Optimization of Conditions for Tritium Labeling of Organic Compounds by Isotope Exchange with Tritium Water,Based on the Concepts of Reactions on the Catalyst Surface

To reveal factors affecting the tritium labeling by isotope exchange with tritium water and to elucidate the reaction mechanism, the concepts of processes involved in heterogeneous catalysis were considered. Conditions were optimized for tritium labeling of deltamethrin, pargiline, trichostatin, ciprofloxacin, and 1,3-O-dibenzylglycerol. Samples with the molar radioactivity of 9.3, 0.5, 1.8, 35.1, and 57.3 Ci mmol⁻¹, respectively, were prepared.__________Translated from Radiokhimiya, Vol. 47, No. 4, 2005, pp. 368–373.Original Russian Text Copyright © 2005 by Shevchenko, Nagaev, Myasoedov.     

Comparative Study of Reaction of Atomic Tritium with Glucosamine and Amino Acids

Reaction of amino acids (glycine and serine) and amino sugar (glucosamine) with atomic tritium generated by thermal dissociation of molecular tritium on a tungsten filament was studied. A frozen aqueous solutions and a freeze-dried mixture of these compounds was bombarded with tritium atoms is a special vacuum unit. The relative yield of the labeled compounds was determined as influenced by the reaction conditions (residual pressure in the system and bombardment time) and target type (frozen solution and freeze-dried mixture). Formation of labeled products is almost independent of the tritium pressure. The ratio of the formation rates of labeled serine and glycine in the frozen solution and freeze-dried mixture bombarded with atomic tritium for 45–270 s was 1.66±0.15 and 1.44±0.13, respectively. At shorter reaction time (15 s), the ratio increases to 3.5±0.2 and 2.0±0.4, respectively. The formation rate of [³H]glucosamine in the mixture is higher at a shorter bombardment time. The radioactivity ratio of labeled glucosamine and glycine formed in frozen solutions and freeze-dried mixture in 15 s was 26.0±2.3 and 6.8±0.6, respectively. At longer reaction time, the relative yield of [³H]glucosamine sharply decreases owing to stronger radiolysis of labeled glucosamine on exposure to tritium beam.__________Translated from Radiokhimiya, Vol. 47, No. 3, 2005, pp. 281–283.Original Russian Text Copyright © 2005 by Badun, Ksenofontov, Lukashina, Pozdnyakova, Fedoseev.     

Synthesis of [3H]Sulfobromophthalein and Related Problems of Tritium Labeling by Solid-Phase Isotope Exchange

Tritium-labeled sulfobromophthalein with a molar radioactivity of 0.5-0.6 PBq mol^-1 was prepared. Various aspects of tritium labeling of organic compounds by solid-phase catalytic isotope exchange are considered. A number of arguments are given in favor of the hypothesis that the degree of isotope exchange mainly depends on the efficiency of tritium spillover in the bulk of the organic substances applied onto the catalyst surface. At present, it can be considered as a reliably proved fact that at temperatures up to 180-200°C the solid-phase isotope exchange mainly occurs via reaction with tritium cations. Apparently, the contribution of the reactions with atomic tritium to labeling is significant only if there is no spillover of tritium cations to the bulk of the organic compound and the substrate withstands heating to 280-300°C.     

Kinetic Features of Labeled Product Formation under the Action of Atomic Tritium on Frozen Solutions and Lyophilically Dried Mixtures of Amino Acids

Formation of labeled amino acids in reactions of their lyophilically dried mixtures and frozen solutions with atomic tritium generated by thermal activation was studied in relation to the reaction time and pressure of molecular tritium. The molar radioactivity of the amino acids is a complex function of the reaction time. At short reaction times, the radioactivity of the amino acid depends primarily on its concentration in the near-surface layer. At long labeling times, in the case of surfactants, the yield of labeled products can decrease owing to side reactions. The influence of water on the rate of formation of labeled products and its role in thermalization of hot tritium atoms in the target is discussed. An increase in the tritium pressure in the system increases the initial radioactivity of the target but affects the yield of the labeled amino acids insignificantly. Recommendations are formulated on optimizing the conditions for labeling of biological macromolecules in their structural studies by tritium planigraphy.     

Tritium Labeling of Androst-4-ene-3,17-dione by Liquid-Phase Selective Hydrogenation and Factors Affecting the Process

Androstenedione labeled with tritium at positions 1 and 2 was prepared. The molar radioactivity of the sample prepared using homogeneous catalysts was as high as 1.4-1.5 PBq mol^{- 1}. [1,2-³H]Androst-4-ene-3,17-dione can be converted into the [1-³H]steroid with the molar radioactivity of 0.9 PBq mol^{- 1}.     

Synthesis of High-Molar-Activity Tritium-Labeled Biologically Active Compounds Containing Aromatic and Heterocyclic Fragments

Procedures are examined for tritium labeling of biologically active compounds. By isotope exchange with tritium water, it is possible to prepare products with the molar radioactivity of about 1 PBq mol^{- 1}. The molar radioactivities of compounds prepared by solid-phase isotope exchange with gaseous tritium at 180-220°C reached 5-6 PBq mol^{- 1}. The degree of labeling varied by a factor of more than 100 depending on the physicochemical properties of the substrate. Selective hydrogenation of a heterocyclic fragment of an organic compound, leaving intact the aromatic fragment, was performed for the first time by solid-phase tritiation.     

Influence of Tritium Spillover on the Efficiency of Labeling of Organic Compounds

Various mechanisms of hydrogen spillover are discussed. A number of evidences are given that tritium incorporation by isotope exchange occurs with the participation of both tritium cations and atomic tritium. The molar radioactivity of the labeled products prepared at temperatures of up to 180°C is, apparently, mainly determined by reactions of the organic compound with tritium cations, and the degree of isotope exchange depends chiefly on the efficiency of tritium spillover to the bulk of the organic substance supported on the catalyst.     

Effect of Various Additives on the Yield and Molar Radioactivity of Labeled Products in Solid-Phase Isotope Exchange

A series of tritium-labeled amino acids were prepared by high-temperature solid-phase isotope exchange in the presence of various additives, namely, of 4-dimethylaminopyridine and rhodium trichloride. These agents strongly affect the degree of isotope exchange. 4-Dimethylaminopyridine provides successful labeling of high-melting compounds; it presumably increases the probability of the reaction of active tritium species with the substrate. The positive effect of rhodium salts may be due to an increase in the thermal stability of amino acids, i.e., in the presence of rhodium trichloride the same yield can be attained at higher temperatures. Therefore, despite modification of the catalyst occurring when rhodium trichloride is reduced, the molar radioactivity of the labeled product appears to be higher than without rhodium salts. Thus, in the presence of rhodium salts, the decrease in the yield of labeled amino acids with increasing temperature is less pronounced than the increase in the molar radioactivity of these substrates, which allows preparation of highly labeled products in reasonable yields.     

Isotope Exchange of Azidothymidine with Tritium

The following modes of isotope exchange of azidothymidine (3'-azido-3'-deoxythymidine) with tritium were studied: solid- and liquid-phase isotope exchange with gaseous tritium and isotope exchange in solution with tritium water. Catalytic reactions of azidothymidine with gaseous tritium in solution result in virtually complete reduction of the azido group to amino group. This reduction also occurs in the course of solid-phase catalytic hydrogenation; the yield of 3'-amino-3'-deoxythymidine ranges from 20 to 70%. The molar radioactivity of tritium-labeled azidothymidine prepared by solid-phase catalytic isotope exchange with gaseous tritium and by isotope exchange in solution with tritium water does not exceed 0.5 Ci mmol^{- 1}.     

Nonaqueous Chlorination of Uranium Metal in Tributyl Phosphate

Low-temperature chlorination of uranium metal in the TBP-TCE-Cl₂ systems was studied. Dissolution of uranium in the dipolar aprotic solvent proceeds with formation of U(IV) compounds. The activation energy of this process is 31.24 kJ mol⁻¹, and relative reaction order with respect to Cl₂ is 2. The effect of TBP concentration on chlorination was examined. The chlorination rate sharply increases at a water content in the TBP-TCE system of 0.2–0.6 vol %.__________Translated from Radiokhimiya, Vol. 47, No. 3, 2005, pp. 239–241.Original Russian Text Copyright © 2005 by Buchikhin, Kuznetsov, Vidanov, Shatalov, Chekmarev.     

Stored energy of a severely deformed interstitial free steel

A Ti-stabilised IF steel subjected to room temperature equal channel angular pressing (ECAP) for 8 passes, route B_C was further cold rolled to 25, 50 and 95% thickness reductions. The evolution of bulk stored energy (350–600 J mol⁻¹) and the associated thermal behaviour was investigated by differential scanning calorimetry (DSC). Local stored energy (5–140 J mol⁻¹) was measured using microhardness, electron back-scattering diffraction (EBSD) and X-ray line profile analysis. The higher stored energy values via calorimetry correspond to energy release from all sources of strain in the material volume as well as Ti precipitation during annealing. An apparent activation energy of 500–550 J mol⁻¹ suggests sluggish recrystallisation due to excess Ti in solid solution.     

Selective Tritium Labeling by a Solid-Phase Procedure

Ways were considered how substrates that contain such reactive centers as halogen atoms and double bonds can be labeled with tritium by solid-phase isotope exchange. The feasibility was demonstrated for tritium labeling by selective solid-phase dehalogenation, hydrogenation, and isotope exchange. Highly labeled vitamin K₁, dihydrofusicoccin, and thyroxine and its derivatives were prepared.     

Synthesis of tritium-labeled Semax

The tert-butyloxycarbonyl derivative of Met-Glu-His-Phe-Pro-Gly was prepared and condensed with either unsaturated or labeled proline. The unsaturated peptide protected by the tert-butyloxycarbonyl group was hydrogenated with gaseous tritium in the presence of a catalyst. The Semax analog labeled in the proline fragment was deprotected by acid hydrolysis. The molar radioactivity of the product (PBq mol^?1) reached 0.93 in the route via labeled proline and 0.37 in the route via unsaturated Semax analog.     

Oxidation of Urea with Nitrous Acid in Nitric Acid Solutions

The kinetics of the reaction between urea and HNO₂ in nitric acid solution was studied spectrophotometrically. It was found that, at a constant ionic strength of the solution μ = 2, in the range of the initial concentrations of urea from 0.01 to 0.1 M, HNO₂, from 0.003 to 0.012 M, and hydrogen ions, from 0.1 to 1.5 M, the rate constant of the reaction is described by the equation -d[HNO₂]/dt = k[HNO₂][CO(NH₂)₂][H⁺] · K([H⁺]K +1)⁻¹, where the rate constant k = 15.6±0.3 l mol⁻¹ min⁻¹ and the protonation constant of urea K = 1.38 l mol⁻¹ at 15°C. From the temperature dependence of the reaction rate in the range of 15–35°C, the activation energy was determined to be 61±5 kJ mol⁻¹. The reaction mechanism involving the reaction of nondissociated HNO₂ molecules and protonated urea species NH₂CONH ₃ ⁺ was suggested.__________Translated from Radiokhimiya, Vol. 47, No. 1, 2005, pp. 57–60.Original Russian Text Copyright © 2005 by Dvoeglazov, Marchenko.     

Synthesis of tritium-labeled 5-fluorouracil and 5-fluorocytosine

The effect of various catalysts and temperature on the solid-phase isotope exchange of 5-fluorouracil and 5-fluorocytosine with tritium was studied. The isotope exchange yielding the desired compounds is accompanied by dehalogenation and hydrogenation of the 5,6-double bond of the pyrimidine ring. Performing the reaction at a temperature below 160°C allowed the process to be carried out selectively, i.e., with the preservation of the functional groups and double bond in the starting compound. The yields of various products formed in the reactions of tritium with the above compounds were estimated. Synthesis conditions were found, and tritium-labeled 5-fluorouracil and 5-fluorocytosine were prepared with the molar radioactivity of 0.45 Ci mmol⁻¹ (16.7 TBq mol⁻¹) and 4.4 Ci mmol⁻¹ (0.16 PBq mol⁻¹), respectively, and with the purity exceeding 98%.     

Solid-Phase Catalytic Hydrogenation of Orotic Acid and 5-Bromouracil with Tritium on a Copper Catalyst

The performance of a copper-based catalyst in solid-phase catalytic hydrogenation of orotic acid and 5-bromouracil with gaseous tritium was studied. Hydrogen isotope exchange in the carboxy group of orotic acid was combined with decarboxylation in a one-pot process. The catalyst performance was judged from the molar radioactivity of [6-³H]uracil and [5-³H]uracil formed by catalytic hydrogenation with gaseous tritium of orotic acid and 5-bromouracil, respectively. In solid-phase catalytic dehalogenation, the performance of the copper-based catalyst is comparable with that of the palladium catalyst, but this level is attained at a higher temperature. To evaluate the performance of the copper catalyst in isotope exchange reactions, additional studies with a wider range of substrates are required.     

Kinetics and Mechanism of Np(IV) Oxidation with Nitric Acid

Neptunium (IV) is oxidized to Np(V) with nitric acid in the presence of U(VI) under conditions of low acidity (<∼0.1 M). The reaction rate is described by the equation d[Np(V)]/dt = k ₁[Np(IV)]/[H⁺]² + k ₂[Np(IV)]²[U(VI)]/[H⁺]³, in which k ₁ = (2.0±0.3) × 10⁻⁵ mol² l⁻² min⁻¹ and k ₂ = (5.50±0.47) × 10⁻² mol l⁻¹ min⁻¹ at 50°C and solution ionic strength μ = 0.5. The activation energies of the two pathways are 148±31 and 122±12 kJ mol⁻¹. The reaction along the main pathway (with the rate constant k ₂) is limited by disproportionation of Np(IV) involving NpOH³⁺ and Np(OH)₂UO ₂ ⁴⁺ complex ions.__________Translated from Radiokhimiya, Vol. 47, No. 3, 2005, pp. 228–233.Original Russian Text Copyright © 2005 by Koltunov, Taylor, Marchenko, Savilova, Dvoeglazov, Zhuravleva.     

Synthesis of tritium-labeled Ganciclovir

The influence of temperature on the solid-phase isotope exchange of Ganciclovir with tritium was studied. Synthesis conditions were found, and tritium-labeled Ganciclovir with the molar radioactivity of 25 Ci mmol⁻¹ (0.925 PBq mol⁻¹) and purity higher than 98% was prepared.     

Monitoring a Flow of “Hot” Tritium Atoms in the Course of Thermal Activation

The possibility of measuring a flow of reactive tritium atoms falling on a target during thermal activation using polyethylene (PE) films was studied. Procedures were developed for pretreatment of PE films before experiment, their subsequent treatment, and radioactivity measurements. The distribution of the flow of tritium atoms in a common reactor as influenced by various modes of atomizer arrangement and different experimental conditions was studied with PE films. The rate of formation of the labeled compound was maximal when the product of the tritium pressure in the system (Pa) by the distance from the atomizer to the target (cm) was 2.     

Tritium labeling of bioorganic compounds by isotope exchange

Procedures for preparing labeled compounds by liquid- and solid-phase methods using gaseous tritium and by isotope exchange with tritium water are considered as different manifestations of a common complex of processes occurring in the presence of tritium, a substrate, and a catalyst. The studies performed allow purposeful optimization of the conditions of tritium labeling of practically any biologically active substance, which makes possible more detailed investigation of the functioning of living objects.