Wilzbach labeling

As a whole, these investigations provided valuable criteria to differentiate between the contributions ofthe various labeling mechanisms, and to determine their relative weight in the formation of the tritiated products. The currently accepted view (cf. Evans, 1966) is that the Wilzbach labeling method is essentially based on radiation-promoted processes, which largely predominate over the reactions of the HeT+ ions, as demonstrated by the fact that several T atoms are incorporated following each /8-decay. Under typical preparative conditions, the... 

The results5,20,22,26-31 of the Wilzbach labeling of a representative number of heterocyclic compounds are shown in Table I. A significant feature of the data is the relatively high specific activity in the crude product immediately after exposure to tritium gas compared with the ultimate activity in the purified compound. For example, the specific activity of crude benzothiophene (777 pCi/mg) dropped to 155 pCi/mg in the purified compound. Comparable data for N-methylpyrrole and trimethylpyridine were 247, dropping to 110, and 423, dropping to... 

M. Wenzel andP. E. Schulze, Tritium Markierung Preparation Measurement and Uses of Wilzbach Labelled Compounds. De Gruyter, Berlin, 1962. 

Comparison of Wilzbach Labeling of Heterocyclics with That of Aromatic Hydrocarbons... 

Although most Wilzbach labeling work published has concerned non-heterocyclic molecules, sufficient heterocyclic compounds have been tritiated by this procedure for satisfactory predictions of labeling patterns... 

Radioactive compounds are obtained in various ways 1. by synthesis in the laboratory 2. biosyntheti-cally, using the specific synthetic capacities of organisms 3. by radiochemical methods, e.g. tritiated compounds by Wilzbach tritiation. The second and third technique often yield unspecific, uniform labeling, but not necessarily an equal degree or labeling of all atoms. For example, in Wilzbach labeling, the C-T bonds are stable, but the O-T and N-T bonds are labile. 

By exposure of the antibiotic actinomycin D (VI) to tritium gas (the Wilzbach labeling method) high specific activity [ H]actinomycin D is produced. After back-exchange of any labile... 

The mechanism of the Wilzbach method applied to the labeling of aromatic compounds was also the subject of a number of investigations, usually involving the measurement of the intramolecular tritium distribution within the labeled aromatic substance. Whilst toluene was the most frequently employed aromatic substrate (Aliprandi et al., 1960 Cacace andPossagno, 1960 Cacace, 1961a Ache etal., 1961, 1962 Ache and Herr, 1962 Ache, 1967), chlorobenzene, nitrobenzene and anisole (Cacace et al., 1960) and anthranilic acid (Crawford and Garnett, 1965 Garnett et al., 1965) have also been studied. 

Regarding the labeling of substituted heterocyclics, little work has been done by the Wilzbach process. Trager and Wacker32 exposed compounds in the pyrimidine series (2)—(4) to tritium unfortunately, the number of curies of tritium used was not reported, although it was constant for all compounds. The maximum level of activity incorporated into the molecules is summarized in Table II. 

Despite the paucity of data for orientation studies with substituted heterocyclics labeled by the Wilzbach method, extensive work has been done with analogous aromatic compounds and, thus, by extrapolation, a number of general predictions can be made for heterocyclic systems. For example, in toluene34 tritium incorporation in the ring predominates over side-chain labeling. 

Alternative methods for accelerating the Wilzbach procedure include irradiation with ultraviolet light,47-49 X-irradiation,45,50 use of microwaves,51 and addition of iodine.52 The accelerated labeling is... 

Fig. 1. Various possibilities for labeling 4-picoline. (1) Dilute DC1 at 100°C, (2) PtOa (self-activated) + D2O, (3) dilute HClat 100°C, (4) C0CI2+ NaBH4 +D2O, (5) Pt02 or NiClg + NaBH4 + DaO, (6) PtOa (self-activated) + H20, (7) dilute DC1 at 100°C, (8) CoCl2 + NaBH4 + HaO, and (9) Wilzbach or TaO. (1), (3), and (7) from self-activation of aqueous solutions of I PtCle.

Chloral hydrate - t ( H-labelled chloral hydrate) has been prepared267 by the Wilzbach technique, using 8Ci of (12mi . ) and subjecting to Tesla excitation for 15 minutes. The specific activity of the product was 30 - 110 mCi/g. 

The Wilzbach procedure is a simple method of labelling organic compounds with tritium The substance is exposed to tritium gas in a sealed glass vessel for several days or weeks. During this period, some of the hydrogen atoms of the molecule to be labelled are substituted by H atoms. Chemical side reactions may take place however at the same time, e.g., unsaturated compounds may be partly or completely hydrogenated. Several firms carry out this type of labelhng on request [312]. 

A method for preparing tritium-labelled oligosaccharides of hyaluronic acid has used the Wilzbach recoil technique the tritiated oligosaccharides are of potential use in studies of the mechanism of hyaluronidases. A four-fold helical form of hyaluronic acid has been detected using the data from fibre-... 

The tritium gas-exposure method for radiation-induced exchange labeling of organic compounds was introduced by Wilzbach in 1957 and has come to be known as the "Wilzbach method" j . In this method the compound to be labeled is exposed to curie amounts of carrier-free tritium gas in a sealed flask for a period of several days to several weeks. The activation energy necessary for exchange of stable hydrogen atoms in the compound with tritium gas is derived from the 6-decay of tritium. Tritium... 

Exposure to tritium gas generally produces nonspecific labeling of organic molecules this technique has been described by Wilzbach (1957). Using the Wilzbach technique, Dutton et (d. (19 ) and Nystrom et ed. (1959) synthesized labeled saturated fatty acids and esters of high specific activities. Unsaturated fatty acids added tritium at the double bonds rather than replacing hydrogens on the carbon chain (Dutton et al., 1962). 

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