Spirostrain-Accelerated Chemiexcitation of Dioxetanes Yields Unprecedented Detection Sensitivity in Chemiluminescence Bioassays

Thymidine Phosphodiester Chemiluminescent Probe for Sensitive and Selective Detection of Ectonucleotide Pyrophosphatase 1

ENPP-1 is a transmembrane enzyme involved in nucleotide metabolism, and its overexpression is associated with various cancers, making it a potential therapeutic target and biomarker for early tumor diagnosis. Current detection methods for ENPP-1 utilize a colorimetric probe, TMP-pNP, which has significant limitations in sensitivity. Here, we present probe CL-ENPP-1, the first nucleic acid-based chemiluminescent probe designed for rapid and highly sensitive detection of ENPP-1 activity. The design of probe CL-ENPP-1 features a phenoxy-adamantyl-1,2-dioxetane luminophore linked to thymidine via a phosphodiesteric bond. Upon cleavage of the enzymatic substrate by ENPP-1, the probe undergoes an efficient chemiexcitation process to emit a green photon. Probe CL-ENPP-1 demonstrates an exceptional signal-to-noise ratio of 15000 and a limit of detection value approximately 4500-fold lower than the widely used colorimetric probe TMP-pNP. A comparison of TMP-pNP activation by ENPP-1 versus alkaline phosphatase (ALP) reveals a complete lack of selectivity. Removal of the self-immolative spacer from probe CL-ENPP-1 resulted in a new chemiluminescent probe, CL-ENPP-2, with an 18.4-fold increase in selectivity for ENPP-1 over ALP. The ability of probe CL-ENPP-2 to detect ENPP-1 activity in mammalian cells was assessed using the human breast cancer cell line MDA-MB-231. This probe demonstrated a 19.5-fold improvement in the signal-to-noise ratio, highlighting its superior ability to detect ENPP-1 activity in a biological sample. As far as we know, to date, CL-ENPP-1 and CL-ENPP-2 are the most sensitive probes for the detection of ENPP-1 catalytic activity. We anticipate that our new chemiluminescent probes will be valuable for various applications requiring ENPP-1 detection, including enzyme inhibitor-based drug discovery assays. The insights gained from our probe design principles could advance the development of more selective probes for ENPP-1 and contribute to future innovations in chemiluminescence research.

Unprecedented Photoinduced-Electron-Transfer Probe with a Turn-ON Chemiluminescence Mode-of-Action

PeT-based fluorescent probes were demonstrated to be powerful tools for detection and imaging, owing to their significant fluorescence enhancement in response to specific targets. While numerous examples of fluorescence-based PeT have been frequently reported, there is not even a single example of a PeT probe that operates via a chemiluminescence mode. Here we report the first PeT-based turn-on probe that acts via a chemiluminescent operation mode. We designed, synthesized, and evaluated a novel chemiluminescent probe, featuring a PeT-based turn-on mechanism. The probe consists of a phenoxy-1,2-dioxetane, linked to an azide unit that acts as a PeT quencher. Upon cycloaddition of a strained cycloalkyne with the azide, a triazole-dioxetane is formed, which undergoes relatively slow chemiexcitation, resulting in a measurement window with an exceptionally high signal-to-noise ratio (over 5000-fold). The PeT-dioxetane probe could effectively detect and image two model proteins labeled with strained cycloalkyne units (Myc-DBCO and Max-DBCO) through either NHS or maleimide conjugations. Comparative analysis shows that our PeT-based chemiluminescent probe significantly outperforms a commercially available fluorescent analog. We anticipate that the insights gained from this study will facilitate the development of additional chemiluminescent probes utilizing various PeT-quenching pathways.

Biocompatible Flash Chemiluminescent Assay Enabled by Sterically Hindered Spiro-Strained-Oxetanyl-1,2-Dioxetane

Chemiluminescence is the emission of light that occurs as a result of a chemical reaction. Depending on the rate of chemiexcitation, light emission can occur as a long-lasting, low-intensity, glow-type reaction or a rapid, highly intense flash-type reaction. Assays using a flash-type mode of action provide enhanced detection sensitivity compared to those using a glow-type mode. Recently, our group discovered that applying spiro-strain to 1,2-dioxetanes significantly increases their chemiexcitation rate, thereby transforming glow-type chemiluminescence into flash-type chemiluminescence. However, further examination of the structure-activity relationships revealed that the spiro-strain severely compromises the chemical stability of the 1,2-dioxetanes. We hypothesized that a combination of spiro-strain, steric hindrance, and an electron-withdrawing effect, will result in a chemically stable spiro-strained dioxetane with an accelerated chemiexcitation rate. Indeed, spiro-fused tetramethyl-oxetanyl exhibited a 128-fold faster chemiexcitation rate compared to adamantyl while maintaining similar chemical stability, with a half-life of over 400 hours in PBS 7.4 buffer at room temperature. Turn-on probes composed of tetramethyl-oxetanyl spiro-dioxetane exhibited significantly improved chemical stability in bacterial and mammalian cell media compared to previously developed dioxetane probes fused to a cyclobutyl unit. The superior chemical stability enables a tetramethyl-oxetanyl dioxetane probe to detect β-Galactosidase (β-gal) activity with enhanced sensitivity in Escherichia coli (E. coli) bacterial assays and leucine aminopeptidase activity in tumoral cell lines. Overall, the development of the tetramethyl-oxetanyl dioxetane luminophore enables us to enhance the detection sensitivity of chemiluminescent probes while maintaining high chemical stability. The results obtained in this study should assist in designing improved chemiluminescent probes and underscore the significance of strain-release techniques in enhancing the detection sensitivity of chemiluminescence assays.

Phenoxy-1,2-dioxetane-based activatable chemiluminescent probes: tuning of photophysical properties for tracing enzymatic activities in living cells

The use of chemiluminophores for tracing enzymatic activities in live-cell imaging has gained significant attention, making them valuable tools for diagnostic applications. Among various chemiluminophores, the phenoxy-1,2-dioxetane scaffold exhibits significant structural versatility and its activation is governed by the chemically initiated electron exchange luminescence (CIEEL) mechanism. This mechanism can be initiated by enzymatic activity, changes in pH, or other chemical stimuli. The photophysical properties of phenoxy-1,2-dioxetanes can be fine-tuned through the incorporation of different substituents on the phenolic ring and by anchoring them with specific triggers. This review discusses the variations in physicochemical properties, including emission maxima, quantum yield, aqueous solubility, and pKa, as influenced by structural modifications, thereby establishing a comprehensive structure-activity relationship. Furthermore, it categorises the probes based on different enzyme classes, such as hydrolase-sensitive probes, oxidoreductase-responsive probes, and transferase-activatable phenoxy-1,2-dioxetanes, offering a promising platform technology for the early diagnosis of diseases and disorders. The summary section highlights key opportunities and limitations associated with applying phenoxy-1,2-dioxetanes in achieving precise and effective enzyme assays.

Chemiexcitation Acceleration of 1,2-Dioxetanes by Spiro-Fused Six-Member Rings with Electron-Withdrawing Motifs

The chemiluminescent light-emission pathway of phenoxy-1,2-dioxetane luminophores attracts growing interest within the scientific community. Dioxetane probes undergoing rapid flash-type chemiexcitation exhibit higher detection sensitivity than those with a slow glow-type chemiexcitation rate. We discovered that dioxetanes fused to non-strained six-member rings, with hetero atoms or inductive electron-withdrawing groups, present both accelerated chemiexcitation rates and elevated chemical stability compared to dioxetanes fused to four-member strained rings. DFT computational simulations supported the chemiexcitation acceleration observed by spiro-fused six-member rings with inductive electron-withdrawing groups of dioxetanes. Specifically, a spiro-dioxetane with a six-member sulfone ring exhibited a chemiexcitation rate 293-fold faster than that of spiro-adamantyl-dioxetane. A turn-ON dioxetane probe for the detection of the enzyme β-galactosidase, containing the six-member sulfone unit, exhibited a S/N value of 108 in LB cell growth medium. This probe demonstrated a substantial increase in detection sensitivity towards E. coli bacterial cells expressing β-galactosidase, with an LOD value that is 44-fold more sensitive than that obtained by the adamantyl counterpart. The accelerated chemiexcitation and the elevated chemical stability presented by dioxetane containing a spiro-fused six-member ring with a sulfone inductive electron-withdrawing group, make it an ideal candidate for designing efficient turn-on chemiluminescent probes with exceptionally high detection sensitivity.

Boosting Chemiexcitation of Phenoxy-1,2-dioxetanes through 7-Norbornyl and Homocubanyl Spirofusion

The chemiluminescent light-emission pathway of phenoxy-1,2-dioxetane luminophores is increasingly attracting the scientific community's attention. Dioxetane probes that undergo rapid, flash-type chemiexcitation demonstrate higher detection sensitivity than those with a slower, glow-type chemiexcitation rate. This is primarily because the rapid flash-type produces a greater number of photons within a given time. Herein, we discovered that dioxetanes fused to 7-norbornyl and homocubanyl units present accelerated chemiexcitation rates supported by DFT computational simulations. Specifically, the 7-norbornyl and homocubanyl spirofused dioxetanes exhibited a chemiexcitation rate 14.2-fold and 230-fold faster than that of spiro-adamantyl dioxetane, respectively. A turn-ON dioxetane probe for the detection of the enzyme β-galactosidase, containing the 7-norbornyl spirofused unit, exhibited an S/N value of 415 at a low enzyme concentration. This probe demonstrated an increase in detection sensitivity toward β-galactosidase expressing bacteria E. coli with a limit-of-detection value that is 12.8-fold more sensitive than that obtained by the adamantyl counterpart. Interestingly, the computed activation free energies of the homocubanyl and 7-norbornyl units were correlated with their CCsC spiro-angle to corroborate the measured chemiexcitation rates.

Mechanically Triggered Bright Chemiluminescence from Polymers by Exploiting a Synergy between Masked 2-Furylcarbinol Mechanophores and 1,2-Dioxetane Chemiluminophores

Mechanoluminescence, or the generation of light from materials under external force, is a powerful tool for biology and materials science. However, direct mechanoluminescence from polymers remains limited. Here, we report a novel design strategy for mechanoluminescent polymers that leverages the synergy between a masked 2-furylcarbinol mechanophore for mechanically triggered release and an adamantylidene-phenoxy-1,2-dioxetane chemiluminophore payload. Ultrasound-induced mechanochemical activation of polymers, in both organic and aqueous solutions, triggers a cascade reaction that ultimately results in bright green light emission. This novel strategy capitalizes on the modularity of the masked 2-furylcarbinol mechanophore system in combination with advances in the design of exceptionally bright and highly tunable adamantylidene-1,2-dioxetane chemiluminophores. We anticipate that this chemistry will enable diverse applications in optoelectronics, sensing, bioimaging, optogenetics, and many other areas.

Revolutionizing Senescence Detection: Advancements from Traditional Methods to Cutting-Edge Techniques

The accumulation of senescent cells is an important factor in the complex progression of aging, with significant implications for the development of numerous diseases. Thus, understanding the fundamental mechanisms of senescence is paramount for advancing preventive and therapeutic approaches to age-related conditions. Important to this pursuit is the precise identification and examination of senescent cells, contingent upon the recognition of specific biomarkers. Historically, detection methods relied on assessing molecular protein and mRNA levels and various staining techniques. While these conventional approaches have contributed substantially to the field, they possess limitations in capturing the dynamic evolution of cellular aging in real time. The emergence of novel technologies has led to a paradigm shift in senescence research. Gene-edited mouse models and the application of advanced probes have revolutionized our ability to detect senescent cells. These cutting-edge methodologies provide a more detailed and accurate means of dynamically monitoring, characterizing and potentially eliminating senescent cells, thus enhancing our understanding of the complex mechanisms of aging. This review comprehensively explores both traditional and innovative senescent cell detection methods, elucidating their advantages, limitations and implications for future investigations and could serve as a comprehensive guide and catalyst for further advancements in the understanding of aging and associated pathologies.

Hyper-Responsive Chemiluminescent Probe Reveals Distinct PYRase Activity in Pseudomonas aeruginosa

Pyrrolidone carboxyl peptidase, commonly known as PYRase, is an exopeptidase that catalytically cleaves an N-terminal pyroglutamic acid from peptides or proteins. The diverse functions of PYRases in bacterial enzymology have prompted the development of various bacterial diagnostic techniques. However, the specific physiological role and activity of this enzyme across the bacterial kingdom remain unclear. Here, we present a functional phenoxy-1,2-dioxetane chemiluminescent probe (PyrCL) that can selectively detect PYRase activity in both Gram-positive and Gram-negative bacteria. The probe activation mechanism is based on the cleavage of a pyroglutamyl substrate, followed by a release of the phenoxy-dioxetane luminophore, which then undergoes efficient chemiexcitation to emit a green photon. Probe PyrCL exhibits an effective turn-on response with superior detection capability in terms of response time and sensitivity compared to existing fluorescence probes. The superior detection sensitivity of the chemiluminescent probe enables us to reveal previously undetected PYRase activity in Streptococcus mutans. Furthermore, it enables the discrimination of Pseudomonas aeruginosa from other Gram-negative bacteria in the tested panel, based on their distinct PYRase activity. We expect that probe PyrCL will have great value for PYRase-based bacteria diagnosis with use in basic research and clinical applications.

Bioorthogonal Reactions in Bioimaging

Visualization of biomolecules in their native environment or imaging-aided understanding of more complex biomolecular processes are one of the focus areas of chemical biology research, which requires selective, often site-specific labeling of targets. This challenging task is effectively addressed by bioorthogonal chemistry tools in combination with advanced synthetic biology methods. Today, the smart combination of the elements of the bioorthogonal toolbox allows selective installation of multiple markers to selected targets, enabling multicolor or multimodal imaging of biomolecules. Furthermore, recent developments in bioorthogonally applicable probe design that meet the growing demands of superresolution microscopy enable more complex questions to be addressed. These novel, advanced probes enable highly sensitive, low-background, single- or multiphoton imaging of biological species and events in live organisms at resolutions comparable to the size of the biomolecule of interest. Herein, the latest developments in bioorthogonal fluorescent probe design and labeling schemes will be discussed in the context of in cellulo/in vivo (multicolor and/or superresolved) imaging schemes. The second part focuses on the importance of genetically engineered minimal bioorthogonal tags, with a particular interest in site-specific protein tagging applications to answer biological questions.