Separation of Streptothricin Antibiotics from Culture Broth with Colorimetric Determination Using Dipicrylamine

Our earlier method for the detection and separation of ε-poly-L-lysine using a yellow anionic dye, the dipicrylamine (DPA^-) anion, was herein optimized for streptothricin antibiotics (ST), which contains the β-lysine oligopeptides moiety, H-[NH-(CH₂)₃-CH(NH₂)-CH₂-CO]_n-. We then applied this method to the detection and separation of ST in a commercially available nourseothricin, a mixture of ST species with n = 1, 2, 3, and 4. The ST species were precipitated with the DPA- anion. The precipitate was found to consist of the salts of the fully protonated ST species, ST^z+ (z = n + 1), with the DPA^- anion. The ST(DPA)_z precipitate was re-dissolved in acetonitrile. The solution was yellowish, and gave an absorption maximum at around 420 nm. Thus, the equivalent concentration of the ST species referred to the charge numbers of ST^z+ can be determined colorimetrically. By the addition of bis(triphenylphosphoranylidene)ammonium chloride, the ST species could be re-precipitated from the acetonitrile solution as hydrochloride salts. All of the ST species were found in the precipitate with high yields. The method was thus successfully applied to the detection and separation of ST species from the culture broth.