Chirality-governed Clusteroluminescence in Polypeptides

Abstract Fluorescent protein-mimics are highly desired for the applications in biomedical fields. In this work, the clusteroluminescence of polypeptides in bulk and solution state was observed and the photoluminescence (PL) intensity of polypeptides can be modulated by the chirality of amino acid residues. Polyglutamates with different optical rotations were synthesized and the racemic polypeptides exhibited a significantly higher PL intensity than the enantiopure ones. This emission is originated from the n-π* transition between C=O groups of polypeptides and enhanced by clusterization of polypeptides. CD and FTIR spectra demonstrated that the enantiopure and racemic polypeptides form α-helix and random coil structures, respectively. The random coils can form chain entanglements and interchain interaction easily because of its high flexibility, leading to the more clusterizations and stronger PL intensity. The rigidity of helical structures restrains the chain entanglements and the formation of intrachain hydrogen bonds between amide groups of backbone impairs the interchain interaction between polypeptides, resulting in lower PL intensity. The PL intensity of polypeptides can also be manipulated by the addition of urea or trifluoroacetic acid. Our study not only elucidate the chirality-based structure-property relationship of clusteroluminescence in peptide-based polymers, but also offers implications for the rational design of fluorecent peptides/proteins.