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

Abstract

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. † Electronic supplementary information (ESI) available: Fits of the persistence length using the wormlike chain model over a series of polypeptoid chain lengths. In addition, the persistence length analysis for a 36-mer polypeptoid consisting of a racemic mixture of the a-chiral side chain found in 2 is also available. Finally, the results of the semiflexible cylinder model fit allowing the contour lengths to fluctuate are reported. See

Impact of Helical Chain Shape in Sequence-Defined Polymers on Polypeptoid Block Copolymer Self-Assembly

“…While the bending radius (local stiffness) of the chain forming the helix remains quite short, the helix itself has a longer persistence length than that of the unstructured chain. 36 This behavior, in which the helix becomes stiff relative to the unstructured chain, yet becomes less space-filling (smaller R g ), emphasizes that standard descriptions of conformational asymmetry do not adequately describe secondary structures such as helices. The helical chain shape and the placement of helical segments relative to the microdomain interface are found to have significant impacts on the final structure due to the specific impacts of space-filling, stretching, and packing interactions.…”

“…Further, the contour length of the helix (77.2 Å) is considerably shorter than the contour length of the chain composing it (130.0 Å). 36 This leads to an important feature: the helix will have a more rodlike character than the equivalent unstructured chain (l p,helix / L C,helix > l p,chain /L C,chain ) (although notably even l p,helix of 10.5 Å is quite short relative to L C,helix of 77.2 Å). Critically, this means that the effective number of segments N helix for the helix will be smaller than the number of segments N chain of the unstructured chain.…”

“…In particular, by incorporating at least 50% of monomers as N-(R-1-phenylethyl)glycine, the polymer is induced into the helical conformation; 32,34,35 if a racemic combination of side groups is used instead (stoichiometric amounts of R-and (S-1-phenylethyl)glycine), the polymer retains an unstructured conformation. 36 Importantly, this method of sterically inducing the helical conformation is remarkably stable across a range of solvents and temperatures in solution and may form entropically stabilized helices (unlike hydrogen-bond-stabilized helices, which transition to an unstructured state above a critical temperature). 34,43 This remarkably environment-independent and stable helix makes it ideal for bulk studies.…”

“…However, the favored cis-amide bond conformation forms a helix with a persistence length of 10.5 Å, nearly twice that of the unstructured chain (at only 6.2 Å). 36 Importantly, this helical persistence length refers to an effective segment of the helix (which will contain several effective chain units). Further, the contour length of the helix (77.2 Å) is considerably shorter than the contour length of the chain composing it (130.0 Å).…”

“…30,31 By incorporating chiral side chains, polypeptoids can be induced to form a helical secondary structure 32−35 while polypeptoids with randomly incorporated, racemic side chains remain unstructured. 36 Helical conformations have been confirmed via a combination of circular dichroism, 32,37 2-D NMR, 33 and crystallography. 38,39 Further, we incorporate the polypeptoid helix at specific locations along the chain backbonespecifically examining the role of chain stretching at the edges of microdomains versus chain packing in the core of microdomains.…”