Copper ions are essential metal ions for life [
1–
3]. Through redox cycling between monovalent copper (Cu
+) and divalent copper (Cu
2+), they play catalytic roles in critical biological processes such as metabolism, proliferation, growth, and respiration [
2,
4–
11]. Concurrently, clinical studies have demonstrated that dysregulation of copper-dependent redox activity is closely associated with pathological conditions including cancer [
7], inflammatory immune responses [
12,
13], major neurodegenerative diseases [
14], and hereditary disorders of copper metabolism [
15,
16]. Owing to the intracellular reducing environment, Cu
+ predominates over Cu
2+ within cells [
17]. Consequently, fluorescence-based detection techniques targeting Cu
+ have achieved significant advancements [
18–
27]. In contrast, Cu
2+ exists at lower concentrations in living cells [
17]. Additionally, Cu
2+ exhibits paramagnetic properties, enabling strong fluorescence signal quenching through electron or energy transfer pathways [
28,
29]. Consequently, most Cu
2+-responsive probes operate
via turn-off mechanisms [
28,
29]. That severely limited the visualization of Cu
2+ and created substantial obstacles for investigating intracellular Cu
2+ distribution, coordination interactions with proteins, and its correlations with various pathological factors.
Driven by the discovery of metal ion-mediated covalent modification of proteins by small molecules, chemists have developed activity-based sensing (ABS) strategies to address the pressing need for detecting intracellular metal ions and their responses to exogenous/endogenous stimuli [
17,
30,
31]. These approaches leverage metal ion-triggered covalent reactions to achieve the spatial anchoring of probes through conjugation with proteins proximal to metal ion sites [
17,
30,
31]. By leveraging Cu
+/Cu
2+-directed acyl imidazole strategies, scientists developed probes CD649 and CD649.2 for intracellular labile copper pools (
Fig. 1) [
17,
30]. Upon Cu
+/Cu
2+coordination, the probes trigger Lewis acid activation of the acyl imidazole moiety, enabling covalent capture of proximal protein nucleophilic residues [
30]. This mechanism allows for the spatial retention of the fluorophore while liberating fluorescence-quenching Cu
2+ ions [
17]. Therefore, this approach enables the real-time tracking of dynamic fluctuations in intracellular labile copper pools [
17]. Notably, the CD649.2 exhibited dual selectivity for copper ions and the divalent oxidation state [
17].
Although progress has been made in ABS-based copper ion probes, current strategies predominantly employ the acyl imidazole core framework, with a notable absence of alternative usable core structures. To address this limitation and explore alternative structural frameworks, we have developed a Cu
2+-directed covalent labeling strategy by using thiophosphoryl pyridinol/pyrimidinol as a reactive and dual Cu
2+-recognizing group. It reacts with nucleophilic lysine under alkaline conditions [
32]. Both sulfur atom of thiophosphoryl and pyridine/pyrimidine rings are widely recognized as potent Cu
2+ chelators and have been extensively utilized in probe design [
33–
35]. Coordination to the Lewis acidic Cu
2+ center enhances the electrophilicity of the thiophosphoryl moiety, thereby inducing covalent conjugation with proteins (
Fig. 1). This mechanism enables detection of Cu
2+ fluctuations in labile copper pools within living cells under exogenous stimuli.
To develop Cu
2+-responsive ABS probes capable of proximal protein labeling, we designed target compounds
Q1–
Q5 featuring a coumarin fluorophore, a diethylene glycol spacer, and thiophosphoryl pyridinol/pyrimidinol that integrate covalent anchoring and Cu
2+-coordination functionalities. The designed compounds were successfully synthesized
via a two-step reaction routine (
Fig. 2).
Next, we compared the activity of
Q1–
Q5 under Cu
2+-catalyzed conditions. Sodium dodecyl sulfate polyacrylamide gel electrophoresis| (SDS-PAGE) fluorescence imaging showed that compounds with
ortho-positioned nitrogen (
Q3, Q4 and
Q5) exhibited activity, whereas those lacking
ortho-nitrogen (
Q1, Q2) showed negligible activity (
Fig. 3A). Furthermore, introduction of additional nitrogen atoms at either the
ortho (
Q4) or
para position (
Q5) enhanced activity compared to
Q3 (
Fig. 3A). Comparative analysis of
Q3 and
Q3a revealed that replacing the sulfur atom with an oxygen atom (
Q3a) abolished the Cu
2+-catalyzed enhancement of labeling activity (
Fig. 3B). These results demonstrated the critical importance of the
ortho-positioned nitrogen atoms and sulfur atom.
To evaluate the Cu
2+ selectivity of
Q3–
Q5, we performed fluorescent labeling assays using HeLa cell lysate as a proteomic substrate
in vitro. The lysate was pre-incubated with various metal ions (250 µmol/L each) under physiological buffer conditions (50 mmol/L HEPES, pH 7.4) for 30 min. For the Fe
2+ and Cu
+ conditions, tris(2-carboxyethyl)phosphine (TCEP, 1000 µmol/L) was included as a reducing agent. Following the metal ion pretreatment, the samples were incubated with
Q3–
Q5 (50 µmol/L) for 2 h at 20 ℃. As shown in Fig. S1 (Supporting information), subsequent fluorescent scanning analysis of SDS-PAGE separated samples demonstrated that
Q3 (Fig. S1A),
Q4 (Fig. S1B), and
Q5 (
Fig. 3C) exhibited Cu
2+-dependent covalent labeling of lysate proteins, with significantly higher efficiency than observed for other metal ions. Notably, even under conditions where Cu
+ concentration is tenfold higher than that of Cu
2+,
Q5 remains inactive (Fig. S2 in Supporting information), demonstrating its high selectivity for Cu
2+.
Next, we examined the effects of various conditions on Cu2+-dependent covalent labeling of proteins, including Cu2+ concentration and reaction time, as well as pH value and competition with copper chelators. As shown in Figs. S3 and S4 (Supporting information), the protein labeling of Q5, catalyzed by Cu2+ ions, exhibits dependence on Cu2+ concentration and reaction time, aligning well with the intended design (Figs. S3A and B). Further experimental results indicate that Q5 labels proteins more readily under alkaline conditions (Fig. S3C). Additionally, Cu2+ chelator tris[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl] amine (TBTA, Fig. S4B) inhibits the Cu2+-dependent protein labeling of Q5 in a concentration-dependent manner (Fig. S4A).
To delineate the underlying mechanism, we first evaluated the effect of Cu
2+ on the hydrolytic activity of thiophosphoryl pyrimidinol. To circumvent potential interference from the coumarin moiety, we modified the
Q5 by replacing the coumarin group with a butynyl group, yielding the redesigned compound
R5 (
Fig. 4A). Hydrolysis kinetics of
R5 (1 µmol/L) were subsequently analyzed in HEPES buffer (pH 8.0). High performance liquid chromatography (HPLC) monitoring revealed slow hydrolysis of
R5 under a mild alkaline condition, with ~30% degradation observed within 92 min (
Fig. 4B). Notably, Cu
2+ (5 µmol/L) dramatically accelerated this process, as evidenced by detectable hydrolysis within 2 min and complete degradation within 92 min (
Fig. 4B). Liquid chromatography-mass spectrometry (LC-MS) analysis identified two hydrolysis products: pyrimidinol (
m/
z 97.0401 [M + H]
+,
Fig. 4C) eluting at 2.9 min (Fig. S5 in Supporting information), and a thiophosphate intermediate (
m/
z 243.0028 [M − H]
−,
Fig. 4D) eluting at 5.9 min (Fig. S5), corresponding to the hydrolysis of the thiophosphoryl pyrimidinol fragment. These results suggest that Cu
2+ promotes the hydrolytic activation of the thiophosphoryl pyrimidinol unit, thereby potentiating the compounds’ covalent protein-labeling activity.
Lysine is highly nucleophilic and predominantly located on protein surfaces, making it an accessible target for covalent reagents and probes [
31,
36,
37]. To examine Cu
2+-dependent lysine modification by
R5, we conducted LC-MS analyses of
R5 (5 µmol/L) and Cbz-Lys (0.5 µmol/L,
Fig. 4A) reaction in the presence of Cu
2+ (25 µmol/L) (Fig. S6A in Supporting information). The product detected (
m/
z 519.1982 [M − H]
−,
Fig. 4E) was confirmed as
R5K (a thiophosphorylated lysine adduct) by comparison with a synthetic standard (Fig. S7 in Supporting information). Critically, no
R5K formation was observed under Cu
2+-free conditions, even after 12 h of incubation during which
R5 had fully been hydrolyzed (Fig. S6B in Supporting information). Collectively, these mechanistic studies demonstrate that Cu
2+ binding enhances the reactivity of the thiophosphorylated pyrimidinol moiety, promoting either hydrolysis or lysine conjugation (
Fig. 4A).
Building on the Cu
2+ selectivity of the compounds, we further investigated their ability to detect changes in intracellular Cu
2+ levels. Among the tested compounds,
Q5 exhibited the highest Cu
2+-catalyzed activity
in vitro (
Fig. 3A). As shown in Figs. S8 and S9 (Supporting information), further cellular experiments and stability assays confirmed the low cytotoxicity of
Q5 toward HeLa cells (50% cytotoxic concentration (CC
50) = 75 ± 0.41 µmol/L, Fig. S8) and its slow hydrolysis under physiological conditions (phosphate buffered saline (PBS), pH 7.4, 37 ℃, Fig. S9). These results prompting the selection of
Q5 for monitoring intracellular Cu
2+ fluctuations. To validate its responsiveness, intracellular Cu
2+ levels were modulated by pre-incubation with Cu(gtsm) (2 µmol/L) (a membrane-permeable Cu
2+ ionophore, Fig. S10 in Supporting information) to elevate Cu
2+ pools or treatment with TETA (100 µmol/L) (triethylenetetramine, a high-affinity Cu
2+ chelator, Fig. S10) to deplete endogenous Cu
2+, followed by incubation with
Q5 (1 µmol/L). Live-cell fluorescence imaging revealed distinct Cu
2+-dependent responses. Cu(gtsm)-treated cells showed markedly enhanced fluorescence compared to untreated control (
Figs. 5A and B). TETA-treated cells exhibited significant fluorescence attenuation (
Figs. 5A and B). The trend that cellular fluorescence intensity changes of
Q5 is similar to that of the Cu
2+ probe
N-aminofluorescein (Fig. S11 in Supporting information) [
38–
40]. These results conclusively demonstrate that
Q5 serves as a reliable probe for monitoring the changes of labile Cu
2+ levels in living cells. Subcellular co-localization experiments demonstrated that
Q5 localizes to endoplasmic reticulum, lysosomes, and mitochondria (Fig. S12 in Supporting information).
In summary, we have successfully developed a novel class of Cu2+-dependent probes based on activity-driven labeling by thiophosphoryl pyridinol/pyrimidinol. Fluorescence labeling assays in cell lysates demonstrated the high selectivity of compounds for Cu2+. Mechanistic studies revealed that Cu2+ enhances the hydrolytic activation of the thiophosphoryl pyrimidinol, which catalyzes compounds to react with lysine residues, thereby enhancing protein-targeted labeling efficacy. Intracellular fluorescence imaging experiments demonstrated that Q5 enables the detection of Cu2+ fluctuations in living cells under exogenous stimuli. This study provided the thiophosphoryl pyridinol/pyrimidinol as a functional group for detecting labile Cu2+ levels in live cells, laying a solid foundation for the broader application of such chemotypes in chemical biology.