A simple copper(II) dppy-based receptor for sensing of L-cysteine and L-histidine in aqueous acetonitrile medium

The development of simple yet efficient receptors that rapidly detect and monitor amino acids with high sensitivity and reliability is crucial for the early-stage identification of various diseases. In this work, we report the synthesis and characterisation of a copper(ii) complex, CuCl ₂L, by employing a 2,6-dipyrazinylpyridine (dppy)-based ligand (L = 2,2′-(4-(3,4,5-trimethoxyphenyl)pyridine-2,6-diyl)dipyrazine). The in situ prepared CuCl ₂L receptor exhibits an instantaneous response to the presence of l-cysteine (Cys) and l-histidine (His) in aqueous acetonitrile (4 : 1 v/v, 10 mM HEPES buffer, pH 7.4). Furthermore, competitive experiments demonstrate the selectivity of CuCl ₂L towards Cys (1 equiv.) in the vicinity of other l-amino acids in the aforementioned solvent conditions. The detection limits for Cys and His are calculated as 0.33 μM and 1.40 μM, respectively. DFT calculations offer a plausible explanation for the observed selectivity of the CuCl ₂L receptor towards Cys and His. They reveal that the most stable conformer of Cu : Cys complex (1 : 1) is a five-membered ring formed through N,S-coordination mode (ΔG = −26.7 kcal mol ⁻¹) over various other possible coordination modes, while comparable ΔG values are only obtained for Cu : His complexes featuring two His moieties.