Reactions of allyl isothiocyanate with alanine, glycine, and several peptides in model systems

The nucleophilic addition reactions of allyl isothiocyanate (AITC) with ala nine, glycine, and five alanine and/or glycine containing di- and tripeptid es were investigated in model aqueous solutions of pH 6, 8, and 10 at 25 de grees C for 2-4 weeks. The formation of primary adducts, i.e., N-allylthioc arbamoyl amino acids (ATC-amino acids) or ATC-peptides, their transformatio n products, i.e., 3-allyl-2-thiohydantoins originating by cyclization of AT C-amino acids or by cleavage of ATC-peptides, and several other minor compo nents were observed. The results revealed that both addition and cleavage r ates rise proportionally to pH,whereas the formation of 2-thiohydantoins fr om ATC-amino acids is controlled by H3O+ concentration. Depending on pH, di fferences in reaction rates of the additions are determined by either pK(a) (NH2) of amino compounds or electrical effects and steric hindrance of the molecules. The latter factors are crucial also for differences in cleavage rates of ATC-peptides. With regard to the pK(a) values and simultaneous AIT C decomposition by aqueous nucleophiles, the reactions with amino acids and oligopeptides are predominant reaction pathways of AITC in solutions of pH 10 and 8, respectively. Reaction mechanism of the cleavage of 2-thiohydant oins from ATC-peptides in alkaline and mild acidic solutions is different f rom the conventional Edman scheme used for anhydrous acid medium.