Nanomolar Affinity Host‐Dye Reporter Pairs from Fluorescently Labeled Oligoarginine Peptides and <i>p</i> ‐Sulfonatocalix[4]arene

Oligoarginine peptides are of interest as cell-penetrating peptides (CPPs) and play important roles in gene regulation and immune response. Complexation of oligoarginine peptides by the supramolecular host p-sulfonatocalix[4]arene (CX4) is well-established and has led to the use of CX4 derivatives as effective counterion activators for arginine-rich CPPs and to the application of CX4 in peptide sensing. Herein, a systematic binding study between oligoarginine peptides with CX4 is reported. The results show that the binding affinity increases with increasing peptide length from ≈10⁴M^-1 for the peptide H-Arg-Arg-OH to nanomolar affinities for peptides with more than four arginine residues. Within the series, C-terminal carboxamides show higher affinities than the respective carboxylates. In addition, the influence of fluorescent dye labeling is investigated with sulforhodamine B (SRB) and fluorescein (FL) dye labels. Efficient fluorescent quenching is observed after complexation of the labeled peptides by CX4, which prompted the exploration of the complexes as reporter pairs for chemosensing applications. The results suggest that the combination of fluorescently labeled oligoarginine peptides with CX4 affords a modular platform of host-dye reporter pairs for the detection of polycationic peptides with tailorable excitation and emission wavelengths and tailorable binding affinity down to the nanomolar affinity range.