Hannah K. Murnen

Hierarchical Self-Assembly of a Biomimetic Diblock Copolypeptoid into Homochiral Superhelices

The aqueous self-assembly of a sequence-specific bioinspired peptoid diblock copolymer into monodisperse superhelices is demonstrated to be the result of a hierarchical process, strongly dependent on the charging level of the molecule. The partially charged amphiphilic diblock copolypeptoid 30-mer, [N-(2-phenethyl)glycine](15)-[N-(2-carboxyethyl)glycine](15), forms superhelices in high yields, with diameters of 624 ± 69 nm and lengths ranging from 2 to 20 μm. Chemical analogs coupled with X-ray scattering and crystallography of a model compound have been used to develop a hierarchical model of self-assembly. Lamellar stacks roll up to form a supramolecular double helical structure with the internal ordering of the stacks being mediated by crystalline aromatic side chain-side chain interactions within the hydrophobic block. The role of electrostatic and hydrogen bonding interactions in the hydrophilic block is also investigated and found to be important in the self-assembly process.

Determination of the persistence length of helical and non-helical polypeptoids in solution

Control over the shape of a polymer chain is desirable from a materials perspective because polymer stiffness is directly related to chain characteristics such as liquid crystallinity and entanglement, which in turn are related to mechanical properties. However, the relationship between main chain helicity in novel biologically derived and inspired polymers and chain stiffness (persistence length) is relatively poorly understood. Polypeptoids, or poly(N-substituted glycines), constitute a modular, biomimetic system that enables precise tuning of chain sequence and are therefore a good model system for understanding the interrelationship between monomer structure, helicity, and persistence length. The incorporation of bulky chiral monomers is known to cause main chain helicity in polypeptoids. Here, we show that helical polypeptoid chains have a flexibility nearly identical to an analogous random coil polypeptoid as observed via small angle neutron scattering (SANS). Additionally, our findings show that polypeptoids with aromatic phenyl side chains are inherently flexible with persistence lengths ranging from 0.5 to 1 nm.

Persistence length of polyelectrolytes with precisely located charges

The conformation of polyelectrolytes in aqueous salt solutions is closely related to their self-assembly properties. In particular, the persistence length has a large impact on how the chain can arrange itself. In this work, biomimetic poly N-substituted glycines (polypeptoids) have been designed to position charged side chains at precise distances from each other to elucidate the relationship between the spacing of the charges along the backbone, the ionic strength, and the persistence length. Using small angle neutron scattering (SANS), it is shown that at low ionic strength, polypeptoids with charged groups located closer to each other along the polymer backbone are stiffer than those with the charged groups spaced further apart. At high ionic strength, the total persistence length decreases for both macromolecules because the electrostatic repulsions between ionized groups are screened. The measured persistence lengths were compared to those calculated using a discrete chain model with bending rigidity, and it is shown that the electrostatic persistence length scales quadratically with the Debye screening length. It is also shown that the bare persistence length of a molecule with alternating ionizable and hydrophilic groups is larger than that of a molecule containing 100% ionizable groups. This difference can be attributed to the longer hydrophilic side chains that may induce local chain stiffening.