What is common between the DNA molecule carrying your genetic information from generations to generations and the plastic bottles you sip your water from? These “soft-matter’’ molecular systems are composed of polymers. Polymers are often relatively long molecular chains with complex and diverse chemical structures. Polymers, whether as the constituents of a biological or a human-made system, can respond to external stimuli on the order of thermal fluctuations, such as mechanical stress, electric field, flow, pH level, or temperature/concentration gradients, etc… They can also transiently adapt geometrical constraints. While all this versatility of polymeric soft-matter, on one hand, provides unprecedented survival and evolutionary capabilities for life (e.g., compaction of meters-long DNA in your cells), on the other hand, create new opportunities to design next-generation materials and pharmaceutical solutions.
Research in our lab focuses on fundamental problems related to polymers in biological and materials science systems, on which we have currently either none or limited understanding. Specifically, we study the kinetic and stimuli-responsive properties of synthetic and biological polymeric systems. Some examples are polyelectrolyte hydrogels’ mechanical and electric response or kinetics of protein-DNA interactions, and their role in chromatin organization, just to name a few. In our investigations, we use Molecular Dynamics (MD) simulations both on atomistic and coarse-grained levels, along with analytical tools of statistical mechanics.
How receptor density and reservoir dimensions effect the diffusion process of ligands initially located at their receptors
Single-molecule (SM) or SPR (Surface Plasmon Resonance) experiments rely on relaxation of concentration quenches of initially surface-bound molecules into confined reservoirs to determine molecular kinetic rates. Similarly, biological processes such exocytosis, in which small molecules are emitted into the intracellular cleft for cellular communication, can be considered to be a relaxation process of an effective concentration quench. We study a model system closely related to the above cases in which weakly interacting Brownian particles are released from their binding sites into a confined volume by using molecular dynamics simulations and scaling arguments. Our results suggest that the rebinding rate of released particles exhibits various power laws until the confined volume is entirely filled by the particles. Furthermore, the cumulative rebinding rate, which is time integration of the rebinding rate, exhibits a novel plateau behaviour. This plateau is a result of the minimal number of collisions between the binding sites and ligands. Our results can have important consequences for molecular signalling as well as for the interpretation of kinetic measurements of ligand-receptor interactions. Read more on Biophysical Journal.
Co-Assembly of Peptide Amphiphiles (PAs) and Lipids into Supramolecular Nanostructures
Co-assembly of binary systems driven by specific non-covalent interactions can greatly expand the structural and functional space of supramolecular nanostructures. Examples include co-assembly of bioactive and nonbioactive molecules to modify the intensity of smart drugs, or functional biomimetic structures. In our work, the self-assembly of peptide amphiphiles (PAs) and fatty acids (lipids) driven primarily by anion−π interactions are focused. With a low content of the lipid, the cylindrical nanofiber morphology is preserved. However, as the aromatic units are placed along the peptide backbone away from the hydrophobic residues, the interactions with the lipids transform the cylindrical supramolecular morphology into ribbon-like structures. Increasing the stoichiometric ratio of the lipid to PA leads to either the formation of large spherical vesicles in the binary systems or a heterogeneous mixture of assemblies. Our findings reveal that specific inter-molecular interactions can drastically change supramolecular morphology and bridge between nano to micro scales. Read more on our work at JACS.
Electrostatic energy conversion with polyelectrolyte gels
Apart, from being the main constituent of contact lenses and pill capsules, hydrogels have also proven themselves pretty effective in other applications including as mechano-electric energy converters or bio-mimetic structures. The hydrogels own their special stimuli-responsive properties to their inherent electrostatic and elastic properties. Their electrostatic nature allows them to adjust their internal interactions reversibly upon application of external stimuli such as electromagnetic fields or mechanical deformations. Our MD simulations with explicit counterions showed that if a polyelectrolyte gel undergoes strain-control deformations, applied mechanical energy is mainly converted into electrostatic energy rather than other energy components (i.e., elastic and steric energies). The energetic change is due to the decreasing translational entropy of counterion gas trapped inside the gel in combination with chain elasticity of PE gels. This scenario is only possible if ionized counterions cannot vacate the gel, which is ensured by the local electro-neutrality condition. Read more on ACS Macro Letters.