The difference spectra of many proteins containing tryptophan are found to exhibit an atypical extremum around 300 mμ under certain conditions. This paper reports on a systematic study of some model compounds, namely, tryptophan and some of its derivatives, which was designed to improve our understanding of this unusual effect. Our study leads to the following conclusions. (1) The extinction difference observed at 300 Mμ when the solvent is changed behaves quantitatively and qualitatively quite differently from the differences observed at shorter wavelengths; for example, changing the aqueous solvent by the addition of an organic solvent caused a characteristic positive difference between about 275 and 295 mμ, but a sometimes negative difference at 300 mμ. (2) The value of the extinction observed at 300 mμ is not changed very much by changes in the nonpolar nature of the medium. It is much more sensitive to changes in the electrostatic environment of the chromophore. (3) There is strong, but not conclusive, evidence that the unusual spectral behavior of indole chromophores at 300 mμ is due to the existence of a minor absorption band, buried under the long-wavelength side of the main π-π* bands. The model compound studies reported in this paper are used in the following paper to interpret, in a self consistent way, the hitherto puzzling spectral behavior of some tryptophyl-containing proteins. © 1969, American Chemical Society. All rights reserved.
