Unexpected Like-Charge Self-Assembly of a Biguanide-Based Antimicrobial Polyelectrolyte

Polyelectrolyte chains dissolved in good solvent are expected to collapse in compact configurations in the presence of multivalent ions. Here, we show that a weakly charged, hydrophilic polyelectrolyte containing biguanide groups self-assembles in water also in the presence of monovalent counterions, even at low salt concentrations. The polymer assembles in a compact, ordered, hairpin-like shape that, with increasing the ionic strength of the solution, can collapse further in three- or five-folded structures. Neither water nor ions mediate the self-assembly which, instead, is driven by the like-charge pairing of the biguanide units. The thermodynamics of the self-assembly show that the self-association is enthalpically driven, is isodesmic (at least at low aggregation number), and is favored by increasing salt concentration. This unique self-assembly behavior may be linked to the well-known polymer's antimicrobial properties and could help in rationalizing its biological activity.