A Pentapeptide with Tyrosine Moiety as Fluorescent Chemosensor for Selective Nanomolar-Level Detection of Copper(II) Ions

Herein, we have investigated principally with the use of UV and fluorescence (steady-state and time-resolved) spectroscopy the interactions between selected pentapeptides with tyrosine residue (EYHHQ, EHYHQ, EHHQY, and KYHHE) and various metal ions (Cu²⁺, Mn²⁺, Co²⁺, Ni²⁺, Zn²⁺, Cr³⁺, Cd²⁺, Ag⁺, Pb²⁺, Sr²⁺, Ba²⁺, Ca²⁺, Mg²⁺, Al³⁺, Fe²⁺, and Ga³⁺) in order to establish the relationship between the position of a tyrosine residue in the peptide sequence and the metal ion-binding properties. Among the peptides studied, EHYHQ was evaluated as an efficient and selective ligand for developing a chemosensor for the detection of copper(II) ions. While significant fluorescence emission quenching was observed for that peptide in the presence of Cu²⁺ cations, other metal cations used at the same and at considerably higher concentrations caused a negligible change of the fluorescence emission spectrum, indicating a high selectivity of EHYHQ for Cu²⁺ ions. Under optimum conditions, fluorescence intensity was inversely proportional to the concentration of Cu²⁺ ions. The limit of detection of Cu²⁺ ions with the use of EHYHQ was determined at the level of 26.6 nM. The binding stoichiometry of the complexes of the studied peptides with Cu²⁺ ions was evaluated spectrophotometrically and fluorimetrically (as in the case of EHYHQ confirmed by mass spectrometry) and found to be 1:2 (Cu²⁺-peptide) for all the investigated systems. Furthermore, the stability constant (K) values of these complexes were determined. The reversibility of the proposed Cu²⁺ ions sensor was confirmed, the pH range where the sensor acts was determined, while its analytical performance was compared with some other reported recently fluorescent sensors. The mechanism of the interactions between EHYHQ and Cu²⁺ was proposed on the basis of NMR spectroscopy investigations.