Poly(arginine)‐Selective Coprecipitation Properties of Self‐Assembling Apoferritin and Its Tb3+ Complex: A New Luminescent Biotool for Sensing of Poly(arginine) and Its Protein Conjugates

The apoferritin protein and apoferritin–Tb³⁺ complex were demonstrated to form oligomeric and polymeric self‐assemblies in neutral aqueous solutions, based on characterization by using luminescence and UV/Vis spectroscopy, dynamic light scattering, and transmission electron microscopy. Addition of a 20‐mer or higher poly(arginine) to the solution resulted in coprecipitation through nanoscale interactions, while biological proteins and other poly(amino acids) rarely yielded precipitates under the conditions employed. The apoferritin–Tb³⁺ complex assembly exhibited a particularly long‐lived green luminescence in aqueous solution, and its poly(arginine)‐selective precipitation behavior was followed by monitoring the changes in luminescence. The poly(arginine)‐tagged albumin precipitated selectively and quantitatively, so that the apoferritin–Tb³⁺ complex can function as a new luminescent biotool for the sensing of poly(arginine) and its protein conjugates.