For years, we've known that a special kind of molecular assembly known as a "polyelectrolyte complex" helps your cells keep themselves organized. These complexes are very good at forming interfaces to keep two liquids separated: your cells use them to create compartments.

 

But for decades, no one knew exactly how the regions looked inside a polyelectrolyte complex. A new study from the University of Chicago's Pritzker School of Molecular Engineering has laid out the internal structure of polyelectrolyte complexes for the first time. First, postdoctoral researchers Artem Rumyantsev (now on the faculty of North Carolina State University) and Heyi Liang developed molecular models and carried out thousands of simulations, as well as theoretical calculations based on statistical mechanics, to understand the most likely way these molecules would assemble. Next, a group led by graduate student Yan Fang and postdoctoral researcher Angelika Neitzel (now at the University of Florida) worked to create precise versions of these molecules in the laboratory and use an advanced technique to determine their structure.

 

One of the few ways to see the fine details of such molecules is with a technique called neutron scattering. This is done by sending beams of neutrons -- the neutral particles that make up atomic nuclei -- at the molecules, and then reconstructing their patterns from the way the neutrons scatter away. But normally, the positively charged and negatively charged chains look the same when you do this. To tell them apart, the researchers used a clever trick. Both chains have hydrogen atoms in them. But the team replaced the hydrogen atoms in the positively charged chains with a very slightly different version of hydrogen, known as deuterium, which shows up differently when the neutrons scatter off it. Using this approach, they could see that the chains did have distinct small-scale repeating patterns, though they were not rigorously ordered over long distances.