Practical Protein Chemistry - A. Darbre 1989

Polypeptide fragmentation by chemical methods
Cleavage at the tryptophan residue
Ozonolysis

During the ozonolysis of Peptides in anhydrous formic acid, the Tryptophan residue is converted into N-formylkynurenine (60) [125, 141, 142, 195, 196]. Subsequent mild acid Treatment results in the Cleavage of the N-formyl group. Brief heating (up to 4 h) of peptides containing the modified tryptophan residue (as kynurenine or N-formylkynurenine) in a bicarbonate buffer leads to the selective Hydrolysis of the peptide bond with a 49–59% yield. It is generally accepted that this peptide bond activation is driven by the intramolecular catalytic effect of the γ-keto group of N-formylkynurenine [143].

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Upon incubation of peptides containing N-formylkynurenine in 0.2 M hydrazine-acetate buffer (pH 3.6) at 100 °C for 2 h, hydrazones (61) are formed, which subsequently cyclize into tetrahydropyridazinones (62) with simultaneous cleavage of the peptide bond. The yield ranges from 35 to 68%, although non-specific cleavage is observed under the action of hydrazine [125–127]. Upon reduction of kynurenine peptides using NaBH₄ [144] or via electrolytic reduction [63], kynurenine is converted into γ-(o-aminophenyl)homoserine (63), followed by the cleavage of the C- and N-peptide bonds of the tryptophan residues with a 20–40% yield.

The cleavage of the tryptophan C-peptide bond via ozonolysis is not widely used in Protein Chemistry due to low yields, as well as The oxidation of Cysteine (to cysteic acid), Methionine (to methionine-S,S-dioxide), and partially Tyrosine.