Characterization of linear and cyclic peptides. As previously described, the in situ generation of TADcontaining peptides (int-16) is qualitatively observed by the characteristic solution color change to carmine (Fig- ). The reactivity of these intermediates (int-16), generated in the presence of Tyr residues, precludes their isolation by reverse-phase HPLC. While optimizing the oxidation and cyclization events using analytical methods that involve diluting reaction aliquots in water (e.g., RP-HPLC analysis with UV-detection), only urazolecontaining peptides (14), cyclic peptides ( ), and minor byproducts were observed. Intriguingly, the carmine color of in situ generated solution of int-16 dissipates within minutes of diluting with the phosphate buffered acetonitrile-water solution. This suggests TAD-peptides (int-16) are rapidly cyclized to give cyclic peptides ( ) under these conditions. Following purification, a comparison of the analytical HPLC chromatograms for acyclic ( ) and cyclic peptides (16) shows that cyclization induces a general polarity change. This property is exemplified by the comparative chromatograms for urazolecontaining peptide 14i (tR = 11.