Single crystals of poly‐L‐lysine

Abstract

AbstractCrystals of poly‐L‐lysine have been grown from aqueous solution in the presence of divalent anions. The most stable of these incorporate the HPO ion and are precipitated by the addition of sodium monohydrogen phosphate to solutions of poly‐L‐lysine HBr. Precipitation at or slightly above room temperature gives rise to single crystals of α‐poly‐L‐lysine HPO4 in the form of hexagonal lamellae about 150 Å thick. The axes of the helical polypeptide molecules are oriented normal to the planes of the lamellae, and since molecular length is about 1100 Å in the α‐helical conformation, these helices must be folded. The a parameter of the hexagonal unit cell is 19.55 Å for crystals immersed in mother liquor, and the lysine side chains are almost fully extended. Precipitation brought about by heating the same solutions to about 75°C produces micro‐crystals of β‐poly‐L‐lysine HPO4. A mode of packing of the anions in these crystals is proposed tentatively on the basis of an intersheet spacing determined from x‐ray powder diffraction patterns. In general, α crystals are transformed to β structures on drying; conditions under which the transition can either be forestalled or reversed are discussed.