of SiO 2 and x Na 2 O·0.1Fe 2 O 3 ·100SiO 2 ( x = 0, 0.1, 1.0) sols and gels showed that (i) Fe 3+ sites in the sols are less rhombicly distorted than in glasses (in the presence of a chelating agent such as acac an effect of changing Fe 3+ /Na + ratio is not detectable) and (ii) in alcogels, ferric ions are observed in axially distorted sites; (iii) UV irradiation of frozen sols and gels leads to the formation of ·C 2 H 5 and CH 3 ĊHOH radicals, which provide information about the degree of hydrolyses of the samples; (iv) if sodium is added as NaNO 3 , the UV irradiation produces ·NO 2 radicals; (v) the addition of 1 mol% nitrate is sufficient to remove residues of carbon compounds on tempering the gels.

Magnetic interactions in Mn(II)-methylpyrazole complexes

Abstract Mn(II) complexes with 5-methylpyrazole (MP) ligands and selected anions are investigated using ESR and susceptibility measurements. Structural consequences of the combined action of MP ligands and anions from the magnetic properties of the unordered systems are deduced. For halogen anions essential local magnetic properties and very large zfs values (| D |=0.45; | E |=0.035 cm −1 ) with respect to Mn(II) are determined. In contrast, by using bifunctional anions, effective spin exchange processes govern the ESR spectra of polycrystalline solids and frozen aqueous solutions. In these, autonomous phases of solid complexes are established during the freezing process over a wide range of concentration of the constituents. These phases as well as the polycrystalline solids are characterized by single ESR lines of unusual width (Δ B PP =6 mT). The magnetic properties of the spin exchanging complexes can be understood in terms of an antiferromagnetic cluster model.