Takuya Terai

Development of a Highly Selective Fluorescence Probe for Hydrogen Sulfide

Hydrogen sulfide (H(2)S) has recently been identified as a biological response modifier. Here, we report the design and synthesis of a novel fluorescence probe for H(2)S, HSip-1, utilizing azamacrocyclic copper(II) ion complex chemistry to control the fluorescence. HSip-1 showed high selectivity and high sensitivity for H(2)S, and its potential for biological applications was confirmed by employing it for fluorescence imaging of H(2)S in live cells.

Development of an Si-Rhodamine-Based Far-Red to Near-Infrared Fluorescence Probe Selective for Hypochlorous Acid and Its Applications for Biological Imaging

A far-red to near-infrared (NIR) fluorescence probe, MMSiR, based on Si-rhodamine, was designed and synthesized for sensitive and selective detection of HOCl in real time. MMSiR and its oxidized product SMSiR have excellent properties, including pH-independence of fluorescence, high resistance to autoxidation and photobleaching, and good tissue penetration of far-red to NIR fluorescence emission. The value of MMSiR was confirmed by real-time imaging of phagocytosis using a fluorescence microscope. wsMMSiR, a more hydrophilic derivative of MMSiR, permitted effective in vivo imaging of HOCl generation in a mouse peritonitis model. This probe is expected to be a useful tool for investigating the wide range of biological functions of HOCl.

Fluorescent probes for bioimaging applications

Fluorescent probes based on small organic molecules have become indispensable tools in modern biology because they provide dynamic information concerning the localization and quantity of the molecules of interest, without the need of genetic engineering of the sample. In this review, following a brief outline of the principle of fluorescence imaging, we recount some recent achievements in the field of small-molecular fluorescent probes. First, probes for metal cations, including those suitable for two-photon imaging, are introduced. Next, methodologies to visualize proteases are discussed, with special emphasis on activity-based probes for use in vivo. All these probes have been confirmed to be applicable to cellular or in vivo imaging.

Development and Application of a Near-Infrared Fluorescence Probe for Oxidative Stress Based on Differential Reactivity of Linked Cyanine Dyes

Reactive oxygen species (ROS) operate as signaling molecules under various physiological conditions, and overproduction of ROS is involved in the pathogenesis of many diseases. Therefore, fluorescent probes for visualizing ROS are promising tools with which to uncover the molecular mechanisms of physiological and pathological processes and might also be useful for diagnosis. Here we describe a novel fluorescence probe, FOSCY-1, operating in the physiologically favorable near-infrared region. The probe consists of two differentially ROS-reactive cyanine dyes connected by a linker; reaction of the more susceptible dye with ROS releases intramolecular fluorescence quenching of the less susceptible dye. We successfully applied this probe to detect ROS produced by HL60 cells and porcine neutrophils and for imaging oxidative stress in a mouse model of peritonitis.

Development of a Highly Sensitive Fluorescence Probe for Hydrogen Peroxide

Hydrogen peroxide is believed to play a role in cellular signal transduction by reversible oxidation of proteins. Here, we report the design and synthesis of a novel fluorescence probe for hydrogen peroxide, utilizing a photoinduced electron transfer strategy based on benzil chemistry to control the fluorescence. The practical value of this highly sensitive and selective fluorescence probe, NBzF, was confirmed by its application to imaging of hydrogen peroxide generation in live RAW 264.7 macrophages. NBzF was also employed for live cell imaging of hydrogen peroxide generated as a signaling molecule in A431 human epidermoid carcinoma cells.

Rational Design of Ratiometric Near-Infrared Fluorescent pH Probes with Various pKa Values, Based on Aminocyanine

Novel ratiometric, near-infrared fluorescent pH probes with various pK(a) values have been designed and synthesized on the basis of aminocyanine bearing a diamine moiety, and their photochemical properties were evaluated. Under acidic conditions, these pH probes showed a 46- to 83-nm red shift of the absorption maximum. This change is sufficiently large to permit their use as ratiometric pH probes, and is reversible, whereas monoamine-substituted aminocyanines showed irreversible changes because of their instability under acidic conditions. Furthermore, the pK(a) values of these probes can be predicted from the calculated pK(a) values of the diamine moieties, obtained from the SciFinder database. This design strategy is very simple and flexible, and should be applicable to develop NIR pH probes for various applications.

Evolution of Group 14 Rhodamines as Platforms for Near-Infrared Fluorescence Probes Utilizing Photoinduced Electron Transfer

The absorption and emission wavelengths of group 14 pyronines and rhodamines, which contain silicon, germanium, or tin at the 10 position of the xanthene chromophore, showed large bathochromic shifts compared to the original rhodamines, owing to stabilization of the LUMO energy levels by σ*-π* conjugation between group 14 atom-C (methyl) σ* orbitals and a π* orbital of the fluorophore. These group 14 pyronines and rhodamines retain the advantages of the original rhodamines, including high quantum efficiency in aqueous media (Φ(fl) = 0.3-0.45), tolerance to photobleaching, and high water solubility. Group 14 rhodamines have higher values of reduction potential than other NIR light-emitting original rhodamines, and therefore, we speculated their NIR fluorescence could be controlled through the photoinduced electron transfer (PeT) mechanism. Indeed, we found that the fluorescence quantum yield (Φ(fl)) of Si-rhodamine (SiR) and Ge-rhodamine (GeR) could be made nearly equal to zero, and the threshold level for fluorescence on/off switching lies at around 1.3-1.5 V for the SiRs. This is about 0.1 V lower than in the case of TokyoGreens, in which the fluorophore is well established to be effective for PeT-based probes. That is to say, the fluorescence of SiR and GeR can be drastically activated by more than 100-fold through a PeT strategy. To confirm the validity of this strategy for developing NIR fluorescence probes, we employed this approach to design two kinds of novel fluorescence probes emitting in the far-red to NIR region, i.e., a series of pH-sensors for use in acidic environments and a Zn(2+) sensor. We synthesized these probes and confirmed that they work well.

Hypoxia-Sensitive Fluorescent Probes for in Vivo Real-Time Fluorescence Imaging of Acute Ischemia

Based on the findings that the azo functional group has excellent properties as the hypoxia-sensor moiety, we developed hypoxia-sensitive near-infrared fluorescent probes in which a large fluorescence increase is triggered by the cleavage of an azo bond. The probes were used for fluorescence imaging of hypoxic cells and real-time monitoring of ischemia in the liver and kidney of live mice.

Design and Synthesis of a Highly Sensitive Off–On Fluorescent Chemosensor for Zinc Ions Utilizing Internal Charge Transfer

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 (Zn(2+)) is the second most abundant heavy metal ion in the human body after iron, and detection of chelatable Zn(2+) in biological studies has attracted much attention. Herein, we present a novel, highly sensitive off-on fluorescent chemosensor for Zn(2+) 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 Zn(2+), chemosensors would offer a better signal-to-noise ratio. However, it is difficult to quench the fluorescence completely before Zn(2+) binding, and most sensor molecules still show very weak fluorescence in the absence of Zn(2+). But even though the sensor shows a weak fluorescence in the absence of Zn(2+), this fluorescence can be further suppressed by selecting an excitation wavelength that is barely absorbed by the Zn(2+)-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 Zn(2+). The usefulness of 1 for monitoring Zn(2+) 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 Zn(2+)-sensitive off-on fluorescent sensor molecule that employs the ICT mechanism; most off-on sensor molecules for Zn(2+) employ the photoinduced electron transfer (PeT) mechanism.

Development of NIR Fluorescent Dyes Based on Si–rhodamine for in Vivo Imaging

We have developed a series of novel near-infrared (NIR) wavelength-excitable fluorescent dyes, SiR-NIRs, by modifying the Si-rhodamine scaffold to obtain emission in the range suitable for in vivo imaging. Among them, SiR680 and SiR700 showed sufficiently high quantum efficiency in aqueous media. Both antibody-bound and free dye exhibited high tolerance to photobleaching in aqueous solution. Subcutaneous xenograft tumors were successfully visualized in a mouse tumor model using SiR700-labeled anti-tenascin-C (TN-C) antibody, SiR700-RCB1. SiR-NIRs are expected to be useful as labeling agents for in vivo imaging studies including multicolor imaging, and also as scaffolds for NIR fluorescence probes.