Salen-based bifunctional chemosensor for copper (II) ions: Inhibition of copper-induced amyloid-β aggregation
Graphical abstract
Introduction
Copper plays critical roles in a variety of fundamental physiological processes including cellular respiration [[1], [2], [3]], antioxidation, neurotransmitter and melanin synthesis [4], and epigenetic modifications [[5], [6], [7]]. Dysregulation of metal homeostasis involving copper ions is also considered as a major cause of diseases including cancer [8], genetic disorders such as Menkes and Wilson’s disease [9], and a variety of neurodegenerative diseases [10].
In one example of the copper-mediated disorders, Alzheimer’s disease (AD) is probably caused by the abnormal aggregation/polymerization/cross-linking of misfolded amyloid-β proteins in which high concentrations of Cu, Zn and Fe are found [[11], [12], [13]]. Copper ions are observed to coordinate to Aβ proteins, forming Cu-Aβ complexes which are detrimental to the central nervous system via generation of toxic Aβ oligomers and producing reactive oxygen species (ROS) [[14], [15], [16], [17], [18], [19], [20], [21]]. Therefore, the modulation of copper ions in the brain represents a possible therapeutic strategy for the treatment of AD. Two copper chelators, clioquinol (CQ) and PBT2, have been used in clinical trials to inhibit copper-induced Aβ aggregation, and were shown to improve cognition in AD patients [[22], [23], [24]].
Given the critical roles of copper in life, several methods such as inductively coupled plasma mass spectrometry (ICP-MS), magnetic resonance imaging (MRI), and positron emitting tomography (PET) have recently been established for detecting and visualizing Cu2+ [7,[25], [26], [27], [28], [29], [30]]. Unfortunately, their widespread applications remain limited due to the complicated synthetic procedures, expensive instrumentation, intricate sample processing, and costly technique workups required [7,[25], [26], [27], [28], [29], [30]]. Therefore, there is an unmet need for simple and inexpensive technologies that incorporate sensitive and selective copper detection. Furthermore, such tools could substantially impact the diagnosis and treatment of diseases associated with the hyper-accumulation of copper.
Herein, we report a salen-based bifunctional ligand N,N’-(1,2-phenylene)bis(1-(1H-imidazol-4-yl)methanimine) (pimi) for Cu2+ detection and capture. This ligand can be easily synthesized with inexpensive starting materials in one step without any catalyst or column purification (Scheme 1). Cu2+ ions were detected using this novel ligand in both aqueous and biological samples within a broad pH range. Readouts including colorimetric detection and sensitive fluorescent on-off responses, were fast (<3 s) with a nanomolar (nM) detection limit. Moreover, the ligand could capture Cu2+ ions directly from copper-Aβ complexes, inhibiting copper-induced aggregation of Aβ, thereby protecting neuronal cells from the toxicity of aggregated Aβ. The combination of favorable solubility, fast response time, broad working pH scope, low detecting limit and easy preparation made the ligand a valuable candidate for the diagnosis and treatment of copper-related diseases.
Section snippets
Materials
The compounds o-phenylenediamine, 4-imidazolecarboxaldehyde and 1, 1, 1, 3, 3, 3-hexafluoro-2-propanol (HFIP) were purchased from Sigma-Aldrich. Aβ42 was purchased from AnaSpec (Aβ42 = DAEFRHDSGYEVHHQKLVFFAEDVGSNKGAIIGLMVGGVVIA). Dulbecco’s Modified Eagle Medium (DMEM), fetal bovine serum (FBS), Penicillin and streptomycin were purchased from Thermo-Fisher Scientific Inc. Other materials were commercially available and of reagent grade. All chemicals were used as received without further
Preparation of pimi
Pimi was synthesized by the condensation of o-phenylenediamine and 4-imidazolecarboxaldehyde in one step with 60% yield (Scheme 1). The starting materials were economical and easily accessible, and the preparation process was straightforward, no catalyst or column purification were needed. The structure of pimi was characterized by 1H NMR, 13C NMR, ESI-MS (Fig. S1-S3) and element analysis. The compound is water-soluble and can be readily dissolved in 0.1% DMSO aqueous solution at the
Conclusion
In summary, we reported a bifunctional salen ligand that could be used for copper detection and capture. The ligand could rapidly detect Cu2+ in aqueous and biological samples via colorimetric and fluorescence response with the detecting limit in low nM range. In comparison with literature-reported fluorescence probes, pimi display significant advantages including favorable solubility, fast response time, accurate sensitivity, and broad working pH scope. In addition, due to the strong binding
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We thank the National Natural Science Foundation of China (No.21576054, 21306026, 81701751), the Fundamental Research Funds for the Central Universities (21617311), and the Science and Technology Program of Guangzhou (201804010440) for financial support.
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