Concentration-Driven Assembly and Sol-Gel Transition of π-Conjugated Oligopeptides

Advances in supramolecular assembly have enabled the design and synthesis of functional materials with well-defined structures across multiple length scales. Biopolymersynthetic hybrid materials can assemble into supramolecular structures with a broad range of structural and functional diversity through precisely controlled noncovalent interactions between subunits. Despite recent progress, there is a need to understand the mechanisms underlying the assembly of biohybrid/synthetic molecular building blocks, which ultimately control the emergent properties of hierarchical assemblies. In this work, we study the concentration-driven self-assembly and gelation of pi-conjugated synthetic oligopeptides containing different pi-conjugated cores (quaterthiophene and perylene diimide) using a combination of particle tracking microrheology, confocal fluorescence microscopy, optical spectroscopy, and electron microscopy. Our results show that pi-conjugated oligopeptides self-assemble into beta-sheet-rich fiber-like structures at neutral pH, even in the absence of electrostatic screening of charged residues. A critical fiber formation concentration cfiber and a critical gel concentration c(gel) are determined for fiber-forming pi-conjugated oligopeptides, and the linear viscoelastic moduli (storage modulus G' and loss modulus G are determined across a wide range of peptide concentrations. These results suggest that the underlying chemical structure of the synthetic pconjugated cores greatly influences the self-assembly process, such that oligopeptides appended to pi-conjugated cores with greater torsional flexibility tend to form more robust fibers upon increasing peptide concentration compared to oligopeptides with sterically constrained cores. Overall, our work focuses on the molecular assembly of pi-conjugated oligopeptides driven by concentration, which is controlled by a combination of enthalpic and entropic interactions between oligopeptide subunits.