The attraction of like-charged biomolecules

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...