Guanidinium can Break and Form Strongly Associating Ion-Complexes

Guanidinium is one of nature's strongest denaturants and is also a motif that appears in several interfacial contexts such as the RGD sequence involved in cell adhesion, cell penetrating peptides, and anti-microbial molecules. It is important to quantify the origin of guanidinium’s ion-specific interactions, so that its unique behavior may be exploited in synthetic applications. Here we show that guanidinium ions can both break and form strongly associating ion-complexes in a context-dependent way. These insights into guanidinium's behavior are elucidated using polyelectrolyte complexes (PECs), where inter-polymer ion-pairs between oppositely charged polymers play an important role in determining material stability. We demonstrate that guanidinium salts are very effective in dissolving the poly(styrenesulfonate)/poly(allylamine) (PSS:PAH) complex, which has one of the highest measured polycation-polyanion association affinities. We also demonstrate that incorporating guanidinium charges directly into the polyelectrolyte results in a complex that remains stable under highly denaturing conditions. The model system of PSS:PAH is used to glean insights into guanidinium's denaturing activity, and to broadly comment on the nature of ion-specific interactions in charged macromolecules.