Spectral studies with histidyl-tryptophan peptides.

A series of linear and cyclic derivatives of histidyl-tryptophan was synthesized for the study of intramolecular indole-imidazolium interaction. The absorption spectrum and the fluorescence intensity of each of the peptides were examined in aqueous solution when the imidazole side chain was protonated (pH 9). The absorption spectra of the protonated peptides show hypochromism around 280 mμ, whereas at shorter wavelengths hyperchromism appears. The hyperchromism is most intense around 240 mμ and it is attributed to the charge-transfer band of the intramolecular indole-imidazolium complex. The fluorescence intensity of all tested histidyl-tryptophan peptides drops markedly with protonation of the imidazole side chain. The change in fluorescence intensity with pH superimposes to the potentio-metric titration curve of the imidazole residue. From the preceeding results it may be concluded that (a) the electron affinity of imidazolium is smaller than that of the positively charged nicotin-amide by 1.2 eV and (b) the indole-imidazolium complex is not of a sandwich type. It is suggested that the charge-transfer bond of this complex is located between the C-2–C-3 bond of the indole and the plus charge center of the imidazolium.