Solvent-Encoded Intrinsic Emission and Self-Assembly of Coiled-Coil-Based Foldamers

Coiled-coil peptides incorporating cyclic β-amino acids, such as trans-(1S,2S)-2-aminocyclopentanecarboxylic acid (trans-ACPC), offer a versatile scaffold for engineering foldamers with tailored self-assembly and photophysical properties. Here, we investigate how solvent polarity and hydrogen-bonding capacity modulate two intrinsic features of such foldamers: autofluorescence and supramolecular organization. A series of trans-ACPC-modified and unmodified coiled-coil peptides were synthesized and characterized by circular dichroism (CD), steady-state and time-resolved fluorescence spectroscopy (SSFS and TRFS), transmission electron microscopy (TEM), and attenuated total reflectance─Fourier-transform infrared (ATR-FTIR) in water, ethanol, and acetonitrile. All peptides exhibited label-free fluorescence, with emission maxima and lifetimes varying systematically with the solvent environment and aggregate morphology. Insertion of a trans-ACPC residue rigidified the backbone, reduced solvent sensitivity, and promoted more homogeneous photophysical responses. TEM and distribution-free statistics show solvent-programmed morphologies, with water favoring extended fibrils, ethanol predominantly yielding globular or shorter twisted aggregates, and acetonitrile producing compact, less ordered clusters with intermediate cross sections (H₂O < ACN < EtOH). ATR-FTIR spectra in the amide I/II region reveal solvent-dependent band positions consistent with reorganized hydrogen-bond networks and through-space interactions among backbone carbonyls, supporting the proposed carbonyl-lock contribution to emission. Across solvents, excitation and emission wavelengths follow the expected solvatochromic ordering (most red-shifted in water, blue-shifted in ethanol, and further in acetonitrile), whereas fluorescence lifetimes are broadly similar in water and acetonitrile and shortened in ethanol, indicating only a partial correlation between supramolecular order and decay kinetics. Thus, the external solvent programs intrinsic emissive states by reshaping backbone packing and hydrogen-bond topology rather than introducing new chromophores. These findings establish a structure-solvent-photophysics relationship for cyclic β-amino acid-containing coiled-coils and highlight their potential as intrinsically emissive nanomaterials and optical probes in environments where external labels are undesirable.