Hypochlorous acid (HOCl) plays an important role in the immune system not only protecting the organism from pathogens, but also, due to its high reactivity, provoking the development and complication of many diseases. Therefore, the development of highly sensitive and selective HOCl biosensors, including fluorescent probes, to better understand the roles of HOCl in living systems, is of great importance. Nevertheless, there are many difficulties associated with HOCl registration in biological probes (chemo- and photostability, cytotoxicity, fluorescence and absorption characteristics, selectivity, sensitivity, etc.). Thus, the development of new chemosensors has been relevant for many years. In this review, we describe classification of small-molecule probes for HOCl detection in biological systems, as well as summarize the results of the development of chemosensors, their photophysical properties, and biological applications. Particular attention is paid to the achievements in this area over the years 2016–2021. A number of problems related to the design and application of small-molecule probes are formulated. Due to the absence of a “gold standard” among the HOCl chemosensors, which is associated with the commercial unavailability or complex methods of synthesis of new sensors as well, some recommendations are given regarding the field of the chemosensor application. Trends in the development of fluorescent chemosensors for HOCl visualization in living cells are outlined.

Hypochlorous acid (HOCl) derived from hydrogen peroxide and chloride anion by myeloperoxidase (MPO) plays a significant role in physiological and pathological processes. Herein we report a phenoxazine-based fluorescent probe Celestine Blue B (CB) that is applicable for HOCl detection in living cells and for assaying the chlorinating activity of MPO. A remarkable selectivity and sensitivity (limit of detection is 32 nM), along with a rapid “turn-on” response of CB to HOCl was demonstrated. Furthermore, the probe was able to detect endogenous HOCl and reactive halogenated species by fluorescence spectroscopy, confocal microscopy, and flow cytometry techniques. Hence, CB is a promising tool for investigating the role of HOCl in health and disease and for screening the drugs capable of regulating MPO activity.

This review has considered the mechanisms of intracellular and intercellular signaling regulation by myeloperoxidase (MPO), an enzyme in neutrophil azurophilic granules, during oxidative/halogenative stress and inflammation development. The stages of enzyme functioning and the formation of reactive halogen species are described. The functioning of MPO and production of reactive halogen species are shown to depend on the activity of NADPH oxidase; the role of NADPH oxidase and reactive oxygen species in the regulation of MPO function is discussed. Particular attention is focused on the role of biological molecules modified by reactive halogen species in modulating NADPH oxidase activity, exocytosis of granular proteins, NETosis, and other neutrophil functions, based on the principle of positive feedback. A special feature of the review is the discussion of the non-canonical function of MPO, namely its signaling role in the regulation of cellular processes, which is not associated with the catalytic activity of the enzyme.

The influence of various mechanical influences (transfusion, stirring, vibration, shaking, etc.) and magnetic installations (used in the application of spin chemistry methods) on colloidal solutions of protein and water, which are often used in pharmaceutical production, was studied. It has been shown that when mechanical influences are applied, physical and chemical properties of water and aqueous colloids of the IgG protein are changed. Magnetic fields do not have a significant effect on water; however, variation in a number of physical and chemical characteristics is observed in protein colloids. Moreover, the effect after exposure to magnetic fields with a frequency of 8 Hz is higher compared to the effect after exposure to magnetic fields with a frequency of 50 Hz. This effect persists even at extremely low concentrations of IgG protein molecules. The measurement system proposed in this work makes it possible to monitor the state of protein molecules in a non-invasive mode. In the future, optical and potentiometric methods built into flow systems can be used at all stages of the production of protein pharmaceuticals.

Reactive oxygen species (ROS) have a crucial role in human physiological and pathophysiological processes. Prolonged exposure to high ROS concentrations may lead to cardiovascular, neurodegenerative, etc. diseases. In this study, gallocyanine has been proposed to register the ROS production. The gallocyanine spectral properties changes under ROS (•О2ˉ, H2O2) and reactive halogen (HOCl) species are analyzed. It is shown that the dye is oxidized in solution, both under the action of ROS and reactive halogen species. Based on the data obtained, it is possible to suggest that superoxide anion radicals make a major contribution to chemical conversion of the dye in suspensions of activated neutrophils. It is that gallocyanine can be used to assess the functional activity of neutrophils, namely, the NADPH-oxidase, as well as to design and test novel therapeutic agents for diseases associated with developing oxidative stress.