Experimental and theoretical studies for instantaneous detection of l-cysteine and l-histidine using a simple Cu(II)-dppy complex

Developing simple and effective receptors for instantaneous detection of l-amino acids is vital for the recognition of life-threatening diseases in their early stage. Here, we report the synthesis and characterization of 2,6-di(pyrazin-2-yl)pyridine (dppy) based ligand L (L = 2,2′-(4-(3,4-diethoxyphenyl)pyridine-2,6-diyl)dipyrazine) and its copper (II) complex, [Cu(NO₃)₂L]. The in-situ prepared [Cu(H₂O)₂L](NO₃)₂ receptor in aqueous acetonitrile (4:1 v/v, 10 mM HEPES buffer, pH 7.4) medium displayed instantaneous responses towards biologically important bio-thiol l-Cysteine (Cys) and essential amino acid l-Histidine (His) over other l-amino acids through UV–Visible absorption spectral studies. Additionally, the competitive experiments confirmed that in-situ prepared [Cu(H₂O)₂L](NO₃)₂ receptor selectively detects Cys (1 equivalent) in the presence of other l-amino acids. The detection limits of Cys and His were calculated to be 8.23 × 10⁻⁷ M and 2.55 × 10⁻⁶ M, respectively. The density functional theory (DFT) calculation shows that selectivity of [Cu(H₂O)₂L](NO₃)₂ towards Cys is due to the reduction of Cu(II) to Cu(I) in the presence of Cys, while Cys is oxidized to cystine (Cys-Cys). For His, selectivity observed due to the formation of [Cu(His)₂] moiety between Cu(II) ion with one His via N,O coordination mode and another His via N,N,O-coordination mode.