The use of proteins in advanced nanotechnological applications requires extended stabilization of the functional protein conformation and enhanced activity. Here we report that simple cationic poly(amino acid)s can significantly increase the activity of the multidomain protein supercomplex Photosystem-I (PS-I) in solution better than other commonly used chemical detergents and anionic poly(amino acid)s. We carried out a systematic analysis using a series of poly(amino acid)s (i.e., poly-L-tyrosine, poly-L-histidine, poly-I.-astral-tic and poly-L-glutamic acid, poly-L-arginine, and poly-L-lysine). Our results show that the polycations poly-L-lysine and poly-L-arginine significantly enhance the photochemical activity of PS-I, whereas negatively charged and hydrophobic poly(amino acid)s did not increase the PS-I functionality in solution. Furthermore, we show that poly-L-lysine can stabilize highly active PS-I in the dry state, resulting in 84% activity recovery. These simple and inexpensive poly(amino acid)s will likely make significant contributions toward a highly active form of the PS-I membrane protein with important applications in nanotechnology and biotechnology.