Misc

Reactive sulfur species (RSS), which include various persulfides and polysulfides, are generated by multiple enzymes in vivo and play critical roles in mammalian physiological processes such as redox signaling, metabolic regulation, radical scavenging and anti-inflammatory responses. Cystathionine β-synthase (CBS), cystathionine γ-lyase (CSE) and 3-mercaptopyruvate sulfurtransferase (3MST) are well known to mediate endogenous production of hydrogen sulfide (H2S), and, together with the mitochondrial isoform of cysteinyl-tRNA synthetase (CARS2), are proposed to be major sources of intracellular persulfides and polysulfides. In mitochondria, enzymes involved in the sulfide oxidation pathway, including sulfide:quinone oxidoreductase (SQOR), persulfide dioxygenase (ETHE1) and thiosulfate sulfurtransferase (TST), also contribute to maintaining and regulating intracellular persulfide levels. Selective inhibitors targeting these enzymes are expected to be powerful tools for elucidating the functions of RSS, as well as having therapeutic potential. In this review, we present a comprehensive overview of these enzymes, focusing on their reaction mechanisms and inhibitors.

Fluorescence imaging is a powerful technique for visualizing biological events in living samples, and new fluorescence-control mechanisms are still needed to extend the scope of biomolecule-targeting fluorogenic probes. Twisted intramolecular charge transfer (TICT) is a unique fluorescence quenching mechanism that depends upon a twisted conformation to promote intramolecular charge separation. Probes utilizing TICT can detect biological molecules/phenomena, such as viscosity, polarity and extended protein structures, that cannot readily be accessed by probes employing other fluorescence-control mechanisms, such as photoinduced electron transfer or spirocyclization. In this review, we summarize recent work on molecular design strategies for TICT-based fluorogenic probes, focusing on structural-modification approaches to control the ease of TICT state formation.

Fluorescence imaging is a powerful technique for visualizing biological events in living samples with high temporal and spatial resolution. Fluorescent probes emitting far-red to near infrared (NIR) fluorescence are particularly advantageous for in vivo imaging due to their high tissue permeability and low autofluorescence, as well as their suitability for multicolor imaging. Among the far-red to NIR fluorophores, Si-rhodamine is one of the most practical fluorophores for the development of tailor-made NIR fluorescent probes because of the relative ease of synthesis of various derivatives, the unique intramolecular spirocyclization behavior, and the relatively high water solubility and high photostability of the probes. This review summarizes these features of Si-rhodamines and presents recent advances in the synthesis and applications of far-red to NIR fluorescent probes based on Si-rhodamines, focusing on live-cell imaging applications such as fluorogenic probes, super-resolution imaging and dye-protein hybrid-based indicators.