The Anomalous Absorption of Tryptophan

The influence of polarizable organic molecules, e.g. organic acids and ionic detergents, on the light absorption of indole, tryptophan and its derivatives was studied. The interaction of these compounds with the chromophore, in addition to the red shift due to solvent perturbation effect, results in an “anomalous” blue shift in the 295 to 310‐nm region. The nature of spectral changes depended on the character of the interacting solvent and solute molecules: a net positive electrostatic field induced red shift, whereas a negative one caused blue shift. The absorption changes increased with the size of nonpolar part of the polarizable molecule. The spectral changes can be assigned to the electrostatic attraction or repulsion of electrons of the indole nitrogen according to the electrostatic character of the polarizable molecule interacting with the chromophore. The energy of indole chromophore‐polarizable molecule interaction was estimated from the data of thermal perturbation difference spectra. The calculated upper limit of this interaction energy was 0.04 eV, which suggests that in aqueous solution van der Waals interactions are formed between the chromophore and the compounds c