Title: Reversible self-assembly of superstructured networks
Authors: Ronit Freeman, Ming Han, Zaida Álvarez, Jacob A. Lewis, James R. Wester, Nicholas Stephanopoulos, Mark T. McClendon, Cheyenne Lynsky, Jacqueline M. Godbe, Hussain Sangji, Erik Luijten, Samuel I. Stupp
Self-assembling materials are of enormous interest due to their ability to form hierarchical structure from micron to millimeter level. One can think of a building as something with hierarchical structure, where the outside is visible, but it also has rooms and other features on the inside. Nature also forms superstructured networks where our tiny cells form together to build tissues and organs inside the body. Scientists often try to recreate these superstructure materials artificially with the long-term goal of tissue regeneration or organ transplants. Although exciting, these artificial superstructured materials are far from their biological counterparts in terms of flexibility, reversibility and stiffness. To bridge this gap, a team of researchers led by Dr. Samuel Stupp and Dr. Erik Luijten from Northwestern University developed a new type of dynamic reversible material with the help of DNA base pairing and peptide self-assembly. These new types of materials self-assemble and disassemble dynamically and reversibly and are thus able to change their shape and properties, which can open a new door to soft material-based superstructured systems. Hydrogels formed from these materials also helped in differentiating between healthy and diseased brain tissues thus with a potential application in monitoring brain microenvironment.
To create the bio-compatible materials, researchers looked into peptides and DNA, both of which are known to produce hierarchical structure inside the body through self-assembly. While DNA uses only hydrogen bonding to form so-called Watson-crick base pairing (https://en.wikipedia.org/wiki/Base_pair), peptides use other methods such as electrostatic interaction, hydrophobic interaction (https://en.wikipedia.org/wiki/Non-covalent_interactions) etc. It was already established before from the group of Dr. Stupp that peptides, when coupled with long hydrophobic tails, undergo a hydrophobic collapse to form self-assembled material in aqueous environment. Long alkyl chains tend to stay away from the hydrophilic environment and interact among themselves to form the assembled structure. Here, the researchers were trying to look how DNA base pairing affect these assembly processes. Self-assembly processes occur from two factors – i) hydrophobic collapse from alkyl tail and peptides and ii) opposite base-pair complementarity of the DNA strand (Fig. 1). The DNA superstructure initially formed a hydrogel and gradually it became stiffer with time indicating ordered structure formation reaching micron scale in both height and width. Scanning electron microscopy (SEM) images revealed the formation of segregated individual fibers with micron size of length (Fig. 2a).
The reversibility of the DNA strand formation was monitored by adding a strand breaking sequence (sequence which can break DNA base pairing resulting in isolation of two strands) and the superstructure returned to initial softer state (Fig. 2b). But the most exciting part was the formation of the hierarchical state upon addition of the anti-invader strand (sequence which can reform DNA base pairing resulting in formation of double strands) proving the reversibility of the process and which is difficult to achieve so far with these kinds of soft materials. Not only that but heating the gel at higher concentration also led to the disruption of hierarchical structures whereas cooling them back slowly returned to their original state.
Next, the researchers sought after computational calculation to figure out the contribution of each factor such as DNA concentration to the self-assembly procedure and the mechanism of formation of bundles. It was calculated and proved experimentally that for low density of DNA monomer, fiber formation does not lead to appreciable bundling whereas the high density of DNA monomer leads to a trapped ‘frozen’ structure without formation of structured bundling. An intermediate density is favorable which should be sufficient enough to form stable fibers but weak enough to induce reversibility. They also found out from the computational calculation that reversible superstructure formation is not limited to DNA base complementarity. To delve into this, they developed molecules with opposite charge complementarity only leaving behind the DNA base pairing. Similar superstructures were also observed from charge complementarity of the peptides and addition of HCl (neutralizes negative charge) or NaOH (neutralizes positive charge) dissolved the stiff gels to a softer state.
Lastly, the researchers investigated the potential biological application of these materials. These superstructures mimic the extracellular matrices (ECM) of the cell and it was observed that by changing the hierarchical architecture of the assemblies, astrocytes could be changed from ‘naïve phenotype’ to ‘reactive phenotype’ (Fig. 3). ‘Naïve phenotype’ generally refers to the astrocytes from healthy brain tissues and have a specific shape and once injured brain tissues change its structure to ‘reactive phenotype’ which has a different shape. Here, by changing the ECM environment the structures of the astrocytes were changed reversibly which can open a new door to the treatment of brain tissues.
These results are very promising and have the potential to change the treatment of neurological diseases in near future. Not only that, but other biological applications can also be explored such as targeted drug-delivery where the superstructures can be triggered to release the drug at a particular location. We can only hope that in the future similar types of research will bring forward new discoveries to combat diseases from different perspectives.
Disclaimer: Figures and articles are re-used purely for academic purposes with no intent of profit.