Visualizing changes in mitochondrial Mg2+ during apoptosis with organelle-targeted triazole-based ratiometric fluorescent sensors

Magnesium (Mg2+): Essential Mineral for Neuronal Health: From Cellular Biochemistry to Cognitive Health and Behavior Regulation

Magnesium (Mg2+) is a crucial mineral involved in numerous cellular processes critical for neuronal health and function. This review explores the multifaceted roles of Mg2+, from its biochemical interactions at the cellular level to its impact on cognitive health and behavioral regulation. Mg2+ acts as a cofactor for over 300 enzymatic reactions, including those involved in ATP synthesis, nucleic acid stability, and neurotransmitter release. It regulates ion channels, modulates synaptic plasticity, and maintains the structural integrity of cell membranes, which are essential for proper neuronal signaling and synaptic transmission. Recent studies have highlighted the significance of Mg2+ in neuroprotection, showing its ability to attenuate oxidative stress, reduce inflammation, and mitigate excitotoxicity, thereby safeguarding neuronal health. Furthermore, Mg2+ deficiency has been linked to a range of neuropsychiatric disorders, including depression, anxiety, and cognitive decline. Supplementation with Mg2+, particularly in the form of bioavailable compounds such as Magnesium-L-Threonate (MgLT), Magnesium-Acetyl-Taurate (MgAT), and other Magnesium salts, has shown some promising results in enhancing synaptic density, improving memory function, and alleviating symptoms of mental health disorders. This review highlights significant current findings on the cellular mechanisms by which Mg2+ exerts its neuroprotective effects and evaluates clinical and preclinical evidence supporting its therapeutic potential. By elucidating the comprehensive role of Mg2+ in neuronal health, this review aims to underscore the importance of maintaining optimal Mg2+ levels for cognitive function and behavioral regulation, advocating for further research into Mg2+ supplementation as a viable intervention for neuropsychiatric and neurodegenerative conditions.

Organelle-targeting ratiometric fluorescent probes: design principles, detection mechanisms, bio-applications, and challenges

Biological species, including reactive oxygen species (ROS), reactive sulfur species (RSS), reactive nitrogen species (RNS), F-, Pd2+, Cu2+, Hg2+, and others, are crucial for the healthy functioning of cells in living organisms. However, their aberrant concentration can result in various serious diseases. Therefore, it is essential to monitor biological species in cellular organelles such as the cell membrane, mitochondria, lysosome, endoplasmic reticulum, Golgi apparatus, and nucleus. Among various fluorescent probes for species detection within the organelles, ratiometric fluorescent probes have drawn special attention as a potential way to get beyond the drawbacks of intensity-based probes. This method depends on measuring the intensity change of two emission bands (caused by an analyte), which produces an efficient internal referencing that increases the detection's sensitivity. This review article discusses the literature publications (from 2015 to 2022) on organelle-targeting ratiometric fluorescent probes, the general strategies, the detecting mechanisms, the broad scope, and the challenges currently faced by fluorescent probes.

Lanthanide-based luminescent probes for biological magnesium: accessing polyphosphate-bound Mg2

Biomolecule-bound Mg2+ species, particularly polyphosphate complexes, represent a large and dynamic fraction of the total cellular magnesium that is essential for cellular function but remains invisible to most indicators. Here we report a new family of Eu(III)-based indicators, the MagQEu family, functionalized with a 4-oxo-4H-quinolizine-3-carboxylic acid metal recognition group/sensitization antenna for turn-on, luminescence-based detection of biologically relevant Mg2+ species.

Ratiometric and Discriminative Visualization of Autophagy and Apoptosis with a Single Fluorescent Probe Based on the Aggregation/Monomer Principle

Autophagy and apoptosis play a central role in maintaining homeostasis in mammals. Therefore, discriminative visualization of the two cellular processes is an important and challenging task. However, fluorescent probes enabling ratiometric visualization of both autophagy and apoptosis with different sets of fluorescence signals have not been developed yet. In this work, we constructed a versatile single fluorescent probe (NKLR) based on the aggregation/monomer principle for the ratiometric and discriminative visualization of autophagy and apoptosis. NKLR can simultaneously perform two-color imaging of RNA (deep red channel) and lysosomes (yellow channel) in aggregation and monomer states, respectively. During autophagy, NKLR migrated from cytoplasmic RNA and nuclear RNA to lysosomes, showing enhanced yellow emission and sharply decreased deep red fluorescence. Moreover, this migration process was reversible upon the recovery of autophagy. Comparatively, during apoptosis, NKLR immigrated from lysosomes to RNA, and the yellow emission decreased and even disappeared, while the fluorescence of the deep red channel slightly increased. Overall, autophagy and apoptosis could be discriminatively visualized via the fluorescence intensity ratios of the two channels. Meanwhile, the cells in three different states (healthy, autophagic, apoptotic) could be distinguished by three point-to-point fluorescence images via the localization and emission color of NKLR. Therefore, the probe NKLR can serve as a desirable molecular tool to reveal the in-depth relation between autophagy and apoptosis and facilitate the study on the two cellular processes.

Usnic Acid-Mediated Exchange of Protons for Divalent Metal Cations across Lipid Membranes: Relevance to Mitochondrial Uncoupling

Usnic acid (UA), a unique lichen metabolite, is a protonophoric uncoupler of oxidative phosphorylation, widely known as a weight-loss dietary supplement. In contrast to conventional proton-shuttling mitochondrial uncouplers, UA was found to carry protons across lipid membranes via the induction of an electrogenic proton exchange for calcium or magnesium cations. Here, we evaluated the ability of various divalent metal cations to stimulate a proton transport through both planar and vesicular bilayer lipid membranes by measuring the transmembrane electrical current and fluorescence-detected pH gradient dissipation in pyranine-loaded liposomes, respectively. Thus, we obtained the following selectivity series of calcium, magnesium, zinc, manganese and copper cations: Zn2+ > Mn2+ > Mg2+ > Ca2+ >> Cu2+. Remarkably, Cu2+ appeared to suppress the UA-mediated proton transport in both lipid membrane systems. The data on the divalent metal cation/proton exchange were supported by circular dichroism spectroscopy of UA in the presence of the corresponding cations.

Restoring cellular magnesium balance through Cyclin M4 protects against acetaminophen-induced liver damage

Acetaminophen overdose is one of the leading causes of acute liver failure and liver transplantation in the Western world. Magnesium is essential in several cellular processess. The Cyclin M family is involved in magnesium transport across cell membranes. Herein, we identify that among all magnesium transporters, only Cyclin M4 expression is upregulated in the liver of patients with acetaminophen overdose, with disturbances in magnesium serum levels. In the liver, acetaminophen interferes with the mitochondrial magnesium reservoir via Cyclin M4, affecting ATP production and reactive oxygen species generation, further boosting endoplasmic reticulum stress. Importantly, Cyclin M4 mutant T495I, which impairs magnesium flux, shows no effect. Finally, an accumulation of Cyclin M4 in endoplasmic reticulum is shown under hepatoxicity. Based on our studies in mice, silencing hepatic Cyclin M4 within the window of 6 to 24 h following acetaminophen overdose ingestion may represent a therapeutic target for acetaminophen overdose induced liver injury.

Recent progress in developing fluorescent probes for imaging cell metabolites

Cellular metabolites play a crucial role in promoting and regulating cellular activities, but it has been difficult to monitor these cellular metabolites in living cells and in real time. Over the past decades, iterative development and improvements of fluorescent probes have been made, resulting in the effective monitoring of metabolites. In this review, we highlight recent progress in the use of fluorescent probes for tracking some key metabolites, such as adenosine triphosphate, cyclic adenosine monophosphate, cyclic guanosine 5'-monophosphate, Nicotinamide adenine dinucleotide (NADH), reactive oxygen species, sugar, carbon monoxide, and nitric oxide for both whole cell and subcellular imaging.

Advances in imaging of understudied ions in signaling: A focus on magnesium

The study of metal ions in the context of cell signaling has historically focused mainly on Ca2+, the second messenger par excellence. But recent studies support an emerging paradigm in which other metals, including magnesium and d-block metals, play a role in signal transduction as well. Armed with the right indicators, fluorescence microscopy offers a unique combination of spatial and temporal resolution perfectly suited to reveal metal transients in real time, while also helping identify possible sources of ion mobilization and molecular targets. With a focus on Mg2+, we highlight recent advancements in the development of molecular indicators and imaging strategies for the study of metal ions in signaling. We discuss remaining conceptual and technical challenges in the field, and we illustrate through the case of Mg2+ how the study of nontraditional ions in signaling is inspiring technological developments applicable more broadly to the study of metals in biology.

Novel Insights into the Role of Copper in Critical Life Stages of Oysters Revealed by High-Resolution NanoSIMS Imaging

Copper (Cu) is a key trace element for many biological processes, but there is little available information regarding its requirements and functions in critical life stages of marine bivalves, during which dramatic morphological and physiological changes occur. In this study, the ontogeny pattern of Cu accumulation across the life history of oysters was explored for the first time and the distributions of Cu in oysters at critical life stages (pediveliger and early settled spat) were in situ mapped by nanoscale secondary ion mass spectrometry (NanoSIMS) with high lateral resolution. We first demonstrated that the late pelagic stage to early settled stage was the critical stage requiring Cu during oyster development. NanoSIMS imaging revealed a significant elevation of intercellular Cu levels along with mitochondrial calcium overload and obvious structural degradation of mitochondria in velar cells at the pediveliger stage, implying the possible role of Cu in cell apoptosis of the velum during larval metamorphosis. Furthermore, an obvious enrichment of Cu together with calcium was observed in the nucleus and mitochondria of ciliated cells of gill tissue at the early settled stage. Their accumulation in the gill cells was significantly higher than that in the juveniles, indicating the potential role of Cu in sustaining the fast growth of the gill filament and the concomitant acceleration of energy metabolism during early benthic development. Our findings offer new insights into the understanding of the interactions between trace metals and marine bivalves.

Magnesium Is a Key Player in Neuronal Maturation and Neuropathology

Magnesium (Mg) is the second most abundant cation in mammalian cells, and it is essential for numerous cellular processes including enzymatic reactions, ion channel functions, metabolic cycles, cellular signaling, and DNA/RNA stabilities. Because of the versatile and universal nature of Mg²⁺, the homeostasis of intracellular Mg²⁺ is physiologically linked to growth, proliferation, differentiation, energy metabolism, and death of cells. On the cellular and tissue levels, maintaining Mg²⁺ within optimal levels according to the biological context, such as cell types, developmental stages, extracellular environments, and pathophysiological conditions, is crucial for development, normal functions, and diseases. Hence, Mg²⁺ is pathologically involved in cancers, diabetes, and neurodegenerative diseases, such as Parkinson's disease, Alzheimer's disease, and demyelination. In the research field regarding the roles and mechanisms of Mg²⁺ regulation, numerous controversies caused by its versatility and complexity still exist. As Mg²⁺, at least, plays critical roles in neuronal development, healthy normal functions, and diseases, appropriate Mg²⁺ supplementation exhibits neurotrophic effects in a majority of cases. Hence, the control of Mg²⁺ homeostasis can be a candidate for therapeutic targets in neuronal diseases. In this review, recent results regarding the roles of intracellular Mg²⁺ and its regulatory system in determining the cell phenotype, fate, and diseases in the nervous system are summarized, and an overview of the comprehensive roles of Mg²⁺ is provided.

Structures of the Mitochondrial CDP-DAG Synthase Tam41 Suggest a Potential Lipid Substrate Pathway from Membrane to the Active Site

In mitochondria, CDP-diacylglycerol (CDP-DAG) is a crucial precursor for cardiolipin biosynthesis. Mitochondrial CDP-DAG is synthesized by the translocator assembly and maintenance protein 41 (Tam41) through an elusive process. Here we show that Tam41 adopts sequential catalytic mechanism, and report crystal structures of the bulk N-terminal region of Tam41 from Schizosaccharomyces pombe in the apo and CTP-bound state. The structure reveals that Tam41 contains a nucleotidyltransferase (NTase) domain and a winged helix domain. CTP binds to an "L"-shaped pocket sandwiched between the two domains. Rearrangement of a loop region near the active site is essential for opening the CTP-binding pocket. Docking of phosphatidic acid/CDP-DAG in the structure suggests a lipid entry/exit pathway connected to the "L"-shaped pocket. The C-terminal region of SpTam41 contains a positively charged amphipathic helix crucial for membrane association and participates in binding phospholipids. These results provide detailed insights into the mechanism of CDP-DAG biosynthesis in mitochondria.

Fluorescent probes for organelle-targeted bioactive species imaging

The dynamic fluctuations of bioactive species in living cells are associated with numerous physiological and pathological phenomena. The emergence of organelle-targeted fluorescent probes has significantly facilitated our understanding on the biological functions of these species. This review describes the design, applications, challenges and potential directions of organelle-targeted bioactive species probes.

Bioactive species, including reactive oxygen species (ROS, including O₂˙^–, H₂O₂, HOCl, ¹O₂, ˙OH, HOBr, etc.), reactive nitrogen species (RNS, including ONOO^–, NO, NO₂, HNO, etc.), reactive sulfur species (RSS, including GSH, Hcy, Cys, H₂S, H₂S_n, SO₂ derivatives, etc.), ATP, HCHO, CO and so on, are a highly important category of molecules in living cells. The dynamic fluctuations of these molecules in subcellular microenvironments determine cellular homeostasis, signal conduction, immunity and metabolism. However, their abnormal expressions can cause disorders which are associated with diverse major diseases. Monitoring bioactive molecules in subcellular structures is therefore critical for bioanalysis and related drug discovery. With the emergence of organelle-targeted fluorescent probes, significant progress has been made in subcellular imaging. Among the developed subcellular localization fluorescent tools, ROS, RNS and RSS (RONSS) probes are highly attractive, owing to their potential for revealing the physiological and pathological functions of these highly reactive, interactive and interconvertible molecules during diverse biological events, which are rather significant for advancing our understanding of different life phenomena and exploring new technologies for life regulation. This review mainly illustrates the design principles, detection mechanisms, current challenges, and potential future directions of organelle-targeted fluorescent probes toward RONSS.

A novel diarylethene-based fluorescent "turn-on" sensor for the selective detection of Mg2

A new photochromic diarylethene derivative with a 4-methylphenol unit has been designed and synthesized. It displayed distinct photochromism and fluorescent ''turn on'' features to Mg2+ in acetonitrile solution. With the addition of Mg2+, there was an obvious increase of fluorescent emission intensity at 552 nm, accompanied by a clear change of fluorescent color from dark purple to green. Meantime, the 1 : 1 stoichiometry between the derivative and Mg2+ was verified by Job's plot and HRMS. Furthermore, the sensor was successfully applied in the detection of Mg2+ in practical samples. Moreover, based on the multiple-responsive fluorescence switching behaviors, it also could be used to construct a molecular logic circuit with UV/vis lights and Mg2+/EDTA as input signals and the emission at 552 nm as the output signal.

Highly Selective, Red Emitting BODIPY-Based Fluorescent Indicators for Intracellular Mg2+ Imaging

Most fluorescent indicators for Mg2+ suffer from poor selectivity against other divalent cations, especially Ca2+, thus do not provide reliable information on cellular Mg2+ concentrations in processes in which such metals are involved. We report a new set of highly selective fluorescent indicators based on alkoxystyryl-functionalized BODIPY fluorophores decorated with a 4-oxo-4H-quinolizine-3-carboxylic acid metal binding moiety. The new sensors, MagQ1 and MagQ2, display absorption and emission maxima above 600 nm, with a 29-fold fluorescence enhancement and good quantum yields (Φ > 0.3) upon coordination of Mg2+ in aqueous buffer. Fluorescence response to Mg2+ is not affected by the presence of competing divalent cations typically present in the cellular milieu, and displays minimal pH dependence in the physiologically relevant range. The choice of alkoxy groups decorating the styryl BODIPY core does not influence the basic photophysical and metal binding properties of the compounds, but has a marked effect on their intracellular retention and thus in their applicability for detection of cellular Mg2+ by fluorescence imaging. In particular, we demonstrate the utility of a triethyleneglycol (TEG) functionalization tactic that endows MagQ2 with superior cellular retention in live cells by reducing active extrusion through organic anion transporters, which are thought to cause fast leakage of typical anionic dyes. With enhanced retention and excellent photophysical properties, MagQ2 can be applied in the detection of cellular Mg2+ influx without interference of high concentrations of Ca2+ akin to those involved in signaling.

Energy transfer chemiluminescence for ratiometric pH imaging

Chemiluminescence imaging offers a low background and high sensitivity approach to imaging analytes in living cells and animals. Intensity-based measurements have been developed, but require careful consideration of kinetics, probe localization, and fluctuations in quantum yield, all of which complicate quantification. Here, we report a ratiometric strategy for quantitative chemiluminescence imaging of pH. The strategy relies on an energy transfer cascade of chemiluminescence emission from a spiroadamantane 1,2-dioxetane to a ratiometric pH indicator via fluorescent dyes in Enhancer solutions. Monitoring the pH-dependent changes in chemiluminescence emission at multiple wavelengths enables ratiometric imaging and quantification of pH independent from variations due to kinetics and probe concentration.

Separately enhanced dual emissions of the amphiphilic derivative of 2-(2'-hydroxylphenyl) benzothiazole by supramolecular complexation

Here, we report separately enhanced dual emissions of the amphiphilic derivative of 2-(2'-hydroxyphenyl)benzothiazole (denoted as HBT-11) by supramolecular complexation with cyclodextrins (CDs). When dispersed in water, HBT-11 shows two relatively weak emission bands, which can be assigned to the emissions of enol- and keto-forms, the two tautomers, owing to excited-state intramolecular proton transfers. Upon the addition of α-CD and β-CD, the keto- and enol-emissions, respectively, are separately enhanced; the enhancement effect is due to the formation of HBT-11/α-CD and HBT-11/β-CD complexes through multiple hydrogen bonding and host-guest interactions, respectively. It is worth to note that the keto-emission caused by the complex of HBT-11/α-CD has a much shorter wavelength compared with that of the aggregates formed by pure HBT-11. To the best of our knowledge, this is the first time that a study on keto-emission of the isolated HBT chromophore has been reported.

Structural and spectroscopic insight into the metal binding properties of the o-aminophenol-N,N,O-triacetic acid (APTRA) chelator: implications for design of metal indicators

The o-aminophenol-N,N,O-triacetic acid (APTRA) chelator is employed extensively as a metal-recognition moiety in fluorescent indicators for biological free Mg(2+), as well as in low-affinity indicators for the detection of high levels of cellular Ca(2+). Despite its widespread use in sensor design, the limited metal selectivity of this chelating moiety can lead to binding of competing cations that complicate the fluorescence-based detection of metals of interest in complex samples. Reported herein are the structural characterization of APTRA complexes with various biologically relevant cations, and the thermodynamic analysis of complex formation with Mg(2+), Ca(2+) and Zn(2+). Our results indicate that the low affinity of APTRA for Mg(2+), which makes it a suitable metal-recognition moiety for sensitive analysis of typical millimolar levels of this metal in cells, stems from a much higher enthalpic cost of Mg(2+) binding compared to that of other cations. The results are discussed in the context of indicator design, highlighting the aspects that may aid the future development of fluorescent sensors with enhanced metal selectivity profiles.

Enhanced selectivity for Mg2+with a phosphinate-based chelate: APDAPversusAPTRA

AnO-methylene-methylphosphinate analogue of APTRA shows a much greater reduction in affinity for Ca²⁺than Mg²⁺, offering a way to improved magnesium-selective ligands.

Visualization of long-term Mg2+ dynamics in apoptotic cells using a novel targetable fluorescent probe

Long-term Mg²⁺ imaging during apoptosis using a HaloTag-coupled Mg²⁺ probe demonstrated a Mg²⁺ concentration increase caused by dissociation of Mg²⁺ from ATP.

Mg²⁺ plays important roles in many physiological processes. However, the underlying molecular mechanisms, especially in the apoptotic pathway, remain unclear due to the diffusion of Mg²⁺ probes, which hinders long-term imaging in specific organelles. We developed an immobilized Mg²⁺ probe, MGH, which is covalently conjugated with the HaloTag protein in various organelles. HaloTag-coupled MGH enabled long-term imaging of intracellular local Mg²⁺ dynamics for 24 h. To exploit this remarkable property, MGH was applied to the investigation of intracellular Mg²⁺ dynamics during apoptosis. Time-lapse imaging revealed an increase in the Mg²⁺ concentration after apoptotic cell shrinkage. Combined imaging analyses of intracellular Mg²⁺ and ATP concentrations strongly suggested that this Mg²⁺ concentration increase was caused by the dissociation of Mg²⁺ from ATP, along with a decrease in the intracellular ATP concentration. Thus, this protein-coupled Mg²⁺ probe could be a new chemical tool to elucidate intracellular Mg²⁺ dynamics with high spatiotemporal resolution.

The Involvement of Mg2+ in Regulation of Cellular and Mitochondrial Functions

Mg²⁺ is an essential mineral with pleotropic impacts on cellular physiology and functions. It acts as a cofactor of several important enzymes, as a regulator of ion channels such as voltage-dependent Ca²⁺ channels and K⁺ channels and on Ca²⁺-binding proteins. In general, Mg²⁺ is considered as the main intracellular antagonist of Ca²⁺, which is an essential secondary messenger initiating or regulating a great number of cellular functions. This review examines the effects of Mg²⁺ on mitochondrial functions with a particular focus on energy metabolism, mitochondrial Ca²⁺ handling, and apoptosis.

4-(Benzo[d]thiazol-2-yl)-N,N-dimethylaniline

The whole molecule of the title compound, C15H14N2S, is approximately planar, with an r.m.s. deviation of 0.0382 Å from the best-fit mean plane through all 18 non-H atoms. In the crystal, dimers form through π–π stacking interactions between the benzene rings of adjacent benzothiazole ring systems, with a centroid–centroid separation of 3.6834 (16) Å.

Mitochondrial Mg(2+) homeostasis decides cellular energy metabolism and vulnerability to stress

Cellular energy production processes are composed of many Mg²⁺ dependent enzymatic reactions. In fact, dysregulation of Mg²⁺ homeostasis is involved in various cellular malfunctions and diseases. Recently, mitochondria, energy-producing organelles, have been known as major intracellular Mg²⁺ stores. Several biological stimuli alter mitochondrial Mg²⁺ concentration by intracellular redistribution. However, in living cells, whether mitochondrial Mg²⁺ alteration affect cellular energy metabolism remains unclear. Mg²⁺ transporter of mitochondrial inner membrane MRS2 is an essential component of mitochondrial Mg²⁺ uptake system. Here, we comprehensively analyzed intracellular Mg²⁺ levels and energy metabolism in Mrs2 knockdown (KD) cells using fluorescence imaging and metabolome analysis. Dysregulation of mitochondrial Mg²⁺ homeostasis disrupted ATP production via shift of mitochondrial energy metabolism and morphology. Moreover, Mrs2 KD sensitized cellular tolerance against cellular stress. These results indicate regulation of mitochondrial Mg²⁺via MRS2 critically decides cellular energy status and cell vulnerability via regulation of mitochondrial Mg²⁺ level in response to physiological stimuli.

A pillar[5]arene-based [2]rotaxane lights up mitochondria

Subcellular organelle-specific reagents for simultaneous targeting, imaging and treatment are highly desirable for cancer therapy. However, it remains a challenge to fabricate a single molecular platform containing a targeting group, imaging and therapeutic agents through traditional synthesis. Due to their superior sensitivity and photostability, fluorescent probes with aggregation-induced emission (AIE) characteristics have attracted more and more attention in studying the process of translocation, drug release, and excretion of nanomedicines in vitro or in vivo. We construct a pillar[5]arene-based [2]rotaxane (R1) by employing tetraphenylethene (TPE) and triphenylphosphonium (TPP) moieties as stoppers; the TPE unit retains the aggregation-induced emission (AIE) attribute and the TPP group is used as a mitochondria-targeting agent. R1 exhibits enhanced AIE, high specificity to mitochondria, and superior photostability. By introducing doxorubicin (DOX) into R1, prodrug R2 is constructed as a dual-fluorescence-quenched Förster resonance energy transfer (FRET) system, in which the TPE-based axle acts as a donor fluorophore and the DOX unit acts as the acceptor. Upon hydrolysis of R2 in endo/lysosomes, the fluorescences of the carrier and the drug recover. R1 is further utilized as a drug delivery platform to conjugate other anticancer drugs containing amine groups through imine formation to prepare prodrugs. The anticancer drugs are released from these prodrugs in the cells upon hydrolysis of the pH-responsive imine bonds.

Bright, red emitting fluorescent sensor for intracellular imaging of Mg2+

A new fluorescent sensor with excellent turn-on ratio, low energy excitation and emission over 600 nm enables Mg²⁺detection in live cells.

Targeting mitochondrial DNA with a two-photon active Ru(ii) phenanthroline derivative

A novel phenanthroline Ru(ii) derivative for targeting mitochondrial DNA was designed and its potential applications in biological processes were highlighted.