Controlling shape selection and nanostructure in chiral assemblies

LAURA LEAY: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on the hot topics in materials research. My name is Laura Leay. For many of us, a fundamental understanding of nature was what led us to become scientists. In biology there are many mysteries to solve surrounding the self-assembly of molecules and larger structures. One collaboration led by a group at Northwestern University has now gained insight into biochirality. 

MONICA OLVERA DE LA CRUZ: You can imagine a chain of carbons and a head group that is an amino acid. Amino acids can be right-handed or left-handed. They can assemble differently depending on that chirality. 

LAURA LEAY: Monica Olvera de la Cruz works on soft matter and self-assembly so she’s intimately familiar with these molecules.   

MONICA OLVERA DE LA CRUZ: They have two parts that they prefer: one the water; another one is hydrophobic. And they self-assemble to hide the parts of them that don’t want to be in the water. So these molecules form many structures that are very similar to structures that are found in biological systems, for example cell-membranes. 

LAURA LEAY: These amphiphilic molecules were submerged in solution and the pH was varied. Using the intense photon source at Argonne National Lab, X-ray scattering showed that at low pH, molecules with a 16-carbon tail formed interdigitated crystalline bilayers but as the pH increased these lipid bilayers roll up. At very high pH this scroll shape becomes stretched out, forming a spiral. Sumit Kewalramani has a great example for visualizing this.  

SUMIT KEWALRAMANI: You have these ribbons on gift wrapping, right. So they’re elongated. They have a natural curvature based on how they have been manufactured. Similarly, chirality gives that curvature, and these are elongated, so by themselves they twist into helices. Instead of a ribbon, if I extend it in both dimensions you cannot make a helix; you will have to have overlaps between them. 

LAURA LEAY: The scroll and spiral shapes were also seen for molecules with 12- and 14-carbon tails. The degree of ionization is what controls the shape and this in turn is controlled by the external conditions. This phenomenon is incorporated into a membrane energetics model.  

SUMIT KEWALRAMANI: The membrane energetics model considers just two parts. One is the electrostatic energy, because all the molecules are charged although screened because they are in a solution with ions. And the other part is: they don’t like water, the tails. On the edges the tails are exposed to water so they want to minimize their perimeters so as to reduce contact with the water. When they are highly charged they want to be extended: that reduces the electrostatic energy. But when you screen the charges they can grow in both directions because that reduces the perimeters. 

MONICA OLVERA DE LA CRUZ: The simulation that Sumit is talking about was very illuminating. That told us more about in which regime we were expecting to have something that will not grow laterally. You know, if it was strongly charged then it could only grow in one dimension but not laterally because that was creating too much electrostatic repulsion. It’s like if you put charge more and more and more in the region, you have to cancel it by some of the solution ions and that causes a lot of entropy decrease so to balance that they just decide to grow in a limited way.

LAURA LEAY: The model moves us one step closer to building entire cells in the lab and could lead to the development of nanotechnology such as drug delivery and electronics. These discoveries were made possible by a multidisciplinary team using molecular dynamics to help interpret the x-ray scattering data. 

SUMIT KEWALRAMANI: From the X-ray we got information on the long length scales, like what is the radius of these helices, what is the pitch of these helices. MD simulations were used to study the local packing and we fit those wide angle diffraction data with different parameters in MD models and we were able to figure out, yes the molecules are tilted.   

LAURA LEAY: Other experimental techniques also really helped determine the structures, as Michael Bedzyk explains. 

MICHAEL BEDZYK: We also complemented the scattering with x-rays with cryo-electron microscopy which was very important for actually seeing the scroll shapes. And likewise, in situ atomic force microscopy. Both of those were really important aspects to really prove that we had these scrolls or twisted ribbons. 

LAURA LEAY: Discoveries like this elegantly demonstrate why multidisciplinary teams can really push the boundaries of what we know. Joseph McCourt, lead author on the paper, explains how complementary these techniques are using an analogy developed by his co-author, Sumit. 

JOSEPH McCOURT: Sumit has a great analogy for, like, how useful it is to have – sort of – real pictures of these cryo-TEM or the atomic force microscopy or electron microscopy images. When we do X-ray scattering we can look at the whole neighborhood of structures and all the different ones they form whereas if you look at AFM – real space images – you’re getting just one house. So you really get to dig in to what one of these houses in the neighborhood looks like, and you’re knocking that door, as opposed to X-ray scattering: you get to see the whole town. And it’s how robust these structures are in an entire group of them. You can zoom in on one of them to get more features. 

LAURA LEAY: This work was published in a recent issue of ACS Central Science. My name is Laura Leay from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on twitter, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.