Spectrophotometric Studies of the Copolymer of l-Tryptophan and l-Glutamic Acid in an Aqueous Solution

Abstract

Abstract The synthetic copolymer of l-tryptophan and l-glutamic acid exhibited two specific bands, around 292 mμ and 234 mμ, in the difference spectrum at pH 4.68 vs. pH 7.60 in a 0.1m phosphate buffer. The former, that at 292 mμ, might be caused by the red shift of the ordinary absorption band around 280 mμ, and the latter, that at 234 mμ, by the red shift of the far-ultraviolet absorption band, around 220 mμ, of the tryptophanyl residue in the copolymer. This specific peak at 234 mμ in the difference spectrum of the tryptophan copolymer may be distinguished from that at 222 mμ, which is due to the hyperchromisity (n→π* transition) of the amide group in the peptide bond reported with poly-l-glutamic acid. The peak at 230–235 mμ appearing in the difference spectrum between native and denaturated proteins may be due to the same red shift (π→π* transition) of the far-ultraviolet absorption band of the tryptophanyl residue as that shown by the tryptophan copolymer. The difference in absorption at 234 mμ was five times larger than that at 292 mμ, and both of them increase proportionally with a lowering of the pH. The relation between the difference absorption and the pH was quite similar to that between the helical content and the pH. The relative fluorescence intensity of the tryptophanyl residue was also depressed with a lowering of the pH, and the profile of the pH-dependence curve on the fluorescence intensity was almost identical with the titration curve of the copolymer. Therefore, it may be said that the appearance of the difference spectrum at 234 mμ corresponds to the formation of the α-helix in polypeptide and that the protonation of the glutamyl residue induces the quenching of the fluorescence of the tryptophanyl residue.