High-contrast fluorescence sensing of aqueous Cu(I) with triarylpyrazoline probes: dissecting the roles of ligand donor strength and excited state proton transfer

Rationally Designed Cu(I) Ligand to Prevent CuAβ-Generated ROS Production in the Alzheimer's Disease Context

In the context of Alzheimer's disease, copper (Cu) can be loosely bound to the amyloid-β (Aβ) peptide, leading to the formation of CuAβ, which can catalytically generate reactive oxygen species that contribute to oxidative stress. To fight against this phenomenon, the chelation therapy approach has been developed and consists of using a ligand able to remove Cu from Aβ and to redox-silence it, thus stopping the reactive oxygen species (ROS) production. A large number of Cu(II) chelators has been studied, allowing us to define and refine the properties required to design a "good" ligand, but without strong therapeutic outcomes to date. Those chelators targeted the Cu(II) redox state. Herein, we explore a parallel and relevant alternative pathway by designing a chelator able to target the Cu(I) redox state. To that end, we designed LH2 ([1N3S] binding set) and demonstrated that (i) it is perfectly able to extract Cu(I) from Cu(I)Aβ even in the presence of an excess of Zn(II) and (ii) it redox-silences the Cu, preventing the formation of ROS. We showed that LH2 that is sensitive to oxidation can efficiently replace the [Zn(II)L] complex without losing its excellent ability to stop the ROS production while increasing its resistance to oxidation.

Bioinspired Luminescent Europium-Based Probe Capable of Discrimination between Ag+ and Cu. Inorg Chem 2021;60:10791-10798. [PMID: 34236828 DOI: 10.1021/acs.inorgchem.1c01486]

Due to their similar coordination properties, discrimination of Cu⁺ and Ag⁺ by water-soluble luminescent probes is challenging. We have synthesized LCC4^Eu, an 18 amino acid cyclic peptide bearing a europium complex, which is able to bind one Cu⁺ or Ag⁺ ion by the side chains of two methionines, a histidine and a 3-(1-naphthyl)-l-alanine. In this system, the naphthyl moiety establishes a cation-π interaction with these cations. It also acts as an antenna for the sensitization of Eu³⁺ luminescence. Interestingly, when excited at 280 nm, LCC4^Eu behaves as a turn-on probe for Ag⁺ (+150% Eu emission) and as a turn-off probe for Cu⁺ (-50% Eu³⁺ emission). Shifting the excitation wavelength to 305 nm makes the probe responsive to Ag⁺ (+380% Eu³⁺ emission) but not to Cu⁺ or other physiological cations. Thus, LCC4^Eu is uniquely capable of discriminating Ag⁺ from Cu⁺. A detailed spectroscopic characterization based on steady-state and time-resolved measurements clearly demonstrates that Eu³⁺ sensitization relies on electronic energy transfer from the naphthalene triplet state to the Eu³⁺ excited states and that the cation-π interaction lowers the energy of this triplet state by 700 and 2400 cm^-1 for Ag⁺ and Cu⁺, respectively. Spectroscopic data point to a modulation of the efficiency of the electronic energy transfer caused by the differential red shift of the naphthalene triplet, deciphering the differential luminescence response of LCC4^Eu toward Ag⁺ and Cu⁺.

Porphyrinic Probe for Fluorescence "Turn-On" Monitoring of Cu+ in Aqueous Buffer and Mitochondria

A zinc(II) porphyrin derivative (ZPSN) was designed and synthesized, and this probe exhibited rapid, selective and reversible binding to Cu⁺ for fluorescence monitoring in pure aqueous buffer. The detection mechanism is based on Cu⁺-activated disruption of axial coordination between the pyridyl ligand and the zinc center, which changes the molecular geometry and inhibits intramolecular electron transfer (ET), leading to fluorescence enhancement of the probe. The proposed sensing mechanism was supported by UV-vis spectroscopy/fluorescence spectral titration, NMR spectroscopy, mass spectrometry, and time-resolved fluorescence decay studies. The dissociation constant was calculated to be 6.53 × 10^-11 M. CLSM analysis strongly suggested that ZPSN could penetrate live cells and successfully visualize Cu⁺ in mitochondria. This strategy may establish a design and offer a potential building block for construction of other metal sensors based on a similar mechanism.

Development of new peptide-based receptor of fluorescent probe with femtomolar affinity for Cu(+) and detection of Cu(+) in Golgi apparatus

Developing fluorescent probes for monitoring intracellular Cu(+) is important for human health and disease, whereas a few types of their receptors showing a limited range of binding affinities for Cu(+) have been reported. In the present study, we first report a novel peptide receptor of a fluorescent probe for the detection of Cu(+). Dansyl-labeled tripeptide probe (Dns-LLC) formed a 1:1 complex with Cu(+) and showed a turn-on fluorescent response to Cu(+) in aqueous buffered solutions. The dissociation constant of Dns-LLC for Cu(+) was determined to be 12 fM, showing that Dns-LLC had more potent binding affinity for Cu(+) than those of previously reported chemical probes for Cu(+). The binding mode study showed that the thiol group of the peptide receptor plays a critical role in potent binding with Cu(+) and the sulfonamide and amide groups of the probe might cooperate to form a complex with Cu(+). Dns-LLC detected Cu(+) selectively by a turn-on response among various biologically relevant metal ions, including Cu(2+) and Zn(2+). The selectivity of the peptide-based probe for Cu(+) was strongly dependent on the position of the cysteine residue in the peptide receptor part. The fluorescent peptide-based probe penetrated the living RKO cells and successfully detected Cu(+) in the Golgi apparatus in live cells by a turn-on response. Given the growing interest in imaging Cu(+) in live cells, a novel peptide receptor of Cu(+) will offer the potential for developing a variety of fluorescent probes for Cu(+) in the field of copper biochemistry.

Rational Design of a Water-Soluble, Lipid-Compatible Fluorescent Probe for Cu(I) with Sub-Part-Per-Trillion Sensitivity

Knowledge-driven optimization of the ligand and fluorophore architectures yielded an ultrasensitive Cu(i)-selective fluorescent probe featuring a 180-fold fluorescence contrast and 41% quantum yield.

Fluorescence probes represent an attractive solution for the detection of the biologically important Cu(i) cation; however, achieving a bright, high-contrast response has been a challenging goal. Concluding from previous studies on pyrazoline-based fluorescent Cu(i) probes, the maximum attainable fluorescence contrast and quantum yield were limited due to several non-radiative deactivation mechanisms, including ternary complex formation, excited state protonation, and colloidal aggregation in aqueous solution. Through knowledge-driven optimization of the ligand and fluorophore architectures, we overcame these limitations in the design of CTAP-3, a Cu(i)-selective fluorescent probe offering a 180-fold fluorescence enhancement, 41% quantum yield, and a limit of detection in the sub-part-per-trillion concentration range. In contrast to lipophilic Cu(i)-probes, CTAP-3 does not aggregate and interacts only weakly with lipid bilayers, thus maintaining a high contrast ratio even in the presence of liposomes.

Thiophene aldehyde-diamino uracil Schiff base: A novel fluorescent probe for detection and quantification of cupric, silver and ferric ions

A new Schiff base from the condensation of 5,6-diamino-1,3-dimethyluracil with 5-methylthiophene-2-carboxaldehyde was synthesized. The compound was characterized by spectral data (UV-Vis, IR, (1)H NMR, fluorescence, MS). Ethanolic solutions of the Schiff base exhibit a strong fluorescence emission at 385 nm (λex=341 nm), and have been employed as a "turn-off" fluorescent probe for selective detection of Ag(+), Cu(2+) and Fe(3+) ions in presence of other cations such as Na(+), K(+), Ca(2+) and Mg(2+) ions abundant in natural water. The interaction between the tested compound and copper, silver or iron ions is associated with a significant fluorescence decrease, showing detection limits of 2.1-14.2 ppb. Under optimal conditions, the developed sensor was successfully employed to determine Ag(+), Cu(2+) and Fe(3+) ions in real samples and proved to be selective and sensitive.

1,3,5-Triarylpyrazolines — pH-driven off-on-off molecular logic devices based on a “receptor1-fluorophore-spacer-receptor2” format with internal charge transfer (ICT) and photoinduced electron transfer (PET) mechanisms

The excited state photophysical properties of the 1,3,5-triarylpyrazolines 1–4 were studied in methanol and 1:1 (v/v) methanol–water, as well as 1:4 (v/v) methanol–water and water by fluorescence spectroscopy. The molecules 2–4 incorporate a “receptor1-fluorophore-spacer-receptor2” format while 1 is a reference compound based on a “fluorophore-receptor1” design. The molecular probes operate according to photoinduced electron transfer (PET) and internal charge transfer (ICT) processes. At basic and neutral pHs, 2–4 are essentially nonfluorescent due to PET from the electron-donating dimethylamino moiety appended on the 5-phenyl ring to the excited state of the 1,3,5-triarylpyrazoline fluorophore. At proton concentrations of 10−3 mol/L, the dimethylamino unit is protonated resulting in a strong blue fluorescence about 460 nm with significant quantum yields up to 0.54. At acid concentrations above 10−2 mol/L, fluorescence quenching is observed by an ICT mechanism due to protonation of the pyrazoline chromophore. Symmetrical off-on-off fluorescence–pH profiles are observed, spanning six log units with a narrow on window within three pH units. Hence, 2–4 are novel examples of ternary photonic pH sensing molecular devices.

H+-Assisted fluorescent differentiation of Cu+ and Cu2+: effect of Al3+-induced acidity on chemical sensing and generation of two novel and independent logic gating pathways

A novel logic gate containing protecting groups interacts with various species (acetonitrile) with fluorescence responses relating to ligand non innocence.

Probing ternary complex equilibria of crown ether ligands by time-resolved fluorescence spectroscopy

Ternary complex formation with solvent molecules and other adventitious ligands may compromise the performance of metal-ion-selective fluorescent probes. As Ca(II) can accommodate more than 6 donors in the first coordination sphere, commonly used crown ether ligands are prone to ternary complex formation with this cation. The steric strain imposed by auxiliary ligands, however, may result in an ensemble of rapidly equilibrating coordination species with varying degrees of interaction between the cation and the specific donor atoms mediating the fluorescence response, thus diminishing the change in fluorescence properties upon Ca(II) binding. To explore the influence of ligand architecture on these equilibria, we tethered two structurally distinct aza-15-crown-5 ligands to pyrazoline fluorophores as reporters. Due to ultrafast photoinduced electron-transfer (PET) quenching of the fluorophore by the ligand moiety, the fluorescence decay profile directly reflects the species composition in the ground state. By adjusting the PET driving force through electronic tuning of the pyrazoline fluorophores, we were able to differentiate between species with only subtle variations in PET donor abilities. Concluding from a global analysis of the corresponding fluorescence decay profiles, the coordination species composition was indeed strongly dependent on the ligand architecture. Altogether, the combination of time-resolved fluorescence spectroscopy with selective tuning of the PET driving force represents an effective analytical tool to study dynamic coordination equilibria and thus to optimize ligand architectures for the design of high-contrast cation-responsive fluorescence switches.

Robust affinity standards for Cu(I) biochemistry

The measurement of reliable Cu(I) protein binding affinities requires competing reference ligands with similar binding strengths; however, the literature on such reference ligands is not only sparse but often conflicting. To address this deficiency, we have created and characterized a series of water-soluble monovalent copper ligands, MCL-1, MCL-2, and MCL-3, that form well-defined, air-stable, and colorless complexes with Cu(I) in aqueous solution. X-ray structural data, electrochemical measurements, and an extensive network of equilibrium titrations showed that all three ligands form discrete Cu(I) complexes with 1:1 stoichiometry and are capable of buffering Cu(I) concentrations between 10(-10) and 10(-17) M. As most Cu(I) protein affinities have been obtained from competition experiments with bathocuproine disulfonate or 2,2'-bicinchoninic acid, we further calibrated their Cu(I) stability constants against the MCL series. To demonstrate the application of these reagents, we determined the Cu(I) binding affinity of CusF (log K = 14.3 ± 0.1), a periplasmic metalloprotein required for the detoxification of elevated copper levels in Escherichia coli . Altogether, this interconnected set of affinity standards establishes a reliable foundation that will facilitate the precise determination of Cu(I) binding affinities of proteins and small-molecule ligands.

Synthetic fluorescent probes for monovalent copper

Fluorescent probes are powerful and cost-effective tools for the detection of metal ions in biological systems. Compared to non-redox-active metal ions, the design of fluorescent probes for biological copper is challenging. Within the reducing cellular environment, copper is predominantly present in its monovalent oxidation state; therefore, the design of fluorescent probes for biological copper must take into account the rich redox and coordination chemistry of Cu(I). Recent progress in understanding the underlying solution chemistry and photophysical pathways led to the development of new probes that offer high fluorescence contrast and excellent selectivity towards monovalent copper.