Variation of formal hydrogen-bonding networks within electronically delocalized π-conjugated oligopeptide nanostructures

Evolution of π-Peptide Self-Assembly: From Understanding to Prediction and Control

Supramolecular materials derived from the self-assembly of engineered molecules continue to garner tremendous scientific and technological interest. Recent innovations include the realization of nano- and mesoscale particles (0D), rods and fibrils (1D), sheets (2D), and even extended lattices (3D). Our research groups have focused attention over the past 15 years on one particular class of supramolecular materials derived from oligopeptides with embedded π-electron units, where the oligopeptides can be viewed as substituents or side chains to direct the assembly of the central π-electron cores. Upon assembly, the π-systems are driven into close cofacial architectures that facilitate a variety of energy migration processes within the nanomaterial volume, including exciton transport, voltage transmission, and photoinduced electron transfer. Like many practitioners of supramolecular materials science, many of our initial molecular designs were designed with substantial inspiration from biologically occurring self-assembly coupled with input from chemical intuition and molecular modeling and simulation. In this feature article, we summarize our current understanding of the π-peptide self-assembly process as documented through our body of publications in this area. We address fundamental spectroscopic and computational tools used to extract information regarding the internal structures and energetics of the π-peptide assemblies, and we address the current state of the art in terms of recent applications of data science tools in conjunction with high-throughput computational screening and experimental assays to guide the efficient traversal of the π-peptide molecular design space. The abstract image details our integrated program of chemical synthesis, spectroscopic and functional characterization, multiscale simulation, and machine learning which has advanced the understanding and control of the assembly of synthetic π-conjugated peptides into supramolecular nanostructures with energy and biomedical applications.

Electroactive peptide-based supramolecular polymers

The electroactivity as a supramolecular feature of intelligently designed self-assembled systems stimulates a wide interest in development of new stimuli-responsive biomaterials. A diverse set of nanostructures are fabricated through programmed self-assembly of molecules for functional materials. Electroactive groups are conjugated as a functional moiety for organic semiconductor applications. In this review, we present recent examples of self-assembling peptide molecules and electroactive units for supramolecular functional electronic ​and optical materials with potential biomedical and bioelectronics applications.

Revealing the Sequence-Structure-Electronic Property Relation of Self-Assembling π-Conjugated Oligopeptides by Molecular and Quantum Mechanical Modeling

Self-assembled nanoaggregates of π-conjugated synthetic peptides present a biocompatible and highly tunable alternative to silicon-based optical and electronic materials. Understanding the relationship between structural morphology and electronic properties of these assemblies is critical for understanding and controlling their mechanical, optical, and electronic responses. In this work, we combine all-atom classical molecular simulations with quantum mechanical electronic structure calculations to ascertain the sequence-structure-electronic property relationship within a family of Asp-X-X-quaterthiophene-X-X-Asp (DXX-OT4-XXD) oligopeptides in which X is one of the five amino acids {Ala, Phe, Gly, Ile, Val} ({A, F, G, I, V}). Molecular dynamics simulations reveal that smaller amino acid substituents (A, G) favor linear stacking within a peptide dimer, whereas larger groups (F, I, V) induce larger twist angles between the peptides. Density functional theory calculations on the dimer show the absorption spectrum to be dominated by transitions between carbon and sulfur p orbitals. Although the absorption spectrum is largely insensitive to the relative twist angle, the highest occupied molecular orbital strongly localizes onto one molecule within the dimer at large twist angles, impeding the efficiency of transport between molecules. Our results provide a fundamental understanding of the relation between peptide orientation and electronic structure and offer design precepts for rational engineering of these systems.

Torsional Impacts on Quaterthiophene Segments Confined within Peptidic Nanostructures

The co-assembly behavior of peptide-π-peptide and peptide-alkyl-peptide triblock molecules that form one-dimensional (1D) nanostructures under acidic, aqueous environments is dependent on the peptide sequence and the torsional constraints imposed within the nanomaterial volume. Although a hydrophilic tripeptide sequence (Asp-Asp-Asp, DDD-) previously promoted isolation/dilution of minority π-electron components in the matrix of aliphatic peptides, a β-sheet promoting sequence (Asp-Val-Val, DVV-) led to blocks of the two components distributed within larger 1D self-assembled nanostructures. Furthermore, torsional restrictions exerted on the oligoaromatic π-electron unit by the self-assembly process can lead to changes in its conformation (for example, planarity), which has ramifications on its functionality within the peptide matrix. Here, we study this impact on thiophene-based π-electron units with inherently different geometries, viz., relatively planar 2,2':5',2″:5″,2‴-quaterthiophene and 3″,4'-dimethyl-2,2':5',2″:5″,2‴-quaterthiophene, which is twisted at the core bithiophene unit due to the presence of two methyl groups. These peptides were co-assembled at 5 and 20 mol % with peptide- n-decyl-peptide triblock molecules, and the resultant assemblies were studied using UV-vis absorption, photoluminescence, and circular dichroism spectroscopies. We found that torsional restriction in dimethylated quaterthiophene units can impact the stacking behavior of these 1D peptide nanoassemblies and have consequences on their photophysical properties. Additionally, these insights help in the understanding of the dependence of the optoelectronic properties of these materials on both the intrinsic conformation of π-units and the geometric constraints imposed by their immediate local environment under aqueous conditions.

Evidence for Prenucleated Fibrilogenesis of Acid-Mediated Self-Assembling Oligopeptides via Molecular Simulation and Fluorescence Correlation Spectroscopy

An important step in controlling biomimetic amyloid systems is understanding the self-assembly reaction kinetics. We are interested in a family of such materials characterized by symmetric sequences of amino acids flanking a π-conjugated functional core. Many of these materials rapidly self-assemble into long fibers upon protonation in an acidic environment. Despite extensive investigation of these materials' properties, little is yet understood regarding their reaction kinetics. Based on previous studies, we have chosen DFAG-4T-GAFD as a representative system and conducted molecular dynamics simulations to show that although large-scale assembly is induced by lowering pH, some degree of assembly is thermodynamically favorable in high-pH nonprotonating environments. These results are consistent with findings for other systems such as DFAG-OPV-GAFD. The nonprotonated aggregation also appears to be concentration dependent, occurring at concentrations of 100 nM and above. Single molecule measurements using fluorescence correlation spectroscopy provide experimental support for these computational predictions. We find evidence of spontaneous aggregation in aqueous solutions of peptides with concentrations as low as 100 nM; however, 10 nM solutions appear to be largely homogeneous solutions of unassembled monomer. These results indicate that the simplest explanations for kinetics of acid-mediated assembly-protonation-induced nucleation by monomeric addition followed by subsequent stages of aggregation and elongation-are inappropriate in this system. In fact, the system only exists as pure monomer in very low concentrations, nucleation actually occurs in the absence of protonating elements at concentrations typically used for experiments, and pH triggered assembly proceeds from these preassembled aggregates. Accordingly, triggered assembly must be considered to operate outside the domain of nucleation-dependent models.

Understanding the conformational analysis of gababutin based hybrid peptides

A new class of gababutin-based tetrapeptide shows a C₁₂/C₁₀ hydrogen-bonded hybrid turn.

Photon management in supramolecular peptide nanomaterials

Self-assembling peptides with covalent pi-electron functionality offer new ways to create delocalized conduits within protein-based nanomaterials. My group's recent research is summarized in this regard, detailing foundational self-assembly and photophysical characterizations that validate the electronic couplings existing within the resulting peptidic nanomaterials. Using these initial studies as a benchmark, ongoing studies to create even more complex photonic energy delocalization schemes are presented, spanning excitonic and Förster energy transfer to low-bandgap dopant sites (whereby 46% of the observed photoluminescence could be quenched by the addition of 1 mol% of an energy acceptor), the creation of charge separated states following photoinduced electron transfer that persisted for over a nanosecond, and use of kinetic control to dictate self-sorting (at long time scales, ca. several hours) or intimate coassembly (at short time scales, ca. several seconds) of multiple peptide components. Peptide coassemblies are described that exhibit both directed exciton migration to low-energy sites and follow-up charge separation events, very much in mimicry with relevant photosynthetic processes.

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. Biopolymer-synthetic 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 π-conjugated synthetic oligopeptides containing different π-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 π-conjugated oligopeptides self-assemble into β-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 cgel are determined for fiber-forming π-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 π-conjugated cores greatly influences the self-assembly process, such that oligopeptides appended to π-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 π-conjugated oligopeptides driven by concentration, which is controlled by a combination of enthalpic and entropic interactions between oligopeptide subunits.

Self-sorted Oligophenylvinylene and Perylene Bisimide Hydrogels

We describe two component hydrogels with networks composed of self-sorted fibres. The component gelators are based on 1,4-distyrylbenzene (OPV3) and perylene bisimide (PBI) units. Self-sorted gels can be formed by a slow decrease in pH, which leads to sequential assembly. We demonstrate self-sorting by NMR, rheology and small angle X-ray scattering (SAXS). Photoconductive xerogels can be prepared by drying these gels. The wavelength response of the xerogel is different to that of the PBI alone.

Nonresonant and Local Field Effects in Peptidic Nanostructures Bearing Oligo(p-phenylenevinylene) Units

Peptide nanostructures with built-in electronic functions offer a new platform for biomaterial science. In this report, we interrogate the influences of the immediate peptide environment around oligo(p-phenylenevinylene) (OPV3) electronic units embedded within one-dimensional peptide nanostructures on the resulting photophysics as assessed by UV-vis, photoluminescence (PL), and circular dichroism spectroscopies. To do so, we studied peptide-core-peptide molecules where the core was either OPV3 or an aliphatic n-decyl chain. Coassemblies of these molecules wherein the π-core was diluted as a minority component within a majority aliphatic matrix allowed for the variation of interchromophore exciton coupling commonly found in homoassemblies of peptide-OPV3-peptide monomers. Upon coassembly of the peptides, a hydrophilic tripeptide sequence (Asp-Asp-Asp-, DDD-) promoted the dilution/isolation of the peptide-π-peptide molecules in the majority peptide-decyl-peptide matrix whereas a hydrophobic tripeptide sequence (Asp-Val-Val-, DVV-) promoted the formation of self-associated stacks within the nanostructures. We also performed temperature variation studies to induce the reorganization of π-electron units in the spatially constrained n-decyl environment. This study elucidates the nonresonant (e.g., conformational) and local peptide field effects enforced within the internal environment of peptide nanomaterials and how they lead to varied photophysical properties of the embedded π-electron cores. It offers new insights on tuning the optoelectronic properties of these types of materials on the basis of the local electronic and steric environment available within the nanostructures.

Control of the hierarchical assembly of π-conjugated optoelectronic peptides by pH and flow

Coarse-grained molecular simulations reveal the influence of pH and flow on the self-assembly of DFAG-OPV3-GAFD optoelectronic peptides.

Reconstructive Phase Transition in Ultrashort Peptide Nanostructures and Induced Visible Photoluminescence

A reconstructive phase transition has been found and studied in ultrashort di- and tripeptide nanostructures, self-assembled from biomolecules of different compositions and origin such as aromatic, aliphatic, linear, and cyclic (linear FF-diphenylalanine, linear LL-dileucine, FFF-triphenylalanine, and cyclic FF-diphenylalanine). The native linear aromatic FF, FFF and aliphatic LL peptide nanoensembles of various shapes (nanotubes and nanospheres) have asymmetric elementary structure and demonstrate nonlinear optical and piezoelectric effects. At elevated temperature, 140-180 °C, these native supramolecular structures (except for native Cyc-FF nanofibers) undergo an irreversible thermally induced transformation via reassembling into a completely new thermodynamically stable phase having nanowire morphology similar to those of amyloid fibrils. This reconstruction process is followed by deep and similar modification at all levels: macroscopic (morphology), molecular, peptide secondary, and electronic structures. However, original Cyc-FF nanofibers preserve their native physical properties. The self-fabricated supramolecular fibrillar ensembles exhibit the FTIR and CD signatures of new antiparallel β-sheet secondary folding with intermolecular hydrogen bonds and centrosymmetric structure. In this phase, the β-sheet nanofibers, irrespective of their native biomolecular origin, do not reveal nonlinear optical and piezoelectric effects, but do exhibit similar profound modification of optoelectronic properties followed by the appearance of visible (blue and green) photoluminescence (PL), which is not observed in the original peptides and their native nanostructures. The observed visible PL effect, ascribed to hydrogen bonds of thermally induced β-sheet secondary structures, has the same physical origin as that of the fluorescence found recently in amyloid fibrils and can be considered to be an optical signature of β-sheet structures in both biological and bioinspired materials. Such PL centers represent a new class of self-assembled dyes and can be used as intrinsic optical labels in biomedical microscopy as well as for a new generation of novel optoelectronic nanomaterials for emerging nanophotonic applications, such as biolasers, biocompatible markers, and integrated optics.