Kinetics and mechanism of decomposition of N-chloroamino acids. II: conformationally restricted models

Chloramine-T in radiolabeling techniques. IV. Penta-O-acetyl-N-chloro-N-methylglucamine as an oxidizing agent in radiolabelling techniques

Chloramine-T (CAT) is commonly used in radiolabeling of bioactive molecules by halogenation. CAT is used to release radioactive elemental iodine by oxidation of its salts. Unfortunately, CAT is a strong oxidizing agent and can cause significant damage to peptides and proteins. This may lower the yield of the iodination reaction and may produce undesirable side products. Recently, it was found that the in situ formation of N-chlorosecondary amines, by the addition of secondary amines to CAT prior to exposure to the substrate, can reduce the oxidative damage caused by CAT. To simplify the method, we prepared penta-O-acetyl-N-chloro-N-methylglucamine (NCMGE) as a solid N-chlorosecondary amine. The chemical reactivity of NCMGE toward a model amino acid, 1-aminocyclohexane carboxylic acid, was compared with that of chloramine-T. In the presence of the model amino acid, CAT lost all its chlorine titer within 60 min while NCMGE retained 99% of its chlorine titer. NCMGE was compared to CAT for the iodination of l-tyrosine and leucine enkephalin. For both substrates, the NCMGE method produced larger or equal yields of the monoiodo and diiodo products and less decomposition. It is proposed that the method employing NCMGE to release diatomic iodine is more convenient and efficient for radiolabeling peptides and proteins than currently used methods.

Solvent effects on chemical processes. 11. Solvent effects on the kinetics of decarboxylative dechlorination of N-chloro amino acids in binary aqueous-organic solvents

The phenomenological theory of solvent effects is extended to chemical reaction rates and is tested against experimental data on the decarboxylative dechlorination of N-chloroalanine and N-chloroleucine at 25 degrees C in binary aqueous-organic solvent mixtures. The organic cosolvents studied were methanol, ethanol, l-propanol, 2-propanol, ethylene glycol, propylene glycol, acetonitrile, and dioxane. Reaction rates increased in all cosolvent systems. The kinetic solvent effects could be quantitatively described by the theory, and the parameters of the theory (solvation exchange constants K1 and K2 and cavity surface area parameter delta g++) were found to possess magnitudes reasonable for the physical significance assigned to them. In particular, the delta g++ value is consistent with a recent measurement of the volume of activation delta V++ of the reaction.