Indole canonical structures

Both IR and UV/vis action spectra were used [139] to characterize the radical cations of Trp-containing peptides, in particular AcGly3TrpNH2. Both types of spectra indicated a canonical n-radical cation structure, ruling out the possibilities of forming a zwitterion, or of an ionization site other than the indole jt-system. The spectra are shown in Fig. 15 (compared with the IR spectrum of the protonated peptide). These spectroscopic probes are seen in general as useful structural diagnostics for ionized peptides containing a Tip residue. 

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