## 1 Introduction  
Reactive oxygen (ROS) and nitrogen (RNS) species in cells are produced as a consequence of exogenous and/or endogenous stimuli [1, 2]. The amounts of those species inside cells determine their harmful or beneficial effects [3]. The mitochondrial electron transport chain (ETC) is their primary source in the cell, and ongoing cellular processes are the mitochondrial electron transport chain (ETC) [2, 4, 5]. The antioxidant defense system controls generating free radicals in a healthy cell [1, 6]. The balance between free radicals and their scavengers is critical because free radicals are involved in numerous processes resulting in diseases [4]. Excessive production of free radicals causes the progression of cardiovascular diseases and multiple cancers, including leukemia [2, 7, 8]. In cells, reactive oxygen species (ROS) are neutralized by antioxidant enzymes-superoxide dismutase, ferritin, and ceruoplasmin--occurring in the cytoplasm and produced by mitochondria catalase--produced by peroxisomes, glutathione peroxidase--produced by mitochondria and present in the cytoplasm. These enzymes are supported by other molecules such as vitamin C, active forms of iron and copper in the cytoplasm, vitamins E, A, and \(\beta\) caroten in organelles' plasma membrane and cell membrane. An imbalance between free-radical-generating and radical-scavenging systems results in \"oxidative stress\". In other words, under physiological conditions, the cell antioxidant defense system guarantees an equilibrium state of reactive oxygen species. In addition, they are also crucial in several physiological functions. However, when the balance between the oxidants and antioxidantsis shifted in favor of oxidants, then oxidative stress arises. It can be involved in developing and progressing several serious diseases [9]. Electron spin resonance (ESR) is now the only method for detecting and studying free radicals directly. It is based on the interaction of unpaired electrons (free radicals) with a magnetic field [10; 11]. ESR measurement provides information about the concentration of free radicals (quantitative data) and the type of free radicals (qualitative data) [12; 13; 14]. Free radical concentration is derived from ESR signal intensity, proportional to double integral. A free radical is determined based on a given radical's hyperfine pattern and g-factor value. The decrease in free radical concentration monitored by ESR spectroscopy proves their decay caused by cell redox processes. This process can be multistep, e.g., through reduction, oxidation, or other reactions. Several nitroxyl radicals were examined as in vivo redox status probes and for radioprotection in biological samples [15; 16]. Free radicals are characterized by a very short lifetime. For this reason, spin traps and spin probes are used for their detection [17; 18]. The unpaired electrons of the spin probes must pair with the unpaired electrons because only then the ESR signal disappears. Spin probes are a group of substances particularly useful in biological research [19; 20; 21]. The most used probes are stable nitroxyl radicals, such as 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO) and 4-hydroxy-TEMPO (TEMPOL) [22; 23]. Spin probes can be used to investigate biological membranes and the penetration (endocytosis) of different molecules from the external environment into cells [24; 25; 26; 27]. The properties of stable free radical nitroxides are extensively studied. [PERSON] et al. [28] examined the effects of TEMPO and TEMPOL on components of the mitogen-activated protein kinase signal transduction cascade. The compounds caused tyrosine phosphorylation and activation of Raf-1 protein kinase. Their toxicity was different--TEMPO induced apoptotic cell death, while TEMPOL was noncytotoxic. The genotoxicity of structurally similar nitroxides, TEMPO, and its derivatives (4-hydroxy-TEMPO, 4-oxo-TEMPO, and 4-methoxy-TEMPO) were investigated and quantified by [PERSON] et al. [29] using the mouse lymphoma assay (MLA) and Comet assay in L5178Y Tk+/ - cells.