Molecular crystallization provides an experimentally simple way to arrange molecules on the nanoscale. Synthetic polypeptides present the potential to control ultimate crystal structure by design of primary structure and/or secondary structure adjustment in solution during crystallization. In this work, hexagonal single crystals of synthetic poly-L-lysine (PLL) were produced from aqueous solution by the addition of divalent counterions. It was found that PLL having 100-200 amino acids crystallized in an unfolded-chain form with the crystal thickness controlled directly by PLL molecular weight. PLL chains exhibited chain folding in the crystal state when the PLL was greater than 265 residues. PLL with less than 100 residues were not crystallizable under the same crystal growth conditions. X-ray diffraction data revealed that PLL chains had an alpha-helical secondary structure and hexagonal symmetry in the crystal state. The introduction of divalent counterions is necessary to initiate the crystallization process and stabilize the crystal structure.