Among marine natural products, halogenation of indole rings is common, due to the enzymatic action of haloperoxidases. Recently, bromotryptophan has been found in peptides from lower marine vertebrates, but the biological effects of brominating tryptophan are not yet understood. In studying the biophysical properties of these compounds, we have observed that UV spectra of halogenated indoles and tryptophan in solution show a distinctive red-shift in their λmax values for the band corresponding to their π- π* transition. Time-dependent density functional theory (TDDFT) calculations reveal an increasing trend in the energy of the HOMO-LUMO transitions: 5-< 6- < 7- < 4-bromoindole regioisomers. The energy differences, arising from the relative destabilization of the HOMOs, thus parallel the experimentally observed red-shifts. In solution, the largest red-shift is observed for the 5-regioisomer (Δλmax = 9 nm), followed by the 6-regioisomer (Δλ max = 6 nm). The red shift shows only a very slight dependence on % organic co-solvent, of less than 1 nm. These observations can be applied in a new method to identify the different regioisomers in solution, using the value of the UV red-shift to distinguish these brominated natural products.
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