Ordered structure in dilute solutions of poly-<i>L</i>-lysine as studied by small-angle x-ray scattering

Abstract

The small-angle x-ray scattering curve shows a clear peak for aqueous solutions of poly-L-lysine (PLL) of a degree of polymerization of 406 when the polymer concentration is between 0.02 and 0.06 g/m1. In the presence of a large amount of NaBr (0.3 M), the peak disappears. A PLL sample of a degree of polymerization of 13 shows no scattering peak but a broad shoulder at a different scattering vector. Mixing of the two fractions gave a new shoulder at a scattering vector intermediate between the positions of the mother fractions, suggesting that the peak is due to intermolecular ordering. The interparticle distance (2Dexp), calculated by assuming the Bragg equation, decreases with increasing PLL concentration, and increases with increasing salt concentration. The 2Dexp value for the high molecular weight sample is always smaller (by a factor of 2) than the average intermacroion distance (2D0) calculated from the concentration. This suggests that a two-state structure is maintained. This fact and the dependences of 2Dexp on salt concentration and degree of ionization indicate that the macroions attract each other in dilute solutions through the intermediary of counterions.