Intrinsic Fluorescence in Peptide Amphiphile Micelles with Protein-Inspired Phosphate Sensing

Abstract

Although peptide amphiphile micelles (PAMs) have been widely studied since they were developed in the late 1990s, to the author's knowledge, there have been no reports that PAMs intrinsically fluoresce without a fluorescent tag, according to the aggregation-induced emission (AIE) effect. This unexpected fluorescence behavior adds noteworthy value to both the peptide amphiphile and AIE communities. For PAMs, intrinsic fluorescence becomes another highly useful feature to add to this well-studied material platform that features precise synthetic control, tunable self-assembly, and straightforward functionalization, with clear potential applications in bioinspired materials for bioimaging and fluorescent sensing. For AIE, it is extremely rare and highly desirable for one platform to exhibit precise tunability on multiple length scales in aqeuous solutions, positioning PAMs as uniquely well-suited for systematic AIE mechanistic study and sequencespecific functionalization for bioinspired AIE applications. In this work, the author proposes that AIE occurs across intermolecular emissive pathways created by the closely packed peptide amide bonds in the micelle corona upon self-assembly, with maximum excitation and emission wavelengths of 355 and 430 nm, respectively. Of the three PAMs evaluated here, the PAM with tightly packed random coil peptide conformation and maximum peptide length had the largest quantum yield, indicating that tuning molecular design can further optimize the intrinsic emissive properties of PAMs. To probe the sensing capabilities of AIE PAMs, a PAM was designed to incorporate a protein-derived phosphate-binding sequence. It detected phosphate down to 1 ppm through AIE-enhanced second-order aggregation, demonstrating that AIE in PAMs leverages tunable biomimicry to perform protein-inspired sensing.

Tuning Molecular Motion Enhances Intrinsic Fluorescence in Peptide Amphiphile Nanofibers

“…It was calculated by comparing the fluorescence emission intensity integration versus absorbance for each PA system at various concentrations (Figure S10) to a tryptophan standard solution, which has a QY of 0.14 (Figure S11). C 16 VVAAK 2 had the highest QY at 0.17, which was significantly higher than those of all other systems, and an order of magnitude higher than the previous AIE PA system . C 16 AAGGK 2 and C 16 GGGGK 2 had the next highest QY values at 0.046 and 0.030, respectively.…”

“…It has been shown that less mobile AIE molecules exhibit higher emission because more energy is converted into emission compared to kinetics . The proposed AIE mechanism of PA nanofibers is that the amide bonds in the peptide backbone constitute enough electron density such that when they are rigidly packed in the nanofiber corona, they exhibit AIE . This unexpected AIE has also been observed in other amide-based aggregates …”

“…Region 3 can consist of charged amino acids for solubility, which is true in this case and is commonly used for “filler” PAs in mixed PA assemblies, or it can also often be a protein-derived binding moiety to target cell receptors or ions. Region 3 of the original PA AIE discovery was a protein-derived phosphate-binding moiety . The PA fluorescence signal responded to phosphate binding, unveiling PAs as an exciting molecular-recognition platform for protein-inspired sensing.…”

“…β-sheets lead to an ordered structure of H-bond formations, causing the PAs with higher β-sheet content to be more rigid or less mobile. , β-sheets were also found to promote coplanar electron conjugation. While a more rigid and orderly structure is potentially more fluorescent, it has also been observed in peptide-based AIE systems that a lack of structure can allow the peptide chains to pack with a closer proximity between subfluorophores than the 5 Å intermolecular distance of β-sheets. , Small spacings between subfluorophores enabled space conjugation, which is a noncovalent pathway essential to AIE. Thus, our six PA designs can shed further insights into how to best balance packing and rigidity to optimize fluorescence.…”

Self-Assembled Homopolymeric Spherulites from Small Molecules in Solution

“…As the irradiation pretreatment time increases, the luminescence intensity reached a maximum level (Figure c and Movie S2). We speculate that the luminescence properties of rPBDT spherulites are related to the formation of rigid disulfide hindering the rotation of benzene rings and the supramolecular structure of molecular chains. − The necessity for UV pretreatment can be linked to the insufficiently dense arrangement of PBDT molecules. This lack of density stems from the rapid progression of step-growth polymerization and self-assembly processes, and UV irradiation may disrupt partial disulfide bonds, which leads to the reorganization of PBDT molecules within the spherulite for a more tightly packed structure.…”