AbstractThe charge‐transfer complexes of a poly(L‐tryptophan) sequence with imidazolium hydrochloride and poly(L‐histidine hydrochloride) have been investigated in 2,2,2‐trifluoroethanol by ultraviolet (uv), circular dichroism (CD), and fluorescence techniques. Both complexes exhibit absorption maxima centered at around 275 nm, whereas hypochromism with respect to the combined spectra of the constituents can be observed below 250 nm. All complexes show optical activity in the near uv and in the peptide absorption region, which is discussed in terms of the conformational properties of the donor. A marked decrease of the fluorescence intensity of theL‐tryptophan sequence is observed upon addition of imidazolium hydrochloride and poly(L‐histidine hydrochloride). From the fluorescence data the formation constant of the charge‐transfer complex between theL‐tryptophan sequence and imidazolium ions is