COMMUNICATIONS TO THE EDITOR
Optical Activity of the Charge-Transfer Complex Between Poly-L-Histidine Hydrochloride and Indole
Chargetransfer complexes between indole and imidazolium derivatives including the protonated side chain of histidine have been the subject of recent thermodynamic and spectroscopic investigations.'-4 Intra-as well as intermolecular indole-imidazolium interactions of the charge-transfer type were also found to exist in various histidine-and tryptophan-containing peptides and in several protein~.3.~.6 These complexes are of biological interest because they presumably influence the structure and function of proteins containing tryptophan and histidine, even though the binding forces between these two residues are weak and strongly dependent on pH and orientation. In the present communication we wish to report our preliminary results on the formation and the optical properties of a poly-Lhistidine hydrochlorideindole complex.
The association constant K was determined by measuring the increase in solubility of indole in water on addition of poly-Lhistidine hydrochloride, assuming that it is due t o the formation of a water-soluble complex. For this purpose a saturated, warm solution of pure indole in water in the presence of 0.014 M sodium chloride was allowed to stand at room temperature for several days. The solubility was then determined from uv absorption data. By the same procedure, the solubility of indole in the presence of poly-Lhistidine hydrochloride (conc. 0.014 mole residues/l.) was obtained.
From these data the association constant was calculated to be 2.2 * 0.3 1. mol-1 in