Chemical modification of the tryptophan residue in cobratoxin

The single tryptophan residue in cobratoxin has been converted into N′formyl-kynurenine by ozonization in formic acid, and also modified by reaction with 2-hydroxy-5-nitrobenzyl bromide. The data from amino acid analysis of modified derivatives show that these reactions are specific for tryptophan, and do not affect the other amino acids. The low toxicity of these modified products points to the importance of the intact tryptophan residue in cobratoxin for full activity.