The attraction of like-charged biomolecules
DOI: 10.1063/1.4796901
To each other is beginning to yield its secrets. The like-charge attraction occurs with polyelectrolytes, large molecules that have a net electric charge in an aqueous solution. (See the article by William Gelbart, Robijn Bruinsma, Philip Pincus, and Adrian Parsegian in Physics Today, September 2000, page 38 .) Researchers have long recognized the importance of multiply charged counter-ions—small dissolved ions having the opposite sign of charge as the biomolecule of interest. Now, a group of experimenters led by Gerard Wong (University of Illinois at Urbana-Champaign) has investigated the role of counterions in a series of experiments. They found that charged filamentary actin molecules could self-organize into an unexpected liquid-crystal phase—a stack of two-dimensional rafts—and that divalent (doubly charged) ions provided the crucial cross-linking between both the filaments and the rafts. Divalent ions of magnesium, calcium, strontium, and barium all worked. The ion-induced changes may play an important role in the restructuring and regulation of the cytoskeleton. Studying counterions fashioned from dumbbell-shaped divalent molecules with variable lengths, Wong’s group found that the most effective ones were the smallest. Since the effective screening length can approach molecular dimensions, the smallest ions could fit within the “screening sheath” and create a localized charge inversion that promoted attraction. The larger dumbbells could not, and behaved like two separate monovalent ions. Working again with filamentary actin, Wong and colleagues found that counterions organize themselves into columns and form frozen counterion-density waves between the protein rods. Remarkably, the tiny ions induce the large actin molecules to twist, which facilitates like-charge attractions. (
