Polyion complex (PIC) formation is an attractive method for obtaining molecular assemblies owing to their facile fabrication process in aqueous media, but more insights are required in order to control the higher‐dimensional structures of polypeptide‐based PICs. Herein, the PIC formation behavior of oppositely charged homochiral polypeptides, poly‐l ‐lysine and poly(ethylene glycol)‐b‐poly(l ‐glutamate) (PEG‐PLG), and their secondary structures are carefully studied in water. PIC formation takes place in a polymer concentration‐dependent manner, and clear β‐sheet formation is observed at polymer concentrations ≥0.3 mg mL−1. The results also confirm that multimolecular aggregation is a prerequisite for β‐sheet formation, which indicates that the inner hydrophobic environment of PICs is favorable for β‐sheet formation. Furthermore, the PEG weight fraction, stereoregularity of the polypeptide, and ionic strength of the solutions are found to be key factors for generating a secondary structure, presumably because these factors can contribute to the tuning of the inner environment of PICs. This method of producing water‐soluble nanoassemblies from oppositely charged polypeptides may expedite self‐assembly studies in biological systems and be incorporated into various molecular systems to exploit protein‐mimicking features.
