Molecular assemblies that change photoluminescence color in response to thermal or mechanical stimulation without dissociation into the monomeric states in water are described herein. A dumbbell-shaped amphiphilic compound forms micellar molecular assemblies in water and exhibits yellow photoluminescence derived from excimer formation of the luminescent core, which contains a 2,6-diethynylanthracene moiety. Annealing of the aqueous solution induces a photoluminescence color change from yellow to green (λ_{em, max}=558525 nm). The same photoluminescence color change is also achieved by rubbing the yellow-photoluminescence-emitting molecular assemblies adsorbed on glass substrates with cotton wool in water. The observed green photoluminescence is ascribed to micelles that are distinct from the yellow-photoluminescence-emitting micelles, on the basis of transmission electron microscopy observations, atomic force microscopy observations, and dynamic light scattering measurements. We examined the relationship between the structure of the molecular assemblies and the photophysical properties of the anthracene derivative in water before and after thermal or mechanical stimulation and concluded that thermal or mechanical stimuli-induced slight changes of the molecular-assembled structures in the micelles result in the change in the photoluminescence color from yellow to green in water.

Switching of the luminescence properties of molecular materials in response to mechanical stimulation is of fundamental interest and also has a range of potential applications. Herein, a water-soluble mechanochromic luminescent pyrene derivative having two hydrophilic dendrons is reported. This pyrene derivative is the first example of a mechanochromic luminescent organic compound that responds to relative humidity. Mechanical stimulation (grinding) of this pyrene derivative in the solid state results in a change of the photoluminescence from yellow to green. Subsequent exposure to water vapor induces recovery of the initial yellow photoluminescence. The color change is reversible through at least ten cycles. It is also demonstrated that this compound can be applied as a mechano-sensing material in frictional wear testing for grease, owing to its immiscibility in non-polar solvents and its non-crystalline behavior. Transmission electron microscope and atomic force microscope observations of samples prepared from dilute aqueous solutions of the pyrene derivative on suitable substrates, together with dynamic light scattering measurements for the compound in aqueous solution, indicate that this amphiphilic dumbbell-shaped molecule forms micelles in water.

We present a novel design strategy for off/on fluorescent probes suitable for selective two-step labeling of proteins. To validate this strategy, we designed and synthesized an off/on fluorescent probe, 1–Ni²⁺, which targets a cysteine-modified hexahistidine (His) tag. The probe consists of dichlorofluorescein conjugated with nitrilotriacetic acid (NTA)-Ni²⁺ as the His-tag recognition site and a 2,4-dinitrophenyl ether moiety, which quenches the probe’s fluorescence by photoinduced electron transfer (PeT) from the excited fluorophore to the 2,4-dinitrophenyl ether (donor-excited PeT; d-PeT) and also has reactivity with cysteine. His-tag recognition by the NTA–Ni²⁺ moiety is followed by removal of the 2,4-dinitrophenyl ether quencher by proximity-enhanced reaction with the cysteine residue of the modified tag; this results in a marked fluorescence increase. Addition of His-tag peptide bearing a cysteine residue to aqueous probe solution resulted in about 20-fold fluorescence increment within 10 min, which is the largest fluorescence enhancement so far obtained with a visible light-excitable fluorescent probe for a His-based peptide tag. Further, we successfully visualized CysHis₆-peptide tethered to microbeads without any washing step. The probe also showed a large fluorescence increment in the presence of His₆Cys-tagged enhanced blue fluorescent protein (EBFP), but not His₆-tagged EBFP. We consider this system is superior to large fluorescence tags (e.g., green fluorescent protein: 27 kDa), which can perturb protein folding, trafficking and function, and also to existing small tags, which generally show little fluorescence increase upon target recognition and therefore require a washout step. This strategy should also be applicable to other tags.

Firefly luciferase is widely used as a reporter gene in assays to study gene expression, gene delivery, and so on because of its extremely high signal-to-noise ratio. The availability of a range of bioluminogenic substrates would greatly extend the applicability of the luciferin–luciferase system. Herein, we describe a design concept for functional bioluminogenic substrates based on the aminoluciferin (AL) scaffold, together with a convenient, high-yield method for synthesizing N-alkylated ALs. We confirmed the usefulness of ALs as bioluminogenic substrates by synthesizing three probes. The first was a conjugate of AL with glutamate, Glu–AL. When Glu–AL, the first membrane-impermeable bioluminogenic substrate of luciferases, was applied to cells transfected with luciferase, luminescence was not observed; that is, by using Glu–AL, we can distinguish between intracellular and extracellular events. The second was Cy5–AL, which consisted of Cy5, a near-infrared (NIR) cyanine fluorescent dye, and AL, and emitted NIR light. When Cy5–AL reacted with luciferase, luminescence derived from Cy5 was observed as a result of bioluminescence resonance energy transfer (BRET) from AL to Cy5. The NIR emission wavelength would allow a signal to be observed from deeper tissues in bioluminescence in vivo imaging. The third was biotin–DEVD–AL (DEVD=the amino acid sequence Asp-Glu-Val-Asp), which employed a caspase-3 substrate peptide as a switch to control the accessibility of the substrate to luciferase, and could detect the activity of caspase-3 in a time-dependent manner. This generalized design strategy should be applicable to other proteases. Our results indicate that the AL scaffold is appropriate for a range of functional luminophores and represents a useful alternative substrate to luciferin.

Fluorescence imaging is a powerful tool for the visualization of biological molecules in living cells, tissue slices, and whole bodies, and is important for elucidating biological phenomena. Furthermore, zinc (Zn²⁺) is the second most abundant heavy metal ion in the human body after iron, and detection of chelatable Zn²⁺ in biological studies has attracted much attention. Herein, we present a novel, highly sensitive off–on fluorescent chemosensor for Zn²⁺ by using the internal charge transfer (ICT) mechanism. The rationale of our approach to highly sensitive sensor molecules is as follows. If fluorescence can be completely quenched in the absence of Zn²⁺, chemosensors would offer a better signal-to-noise ratio. However, it is difficult to quench the fluorescence completely before Zn²⁺ binding, and most sensor molecules still show very weak fluorescence in the absence of Zn²⁺. But even though the sensor shows a weak fluorescence in the absence of Zn²⁺, this fluorescence can be further suppressed by selecting an excitation wavelength that is barely absorbed by the Zn²⁺-free sensor molecule. Focusing on careful control of ICT within the 4-amino-1,8-naphthalimide dye platform, we designed and synthesized a new chemosensor (1) that shows a pronounced fluorescence enhancement with a blueshift in the absorption spectrum upon addition of Zn²⁺. The usefulness of 1 for monitoring Zn²⁺ changes was confirmed in living HeLa cells. There have been several reports on 4-amino-1,8-naphthalimide-based fluorescent sensor molecules. However, 1 is the first Zn²⁺-sensitive off–on fluorescent sensor molecule that employs the ICT mechanism; most off–on sensor molecules for Zn²⁺ employ the photoinduced electron transfer (PeT) mechanism.

The prevalence of type 2 diabetes is increasing dramatically throughout the world. Recently, dipeptidyl peptidase 4 (DPP4) was identified as a potential antidiabetes target. Many DPP4 inhibitors, such as sitagliptin and vildagliptin, have been developed and marketed, but superior therapeutic agents are still required. Therefore, we have developed new methodology for screening of DPP4 inhibitors. Absorption-based measurements with para-nitroaniline or fluorescence-based measurements with the coumarin derivative 7-amino-4-methylcoumarin are often used for the screening of protease inhibitors, including DPP4 inhibitors, but these strategies are not sufficiently sensitive because of interfering background absorption and fluorescence, thus giving rise to many false-positive and false-negative results. Therefore, we have designed and synthesised a novel DPP4 probe (Gly-Pro-BCD-Tb; Gly=glycine, Pro=proline, andBCD defines the backbone of the probe comprising an aniline derivative as on/off switch, a 7-amino-4-methyl-2(1H)-quinolinone (cs-124) as antenna moiety, and a diethylenetriamine-N,N,N’,N’’,N’’-pentaacetic acid (DTPA) as chelator moiety, Tb=terbium) for time-resolved fluorescence (TRF) measurements. TRF measurements with Gly-Pro-BCD-Tb showed high sensitivity and reliability in the inhibitory assay relative to Gly-Pro-MCA (MCA=4-methylcoumarin-7-amide), a conventional fluorescence probe for DPP4. Further, we employed our probe for high-throughput DPP4 inhibitor screening with 3841 randomly selected compounds and found that epibestatin, an epimer of bestatin (a well-known anticancer drug and general aminopeptidase inhibitor), showed dose-dependent DPP4 inhibitory activity. Interestingly, bestatin did not exhibit DPP4 inhibitory activity. We believe that this screening system will be useful for the discovery of DPP4 inhibitors with novel structural scaffolds.

We systematically synthesized bioluminogenic substrates bearing an amino group on benzothiazole, quinoline, naphthalene, and coumarin scaffolds. They emit bioluminescence in various colors: red, orange, yellow, and green. An amino-substituted coumarylluciferin derivative, coumarylaminoluciferin (CAL), showed the shortest bioluminescence wavelength among substrates reported so far. Further, the fluorescence of CAL did not exhibit solvatochromism, which suggests that its bioluminescence is not susceptible to environmental factors. We applied CAL as an energy-donor substrate for a bioluminescence resonance energy transfer (BRET) system with click beetle red luciferase (CBRluc), a mutant of firefly luciferase, as the energy-donor enzyme and yellow fluorescent protein (YFP) as the energy-acceptor fluorophore, and obtained a clearly bimodal bioluminescence spectrum. Stable bioluminescence that is not influenced by environmental factors is highly desirable for reliable measurements in biological assays.

In spite of the wide availability of various near-infrared (NIR) fluorophores as labeling reagents, there are few functional NIR fluorescent probes for which change in the absorption and/or fluorescence spectra upon specific reaction with biomolecules is seen. The widely used photoinduced electron-transfer mechanism is unsuitable for NIR fluorophores, such as tricarbocyanines, because their long excitation wavelength results in a small singlet excitation energy. We have reported the unique spectral properties of amine-substituted tricarbocyanines, which were utilized to develop two design strategies. One approach was based on control of the absorption wavelength by using the difference in electron-donating ability before and after a specific reaction with a biomolecule, and the other approach was based on control of the fluorescence intensity by modulating the Förster resonance energy-transfer efficiency through a change in the overlap integral that arises from the change in absorption under acidic conditions. These strategies were validated by obtaining tricarbocyanine-based ratiometric NIR fluorescent probes for esterase and for pH level.

Magnetic resonance imaging (MRI) permits noninvasive three-dimensional imaging of opaque organisms. Gadolinium (Gd³⁺) complexes have become important imaging tools as MRI contrast agents for MRI studies, though most of them are nonspecific and report solely on anatomy. Recently, MRI contrast agents have been reported whose ability to relax water protons is triggered or greatly enhanced by recognition of a particular biomolecule. This new class of MRI contrast agents could open up the possibility of reporting on the physiological state or metabolic activity deep within living specimens. One possible strategy for this purpose is to utilize the increase in the longitudinal water proton r₁ relaxivity that occurs upon slowing the molecular rotation of a small paramagnetic complex, a phenomenon which is known as receptor-induced magnetization enhancement (RIME), by either binding to a macromolecule or polymerization of the agent itself. Here we describe the design and synthesis of a novel β-galactosidase-activated MRI contrast agent, the Gd³⁺ complex [Gd-5], by using the RIME approach. β-Galactosidase is commonly used as a marker gene to monitor gene expression. This newly synthesized compound exhibited a 57 % increase in the r₁ relaxivity in phosphate-buffered saline (PBS) with 4.5 % w/v human serum albumin (HSA) in the presence of β-galactosidase. Detailed investigations revealed that RIME is the dominant factor in this increase of the observed r₁ relaxivity, based on analysis of Gd³⁺ complexes [Gd-5] and [Gd-8], which is generated from [Gd-5] by the activity of β-galactosidase, and spectroscopic analysis of their corresponding Tb³⁺ complexes, [Tb-5] and [Tb-8].

Near-infrared (NIR) fluorescent probes have attracted much attention, but despite the availability of various NIR fluorophores, only a few functional NIR probes, that is, probes whose absorption and/or fluorescence spectra change upon specific reaction with biomolecules, have been developed. However, functional probes operating in the NIR range that can be targeted to protons, metal ions, nitric oxide, β-galactosidase, and cellular stress markers are expected to be effective for fluorescence imaging in vivo. This Focus Review concentrates on these functional NIR probes themselves, not their applications.