The finding of the enhanced polymerization of N,N-carbonyldiimidazole (CDI) activated L-alpha-glutamic acid (Glu) by alkali cations like Na+ and K+ and CDI-activated L-alpha-arginine (Arg) by halide anions like Cl- and other common ions indicates the importance of the weak electrostatic coordination of these common ions on the polymerization of charged amino acids in H2O. Here the copolymerization Of L-arginine-N-carboxyanhydride (L-Arg-NCA) mixed with L-glutamic acid-N-carboxyanhydride (L-Glu-NCA) produced much longer peptides than that of the same concentration Of L-Arg-NCA or L-Glu-NCA, and the optimal proportion of L-Glu-NCA over L-Arg-NCA for the longest peptide formation was 1:1. NaCl affected the sequences of the peptides formed in the mixed L-Arg-NCA with L-Glu-NCA, yielding peptides with significantly more Arg residues. than Glu residues. Enzymatic hydrolysis of the formed peptides uncovered significantly more two residues of the same charges. These data strengthen the importance of weak electrostatic interactions in the polymerization of charged amino acids in H2O.