Fernand Dejehet

Spin labelled arabinogalactan as MRI contrast agent

In this study, we report the synthesis and the evaluation as MRI contrast agent of arabinogalactan/pyrrolidinoxyl radicals (PCA) covalent adduct (SLAG:Spin Labelled ArabinoGalactan). Arabinogalactan was used as targeting device, as it is recognized by the asialoglycoprotein receptor specific to the hepatocytes. The higher relaxivity R1 in water of SLAG, compared with small hydrophilic nitroxyl radicals, was explained by the molecular dynamics study using EPR spectroscopy that showed some immobilization of the radical into the polysaccharide. A binding study on isolated hepatocytes revealed that SLAG still recognizes the asialoglycoprotein receptor. MR imaging was performed using spin-echo T1 weighted images on mice to compare the contrast effect obtained with SLAG and PCA after IV injection (1 mmol/kg free radical). The percent signal enhancement observed in the liver 5 min after IV injection was 40 +/- 3% and 13 +/- 5% for SLAG and PCA, respectively. The signal was also dramatically increased in the renal cortex. This latter effect as well as the prolonged duration of the contrast (+/- 3 h), indicates at least a partial nonselective biodistribution; the high concentration needed to obtain a contrast effect could account for the saturation of the asialoglycoprotein receptor and hence for the apparent nonselective biodistribution.

Evaluation of nonionic nitroxyl lipids as potential organ-specific contrast agents for magnetic resonance imaging

Considering their intrinsic properties of accumulation in the hepatic tissue, we have synthesized nitroxyl-containing lipids as potential organ-specific contrast agents for magnetic resonance imaging (MRI). Their resistance to reduction by ascorbate and in liver homogenates, and their relaxivity in different media were investigated and compared to those of free carboxyl-Proxyl (3-carboxy-2,2,5,5-tetramethylpyrrolidine-1-oxyl) and Tempamaine (4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl). With respect to the reduction rates by ascorbate, the lipid derivatives show the same well-known order of reactivity as carboxy-Proxyl and Tempamine, the five-membered nitroxyls being more stable than the six-membered compounds. However the binding of the piperidinoxyl compounds to the fatty acids confers to those lipid derivatives a markedly increased stability. Similarly, in liver homogenates, the nitroxyl lipids remained unchanged more than 20 min, contrarily to carboxy-Proxyl and Tempamine. The measurements of spin-lattice relaxation time (T1) in biological media have demonstrated a higher relaxivity of nitroxyl lipids, which can be related to their interaction with proteins. Tested in vivo, one of the synthesized compounds (0.75 mmol/kg) produced an enhancement of 44 +/- 12% of the hepatic signal 5 min after intraportal injection in T1-weighted images. The potential applicability of the other nitroxyl lipids as contrast agents for MRI was limited in the in vivo studies by an unexpected toxicity. Work is currently in progress to improve the therapeutic index of the present class of nitroxyl lipids.

An EPR study of γ- and α-irradiated synthetic powdered calcite labelled with 13C

Abstract Samples of 13 C enriched (99%) calcite were exposed to γ ray and α particle irradiations. Due to their g and hyperfine coupling values, the EPR signals could be attributed to four CO - 2 radicals located in different environments, two CO - 3 radicals and two non-carbon centred species (at g = 2.0052 and 2.0036). The isotropic dating signal at g = 2.0007 is attributed to a rapidly tumbling CO - 2 radical. An axial CO - 2 radical is selectively produced by α particles.

ESR dating: A study of 210Po-coated geological and synthetic samples

Abstract Several synthetic (calcite, calcite doped with humic acids, carbonate apatite) and natural (tooth, mollusc, speleothem) samples are irradiated with a pure alpha emitter, 210 Po, adsorbed on the sample grains surface. The ESR spectra of these “ external ” alpha irradiated samples are compared with those of gamma irradiated ones. Both radiations globally induce the same ESR signals. On the contrary, radiolytical effects in pure synthetic calcite of “ external ” and “ internal ” alpha irradiations appear to be different. “ External ” alpha efficiency factors (ESR k -values) are estimated for some ESR lines and range from 0.08 to 0.3.

Location and Motion of Isotropic Paramagnetic Centres in Synthetic Monohydrocalcite

The isotropic CO-2 and CO-3 centres in CaCO3H2O were examined with electron spin resonance (ESR), electron-nuclear double resonance (ENDOR) and thermogravimetric analysis (TGA). From the absence of anisotropy and the small value of the 1H ENDOR splitting ( ~0.3 MHz), the effect of crushing and/or thermal annealing on the ESR and TGA spectra, it is concluded that both isotropic centres are located in the occluded water, surrounding the constituent crystallites of the spherulites. ESR line width and 13C hyperfine coupling parameter measurements yield approximate values for the rotation activation energy (ΔE( CO3-<ΔE( CO2-) with ΔE~ 0.2 eV) and for the vibrational frequency ( ~2.1012 Hz for CO2-).

ESR dating: A study of humic acids incorporated in synthetic calcite

Calcite doped with humic acids (HA) exhibits an ESR signal at g = 2.0043, with a line width (ΔHpp) of about 0.6 mT, that appears to be very radiosensitive. It seems that one may conclude that this line, due to humic acids, contributes at least partly to the line at 2.003 ± 0.001 observed in natural samples. Moreover, upon γ-irradiation, ESR lines at g=2.0052 (ΔHpp∽ 0.06 mT) and at g=2.0035 (ΔHpp ∽ 0.05 mT) which are also present in γ-irradiated pure calcite, are amplified by the presence of humic acids. The line at g = 2.0052 observed in γ-irradiated HA-doped calcite might well belong to the same paramagnetic center as the line at 2.0057 observed in natural samples. In that case, the last one should not be assigned to a humic acid-clay complex.

Paramagnetic centers in γ-irradiated synthetic monohydrocalcite

When spherulitic grains of (CaCO3.H2O)-C-12, C-13 are gamma-irradiated at room temperature, only three isotropic paramagnetic species are produced: 1) the tumbling CO3- radical with g = 2.0115 and A13c = 11.4 gauss; 2) the tumbling CO2- radical with g = 2.0006 and A13c = 148 gauss; and 3) a non-carbon-centered species (SO3-?) with g = 2.0032 exhibiting an hyperfine coupling interaction of 21.2 gauss with two equivalent I = 1/2 nuclei (H). If these rotating species do substitute for the carbonate ion, they are surrounded by a sphere of water molecules. The isotropic CO2-(dating signal) observed in natural carbonates and the CO2-radical in monohydrocalcite have very similar characteristics, suggesting that the first one is also surrounded by water molecules incorporated in the lattice in some parasitic manner, for example together with Mg2+ or Zn2+ ions.

Development of biocompatible implants of fusinite for in vivo EPR oximetry.

The development of oxygen-sensitive paramagnetic materials is being actively pursued because of their potential applications forin vivo electron paramagnetic resonance (EPR) oximetry. Among these materials, fusinite is of particular interest because of the high sensitivity of the EPR linewidth to the partial pressure pO2. Although this material has led to a number of very useful results in experimental systems, its potential use in humans is limited by the need to prove that it will not cause deleterious effects. The strategy used in this study to optimize the biocompatibility of the oxygen-sensitive materials was to prepare small silicon implants containing the fusinite. The use of silicon permits the diffusion of oxygen inside the implant while the material does not have contact with the biological environment. Radiosterilization did not affect the pO2 sensitivity of the material. The feasibility of performing pO2 measurement was verifiedin vivo by perodically inducing ischemia in the gastrocnemius muscle of mice over a period of 6 weeks.

ESR study of internally α-irradiated (210Po nitrate doped) calcite single crystal

Abstract Amongst the many paramagnetic species created in calcite by the α-desintegration of the incorporated Polonium 210 nitrate, four centres could be followed A ( g ∥ = 2.0017, g ⊥ = 2.0060, A ∥ = 68, A ⊥ = 34.4 gauss); B ( g ∥ = 2.0024, g ⊥ = 2.0052); C ( g ∥ = 2.0033, g ⊥ = 1.9996) and D ( g z = 2.0087, g y = 2.0039, g x = 2.0029, A ∥ = 36.4, A ⊥ = 4.5 gauss). A and D centres are respectively assigned to NO 2- 3 and NO 2- 2 species. The unidentified C centre seems specific to an α-irradiation, whereas the unidentified B centre is also observed in γ-irradiation. This work could be of some use, from the archaeometrical point of view, for the study of natural calcite formations originating from caves where nitrate impurities occur.

Possibility of ESR-dating without determination of the annual dose

It should, in principle, be possible to date a sample without determination of annual dose rate, when two ESR centers of different stability are present. The method assumes that the centers are independent and decay according to a first order process and that their concentration is far from saturation. The model is presented with emphasis on its inherent difficulties.