Poly-L-lysine provides a good model compound for understanding protein interactions as the peptide secondary structure and aggregation state can be tuned by changing the solvent conditions and the temperature. In this work, static light scattering experiments have been used to determine the two-body interactions of poly-L-lysine (PLL) and the preferential interaction parameter with salt as a function of ionic strength for the solutions of KSCN, KCl, KNO3, K2SO4, and LiCl at pH 7 and at pH 9. For solutions of KSCN, PLL-PLL interactions become more attractive with increasing ionic strength at either pH 7 or at pH 9 as indicated by a concomitant reduction in the osmotic second virial coefficient (B22). The effect is attributed to the binding affinity of SCN- to positively charged side groups of PLL. Interestingly, PLL-PLL interactions are more repulsive as ionic strength increases in solutions containing either KNO3, or K2SO4 at pH 7 and at pH 9 or solutions of KCl at pH 7. This observation is in contrast to the general finding of polyelectrolyte solutions in which added salt leads to screening of repulsive electrostatic interactions. One explanation for the salt-induced repulsion is that the size of PLL increases when salt concentration is increased. This hypothesis has been confirmed by using dynamic light scattering to monitor the hydrodynamic size of the homopolypeptide. Further, we have studied solutions of LiCl as it is known that preferential binding occurs between the lithium ion and the polypeptide main chain at low salt concentration [1]. Our results are in good agreement with the literature in which the pair interactions of PLL are significantly repulsive for the salt concentration range between 0.1 M and 0.5 M. This finding indicates that the increase in size of PLL might be linked to non-specific preferential interactions between salt ions and the dipole moment of the peptide backbone.

The preferential interaction parameters of PLL with salt, , are negative for all salt solutions at pH 7 and at pH 9, indicating there is a deficiency of salt in the peptide domain relative to the bulk solution. This ion-specific exclusion reflects the salting-out of the peptide hydrophobic groups.

References:

[1] Juan A. Martin, Rosa Olmo, Jesus M. Socorro, M. Dolores Blanco and Jose M. Teijon; Polymer international, 1997, 42, 218-224