Anomalous Emission from Single α-Helical Peptides in Solution

Polypeptides lacking aromatic residues can display in the aggregated state anomalous spectroscopic properties, such as UV absorption and fluorescence emission. Despite the fundamental interest and potential applications of this anomalous luminescence, our understanding of this phenomenon is still poor, and no soluble model system or general design rules to obtain non-aromatic peptide luminogens are available. Here, we show that short peptides derived from zwitterionic single α-helices (SAHs), formed exclusively by non-aromatic lysine and glutamic acid residues are UV-active and luminescent at near-UV wavelengths in solution. We demonstrate that their emission depends on the α-helical folding of the SAHs, which favors intramolecular through-space interactions between the Lys/Glu side chains, leading to non-aromatic fluorescence (NAF). We also show that the introduction of Trp or Tyr residues within the helical polyampholyte framework produces long-wavelength luminescence, red-shifted from the characteristic emission of the aromatic residues in the sequence (λexc = 525 nm; λem = 550 nm).