Mapping the Morphological Landscape of Oligomeric Di-block Peptide-Polymer Amphiphiles

Abstract

Peptide-polymer amphiphiles (PPAs) are tunable hybrid materials that achieve complex assembly landscapes by combining the sequence-dependent properties of peptides with the structural diversity of polymers. Despite their promise as biomimetic materials, determining how polymer and peptide properties simultaneously affect PPA self-assembly remains challenging. We herein present a systematic study of PPA structure-assembly relationships. PPAs containing oligo(ethyl acrylate) and random-coil peptides were used to determine the role of oligomer molecular weight, dispersity, peptide length, and charge density on self-assembly. We observed that PPAs predominantly formed spheres rather than anisotropic particles. Oligomer molecular weight and peptide hydrophilicity dictated morphology, while dispersity and peptide charge affected particle size. These key benchmarks will facilitate the rational design of PPAs that expand the scope of biomimetic functionality within assembled soft materials.

Keywords: Biomimetic; Electron Microscopy; Peptide-Polymer Amphiphile; Self-Assembly; Supramolecular Chemistry.

Figure 1.

Synthesis of peptide-polymer amphiphiles (PPAs). (a) Maleimide-terminated oligomer tails were synthesized by atom transfer radical polymerization (ATRP) followed by substitution of the bromine chain-end and deprotection. (b) PPAs were synthesized through thiol-maleimide coupling, which joins the maleimide-terminated oligomer tail to the N-terminal cysteine of a peptide. (c) The LC chromatogram of a purified PPA shows distinct peaks that correspond to amphiphiles with discrete degrees of polymerization, which (d) can be identified by electrospray ionization (ESI)

Figure 2.

Images from negatively stained (left) and cryogenic (right) transmission electron microscopy (TEM) of major particle types formed by PPA self-assembly. Scale bars represent 50 nm.

Figure 3.

Effect of increasing oligomer tail DP_n on self-assembly of PPAs with peptide XTEN2. (a) Representative negatively stained TEM images of five PPAs with DP_n ranging from 5.3 to 12.9; scale bar represents 100 nm. (b) Normalized particle size distributions of spherical particles (excluding anisotropic particles), measured from negatively stained TEM images. Inset values show average diameter ± standard deviation (to indicate distribution width). Values at right are the total number of spherical particles measured. (c) Peak fitting of spherical particle size distributions: light gray = raw data, blue = Gaussian-fit micelle distribution, yellow = lognormal-fit vesicle distribution, black = sum of blue and yellow curves. Inset text shows peak mode (standard deviation to indicate peak width). (d) Percent of each particle type present in the morphological distribution for each PPA, estimated via peak fitting for the spherical particles.

Figure 4.

Assembly behavior of PPAs with the same average DP_n but different tail length dispersities ranging from broadest (top of each panel) to narrowest (bottom). (a) Molecular weight distributions. (b) Spherical particle size distributions. Inset text shows average particle diameter ± standard deviation and total number of particles measured. (c) Peak fitting for spherical particle size distributions; light gray = raw data, blue = Gaussian fit micelle distribution, yellow = lognormal fit vesicle distribution, black = sum of blue and yellow curves. Inset text shows peak mode (standard deviation indicating peak width). (d) Percent of each particle type present in the morphological distribution for each PPA, estimated via peak fitting for spherical particles.

Figure 5.

Effect of peptide charge and length on self-assembly of PPAs with oligomer tail oEA₅. (a) Representative negatively stained TEM images of PPAs containing each of the selected peptide sequences; scale bar represents 100 nm. (b) Normalized particle size distributions of spherical particles (excluding anisotropic particles), measured from negatively stained TEM images. Inset values show average diameter ± standard deviation. Values at right are the total number of spherical particles measured. (c) Peak fitting of spherical particle size distributions: light gray = raw data, blue = Gaussian-fit micelle distribution, yellow = lognormal-fit vesicle distribution, black = sum of blue and yellow curves. Inset text shows peak mode (standard deviation indicating peak width). (d) Percent of each particle type present in the morphological distribution for each PPA, estimated via peak fitting for spherical particles.

Figure 6.

Fractionation of oEA₅-PAS1 during purification yields a PPA sample with (a) a narrower molecular weight distribution and DP_n of 2.5 ± 1.0. (b) Normalized particle size distributions of spherical particles. Inset text shows average diameter ± standard deviation. (c) Peak fitting scheme; light gray = raw data, blue = Gaussian fit micelle distribution, yellow = lognormal fit vesicle distribution, black = sum of blue and yellow curves. Inset text shows peak mode (standard deviation, indicating peak width). (d) Percent of each particle type present in the morphological distribution for each PPA, estimated via peak fitting.