C-Terminal tryptophan containing peptides have been converted into 1-peptidyl-3,4-dihydro-β-carboline-3-carboxylic acids by acid catalyzed isomerization of their azlactone derivatives. The main properties of these abnormal peptides are a strong absorption around 360 nm an intense yellow fluorescence and ability to generate reactive carbanions when dissolved in basic media. This tryptophan conversion