Radiochemical derivatization

Very little work has been carried out on radiochemical derivatization for analysis of trace amounts of materials. The technique has the advantage of being both selective and sensitive. Die main advantage is that the sample background does not cause interference in the detection as it does in most other methods and which necessitates some degree of clean-up. Also, the reactions used are those for normal derivatization procedures, the only difference being that the reagent is radiolabeled and that appropriate precautions are required for radioactive substances. The few methods described below illustrate the application of this technique. 

The recovery of 1 from the heptane extract of dog plasma by normal phase HPLC was reproducible over the range of plasma concentrations studied. Equivalent overall recoveries were obtained by both radiochemical analysis (83.7 1.8% SE) and electron-capture GLC analysis (84.0 4.9% SE) of the derivatized tetrahydrocannabinol. ... 

Proof that a lysine residue has been modified can be readily obtained because pyridoxyl derivatives of lysine possess characteristic white-blue fluorescence (Ronchi et al. 1969). In addition, they have a distinctive absorption maximum at 325 nm with of 9710 cm (Fisher et al. 1963). Finally, a radiochemical label can be introduced by reducing the pyridoxal-5-phosphate protein complex with tritium-labelled sodium borohydride. The peptide containing the derivatized lysine can therefore be detected either by fluorimetry, spectrophotometry or radiochemical techniques following routine procedures of proteolytic digestion and fractionation. Acid hydrolysis in 6 N HCl for 24 hr of peptides containing pyridoxal-5-phosphate lysine yields pyridoxyl-lysine since phosphate esters are readily hydrolyzed under these conditions. Pyridoxyl-lysine is eluted between lysine and histidine from a 55 cm column of Beckman 50 resin with 0.15 M citrate buffer pH 5.28. 

The first application of copper(I)-catalyzed 1,3-dipolar cycloaddition in preparation of [18F]fluoropeptides was reported by Marik and Sutcliffe in 2006 (Figure 14.9) [92]. Three [18F]fluoroalkynes (n = 1, 2, and 3) were prepared in yields ranging from 36% to 80% by nucleophilic substitution of a p-toluenesulfonyl moiety with [18F]fluoride ion. Reaction of these [18F]fluoroalkynes with various peptides (previously derivatized with 3-azidopropionic acid) via the Cu(I)-mediated 1,3-dipolar cycloaddition provided the desired 18F-labeled peptides in 10 minutes at room temperature with yields of 54-99% and great radiochemical purity (81-99%) [82]. 

The half-life of thromboxane A2 in platelet-poor plasma was determined as described (9). Briefly, radiolabelled thromboxane A2 was generated from human platelets as enzymatic source and immediately dived in the studied plasma. Then, aliquots of plasma were decanted in 80 volumes of methanol to obtain mono-o-methyl thromboxane B2 from thromboxane A2 (10). Derivatized thromboxane B2 was radiochemically assayed (8) and a semi-logarithmic regression leads to half-life determination. 

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