Endogenous Conjugation of Biomimetic Dinitrosyl Iron Complex with Protein Vehicles for Oral Delivery of Nitric Oxide to Brain and Activation of Hippocampal Neurogenesis

Mechanistic and Kinetic Insights into Cellular Uptake of Biomimetic Dinitrosyl Iron Complexes and Intracellular Delivery of NO for Activation of Cytoprotective HO-1

Dinitrosyl iron unit (DNIU), [Fe(NO)2], is a natural metallocofactor for biological storage, delivery, and metabolism of nitric oxide (NO). In the attempt to gain a biomimetic insight into the natural DNIU under biological system, in this study, synthetic dinitrosyl iron complexes (DNICs) [(NO)2Fe(μ-SCH2CH2COOH)2Fe(NO)2] (DNIC-COOH) and [(NO)2Fe(μ-SCH2CH2COOCH3)2Fe(NO)2] (DNIC-COOMe) were employed to investigate the structure-reactivity relationship of mechanism and kinetics for cellular uptake of DNICs, intracellular delivery of NO, and activation of cytoprotective heme oxygenase (HO)-1. After rapid cellular uptake of dinuclear DNIC-COOMe through a thiol-mediated pathway (tmax = 0.5 h), intracellular assembly of mononuclear DNIC [(NO)2Fe(SR)(SCys)]n-/[(NO)2Fe(SR)(SCys-protein)]n- occurred, followed by O2-induced release of free NO (tmax = 1-2 h) or direct transfer of NO to soluble guanylate cyclase, which triggered the downstream HO-1. In contrast, steady kinetics for cellular uptake of DNIC-COOH via endocytosis (tmax = 2-8 h) and for intracellular release of NO (tmax = 4-6 h) reflected on the elevated activation of cytoprotective HO-1 (∼50-150-fold change at t = 3-10 h) and on the improved survival of DNIC-COOH-primed mesenchymal stem cell (MSC)/human corneal endothelial cell (HCEC) under stressed conditions. Consequently, this study unravels the bridging thiolate ligands in dinuclear DNIC-COOH/DNIC-COOMe as a switch to control the mechanism, kinetics, and efficacy for cellular uptake of DNICs, intracellular delivery of NO, and activation of cytoprotective HO-1, which poses an implication on enhanced survival of postengrafted MSC for advancing the MSC-based regenerative medicine.

Electronic Structure and Transformation of Dinitrosyl Iron Complexes (DNICs) Regulated by Redox Non-Innocent Imino-Substituted Phenoxide Ligand

The coupled NO-vibrational peaks [IR νNO 1775 s, 1716 vs, 1668 vs cm-1 (THF)] between two adjacent [Fe(NO)2] groups implicate the electron delocalization nature of the singly O-phenoxide-bridged dinuclear dinitrosyliron complex (DNIC) [Fe(NO)2(μ-ON2Me)Fe(NO)2] (1). Electronic interplay between [Fe(NO)2] units and [ON2Me]- ligand in DNIC 1 rationalizes that "hard" O-phenoxide moiety polarizes iron center(s) of [Fe(NO)2] unit(s) to enforce a "constrained" π-conjugation system acting as an electron reservoir to bestow the spin-frustrated {Fe(NO)2}9-{Fe(NO)2}9-[·ON2Me]2- electron configuration (Stotal = 1/2). This system plays a crucial role in facilitating the ligand-based redox interconversion, working in harmony to control the storage and redox-triggered transport of the [Fe(NO)2]10 unit, while preserving the {Fe(NO)2}9 core in DNICs {Fe(NO)2}9-[·ON2Me]2- [K-18-crown-6-ether)][(ON2Me)Fe(NO)2] (2) and {Fe(NO)2}9-[·ON2Me] [(ON2Me)Fe(NO)2][PF6] (3). Electrochemical studies suggest that the redox interconversion among [{Fe(NO)2}9-[·ON2Me]2-] DNIC 3 ↔ [{Fe(NO)2}9-[ON2Me]-] ↔ [{Fe(NO)2}9-[·ON2Me]] DNIC 2 are kinetically feasible, corroborated by the redox shuttle between O-bridged dimerized [(μ-ONMe)2Fe2(NO)4] (4) and [K-18-crown-6-ether)][(ONMe)Fe(NO)2] (5). In parallel with this finding, the electronic structures of [{Fe(NO)2}9-{Fe(NO)2}9-[·ON2Me]2-] DNIC 1, [{Fe(NO)2}9-[·ON2Me]2-] DNIC 2, [{Fe(NO)2}9-[·ON2Me]] DNIC 3, [{Fe(NO)2}9-[ONMe]-]2 DNIC 4, and [{Fe(NO)2}9-[·ONMe]2-] DNIC 5 are evidenced by EPR, SQUID, and Fe K-edge pre-edge analyses, respectively.

In Situ Forming of Nitric Oxide and Electric Stimulus for Nerve Therapy by Wireless Chargeable Gold Yarn-Dynamos

Endogenous signals, namely nitric oxide (NO) and electrons, play a crucial role in regulating cell fate as well as the vascular and neuronal systems. Unfortunately, utilizing NO and electrical stimulation in clinical settings can be challenging due to NO's short half-life and the invasive electrodes required for electrical stimulation. Additionally, there is a lack of tools to spatiotemporally control gas release and electrical stimulation. To address these issues, an "electromagnetic messenger" approach that employs on-demand high-frequency magnetic field (HFMF) to trigger NO release and electrical stimulation for restoring brain function in cases of traumatic brain injury is introduced. The system comprises a NO donor (poly(S-nitrosoglutathione), pGSNO)-conjugated on a gold yarn-dynamos (GY) and embedded in an implantable silk in a microneedle. When subjected to HFMF, conductive GY induces eddy currents that stimulate the release of NO from pGSNO. This process significantly enhances neural stem cell (NSC) synapses' differentiation and growth. The combined strategy of using NO and electrical stimulation to inhibit inflammation, angiogenesis, and neuronal interrogation in traumatic brain injury is demonstrated in vivo.

Ligand Control of Dinitrosyl Iron Complexes for Selective Superoxide-Mediated Nitric Oxide Monooxygenation and Superoxide-Dioxygen Interconversion

Through nitrosylation of [Fe-S] proteins, or the chelatable iron pool, a dinitrosyl iron unit (DNIU) [Fe(NO)2] embedded in the form of low-molecular-weight/protein-bound dinitrosyl iron complexes (DNICs) was discovered as a metallocofactor assembled under inflammatory conditions with elevated levels of nitric oxide (NO) and superoxide (O2-). In an attempt to gain biomimetic insights into the unexplored transformations of the DNIU under inflammation, we investigated the reactivity toward O2- by a series of DNICs [(NO)2Fe(μ-MePyr)2Fe(NO)2] (1) and [(NO)2Fe(μ-SEt)2Fe(NO)2] (3). During the superoxide-induced conversion of DNIC 1 into DNIC [(K-18-crown-6-ether)2(NO2)][Fe(μ-MePyr)4(μ-O)2(Fe(NO)2)4] (2-K-crown) and a [Fe3+(MePyr)x(NO2)y(O)z]n adduct, stoichiometric NO monooxygenation yielding NO2- occurs without the transient formation of peroxynitrite-derived •OH/•NO2 species. To study the isoelectronic reaction of O2(g) and one-electron-reduced DNIC 1, a DNIC featuring an electronically localized {Fe(NO)2}9-{Fe(NO)2}10 electronic structure, [K-18-crown-6-ether][(NO)2Fe(μ-MePyr)2Fe(NO)2] (1-red), was successfully synthesized and characterized. Oxygenation of DNIC 1-red leads to the similar assembly of DNIC 2-K-crown, of which the electronic structure is best described as paramagnetic with weak antiferromagnetic coupling among the four S = 1/2 {FeIII(NO-)2}9 units and S = 5/2 Fe3+ center. In contrast to DNICs 1 and 1-red, DNICs 3 and [K-18-crown-6-ether][(NO)2Fe(μ-SEt)2Fe(NO)2] (3-red) display a reversible equilibrium of "3 + O2- ⇋ 3-red + O2(g)", which is ascribed to the covalent [Fe(μ-SEt)2Fe] core and redox-active [Fe(NO)2] unit. Based on this study, the supporting/bridging ligands in dinuclear DNIC 1/3 (or 1-red/3-red) control the selective monooxygenation of NO and redox interconversion between O2- and O2 during reaction with O2- (or O2).

Inhibition of amyloid β1-42 peptide aggregation by newly designed cyclometallated palladium complexes

Uncontrolled amyloid aggregation is a frequent cause of neurodegenerative disorders such as prions and Alzheimer's disease (AD). As a result, many drug development approaches focus on evaluating novel molecules that can alter self-recognition pathways. Herein, we designed and synthesized the cyclometallated pyrene (Pd-1 and Pd-3) and anthracene (Pd-2) based palladium complexes ([Pd((L1)Cl] Pd-1, [Pd(L2)Cl](Pd-2), and [Pd(L3)Cl] (Pd-3)). This study explores the effect of these complexes on the aggregation, fibrillation, and amyloid formation of bovine serum albumin (BSA) and Aβ1-42 peptide. Several spectroscopic methods were used to characterize all the Pd-complexes, and the molecular structure of Pd-3 was determined by X-ray crystallography. The secondary structures were studied using circular dichroism (CD) and transmission electron microscopy (TEM), while amyloid aggregation and inhibitory activities were investigated using the Thioflavin-T (ThT) fluorescence assay. Molecular docking of the Pd-complex (Pd-3) was done using fibril (PDB: 2BEG) and monomeric (PDB: 1IYT) peptides using Auto-dock Vina. As a result, the hydrogen bonding and hydrophobic interaction between the aromatic rings of the Pd-complexes and the amino acids of amyloid-β peptides significantly reduced the production of ordered β-sheets of amyloid fibrils and protein aggregation in the presence of Pd-2 and Pd-3 complexes.

Decreased Hippocampal Neurogenesis in Aged Male Wistar Rats Is Not Associated with Memory Acquisition in a Water Maze

Brain aging is associated with a progressive decrease in learning abilities, memory, attention, decision making, and sensory perception. Age-related cognitive disturbances may be related to a decrease in the functional capacities of the hippocampus. This brain region is essential for learning and memory, and the lifelong neurogenesis occurring in the subgranular zone of the dentate gyrus may be a key event mediating the mnemonic functions of the hippocampus. In the present study, we investigated whether age-related changes in hippocampal neurogenesis are associated with learning and memory disturbances. Four- and 24-month-old rats were trained to find a hidden platform in a water maze. Though the older group showed higher latency to search the platform as compared to the younger group, both groups learned the task. However, the density of proliferating (PCNA-positive), differentiating (Dcx-positive), and new neurons (pre-labeled BrdU-positive) was significantly lower in the hippocampus of aged rats as compared to young ones. This inhibition of neurogenesis could be related to increased local production of nitric oxide since the density of neurons expressing neuronal NO-synthase was higher in the aged hippocampus. Thus, we can suggest that an age-related decrease in neurogenesis is not directly associated with place learning in aged rats.

Conjugation of bone grafts with NO-delivery dinitrosyl iron complexes promotes synergistic osteogenesis and angiogenesis in rat calvaria bone defects

Craniofacial/jawbone deformities remain a significant clinical challenge in restoring facial/dental functions and esthetics. Despite the reported therapeutics for clinical bone tissue regeneration, the bioavailability issue of autografts and limited regeneration efficacy of xenografts/synthetic bone substitutes, however, inspire continued efforts towards functional conjugation and improvement of bioactive bone graft materials. Regarding the potential of nitric oxide (NO) in tissue engineering, herein, functional conjugation of NO-delivery dinitrosyl iron complex (DNIC) and osteoconductive bone graft materials was performed to optimize the spatiotemporal control over the delivery of NO and to activate synergistic osteogenesis and angiogenesis in rat calvaria bone defects. Among three types of biomimetic DNICs, [Fe2(μ-SCH2CH2COOH)2(NO)4] (DNIC-COOH) features a steady kinetics for cellular uptake by MC3T3-E1 osteoblast cells followed by intracellular assembly of protein-bound DNICs and release of NO. This steady kinetics for intracellular delivery of NO by DNIC-COOH rationalizes its biocompatibility and wide-spectrum cell proliferation effects on MC3T3-E1 osteoblast cells and human umbilical vein endothelial cells (HUVECs). Moreover, the bridging [SCH2CH2COOH]- thiolate ligands in DNIC-COOH facilitate its chemisorption to deproteinized bovine bone mineral (DBBM) and physisorption onto TCP (β-tricalcium phosphate), respectively, which provides a mechanism to control the kinetics for the local release of loaded DNIC-COOH. Using rats with calvaria bone defects as an in vivo model, DNIC-DBBM/DNIC-TCP promotes the osteogenic and angiogenic activity ascribed to functional conjugation of osteoconductive bone graft materials and NO-delivery DNIC-COOH. Of importance, the therapeutic efficacy of DNIC-DBBM/DNIC-TCP on enhanced compact bone formation after treatment for 4 and 12 weeks supports the potential for clinical application to regenerative medicine.

Neuroprotective Effect of NO-Delivery Dinitrosyl Iron Complexes (DNICs) on Amyloid Pathology in the Alzheimer's Disease Cell Model

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by cognitive impairment, memory loss, and behavioral deficits. β-amyloid1-42 (Aβ1-42) aggregation is a significant cause of the pathogenesis in AD. Despite the numerous types of research, the current treatment efficacy remains insufficient. Hence, a novel therapeutic strategy is required. Nitric oxide (NO) is a multifunctional gaseous molecule. NO displays a neuroprotective role in the central nervous system by inhibiting the Aβ aggregation and rescuing memory and learning deficit through the NO signaling pathway. Targeting the NO pathway might be a therapeutic option; however, NO has a limited half-life under the biological system. To address this issue, a biomimetic dinitrosyl iron complex [(NO)2Fe(μ-SCH2CH2COOH)2Fe(NO)2] (DNIC-COOH) that could stably deliver NO was explored in the current study. To determine whether DNIC-COOH exerts anti-AD efficacy, DNIC-COOH was added to neuron-like cells and primary cortical neurons along with Aβ1-42. This study found that DNIC-COOH protected neuronal cells from Aβ-induced cytotoxicity, potentiated neuronal functions, and facilitated Aβ1-42 degradation through the NO-sGC-cGMP-AKT-GSK3β-CREB/MMP-9 pathway.

Design and synthesis of ruthenium complexes and their studies on the inhibition of amyloid β (1-42) peptide aggregation

Misfolding and protein aggregation have been linked to numerous human neurodegenerative disorders such as Alzheimer's, prions, and Parkinson's. Due to their interesting photophysical properties, ruthenium (Ru) complexes have received considerable attention in studying protein aggregation. In this study, we synthesized the novel Ru complexes ([Ru(p-cymene)Cl(L-1)][PF₆](Ru-1), and [Ru(p-cymene)Cl(L-2)][PF₆](Ru-2)) and investigated their inhibitory activity against the bovine serum albumin (BSA) aggregation and the Aβ_1-42 peptides amyloid formation. Several spectroscopic methods were used to characterize the complexes, and the molecular structure was determined by X-ray crystallography. Amyloid aggregation and inhibition activity were examined using the Thioflavin-T (ThT) assay, and secondary structures were analyzed by circular dichroism (CD) spectroscopy and transmission electron microscopy (TEM). The cell viability assay was carried out on the neuroblastoma cell line, revealing that the Ru-2 complex showed better protective effects against Aβ_1-42 peptide toxicity on neuro-2a cells than the Ru-1 complex. Molecular docking studies elucidate binding sites and interactions between the Ru-complexes and the Aβ_1-42 fibrils. The experimental studies revealed that these complexes significantly inhibited BSA aggregation and Aβ_1-42 amyloid fibril formation at 1:3 and 1:1 equimolar concentrations, respectively. Antioxidant assays demonstrated that these complexes act as antioxidants, protecting from amyloid-induced oxidative stress. Molecular docking studies with the monomeric Aβ_1-42 (PDB: 1IYT) show hydrophobic interaction, and both complexes bind preferably in the central region of the peptide and coordinate with two binding sites of the peptide. Hence, we suggest that the Ru-based complexes could be applied as a potential agent in metallopharmaceutical research against Alzheimer's disease.

A nitrosyl iron complex with 3.4-dichlorothiophenolyl ligands: synthesis, structures and its reactions with targets - carriers of nitrogen oxide (NO) in vivo

In this work, a new binuclear nitrosyl complex with 3.4-dichlorothiophenolyl ligands [Fe₂(SC₆H₃Cl₂)₂(NO)₄] has been synthesized. Nitrosyl iron complexes (NICs) are systems for the storage and delivery of NO in the body. There is a dynamic equilibrium between dinitrosyl iron units bound to low molecular weight ligands and high molecular weight (protein) ligands in vivo. From this point of view, the transformation of the studied complex in DMSO and buffer, as well as in biological systems, has been analyzed. In DMSO, it decomposes into mononuclear NICs, which quickly decay in buffer solutions with NO release. The high molecular weight product is formed as a result of the binding of the complex to bovine serum albumin (the Stern-Volmer constant is 2.1 × 10⁵ M^-1). In this case, the complex becomes a prolonged NO-donor. Such a long-term effect has been observed for the first time. Similarly, in a system with oxyhemoglobin, NO generation is slower; the UV-vis spectra show a gradual formation of methemoglobin. On the other hand, reduced glutathione has little effect on the NO-donor properties of the complex despite the fact that ligand substitution is observed in the system and a binuclear product is formed. Mucin binds the complex, and the decomposition mechanism is different from that for buffer solutions. Thus, these proteins and glutathione are able to participate in the transformation of the complex and modulate its properties as a potential drug.

Delivery of nitric oxide with a pH-responsive nanocarrier for the treatment of renal fibrosis

Fibrosis is an excessive accumulation of extracellular matrix (ECM) that may cause severe organ dysfunction. Nitric oxide (NO), a multifunctional gaseous signaling molecule, may inhibit fibrosis, and delivery of NO may serve as a potential antifibrotic strategy. However, major limitations in the application of NO to treat fibrotic diseases include its nonspecificity, short half-life and low availability in fibrotic tissue. Herein, we aimed to develop a stimuli-responsive drug carrier to deliver NO to halt kidney fibrosis. We manufactured a nanoparticle (NP) composed of pH-sensitive poly[2-(diisopropylamino)ethyl methacrylate (PDPA) polymers to encapsulate a NO donor, a dinitrosyl iron complex (DNIC; [Fe₂(μ-SEt)₂(NO)₄]). The NPs were stable at physiological pH 7.4 but disintegrated at pH 4.0-6.0. The NPs showed significant cytotoxicity to cultured human myofibroblasts and were able to inhibit the activation of myofibroblasts, as indicated by a lower expression level of α-smooth muscle actin and the synthesis of a major ECM component, collagen I, in cultured human myofibroblasts. When given to mice treated with unilateral ureteral ligation/obstruction (UUO) to induce kidney fibrosis, these NPs remained in blood at a stable concentration for as long as 24 h and might enter the fibrotic kidneys to suppress myofibroblast activation and collagen I production, leading to a 70% reduction in the fibrotic area. In summary, our strategy to assemble a NO donor, the iron nitrosyl complex DNIC, into pH-responsive NPs proves effective in treating renal fibrosis and warrants further investigation for its therapeutic potential.

Effect of solvents and glutathione on the decomposition of the nitrosyl iron complex with N-ethylthiourea ligands: An experimental and theoretical study

Dinitrosyl iron complexes (DNICs) are a depot and potential source of free NO in organisms. Their synthetic analog, N-ethylthiourea DNIC [Fe(SC(NH₂)(NHC₂H₅))₂(NO)₂]⁺Cl^-∙[Fe(SC(NH₂)(NHC₂H₅))Cl(NO)₂]⁰ (complex 1), as cardioprotective and cytostatic agent is a promising prodrug for the treatment of socially relevant diseases. In this work, transformation mechanism of complex 1 has been studied in anaerobic aqueous solution (pH = 7.0), DMSO, and ethanol. It was shown that the solvent has a significant effect on the decomposition of complex. According to EPR-spectroscopy, only cationic part of complex is found upon its dissolution in water; only neutral part is retained in DMSO, and both fragments are present in ethanol. Effective generation of NO occurs in an aqueous solution. The structures of the decomposition products were proposed for all solvents, their UV-spectra and rate constants were calculated. From the experimental and theoretical data obtained, it follows that complex 1 is most stable in DMSO. Solutions of complex in a DMSO-water mixture can be used to improve its bioavailability in further in vitro and in vivo studies. Also, we have analyzed its interaction with glutathione (GSH), which can participate in the metabolism of this compound. This study shows that complex 1 reacts with GSH to form a new binuclear DNIC with two GS^--ligands. It was found that the resulting complex is a more prolonged NO-donor than the initial one: k = 6.1∙10^-3·s^-1 in buffer, k = 6.4∙10^-5 s^-1 with GSH. This reaction may prevent S-glutathionylation of the essential enzyme systems and is important for metabolism of complex, associated with its antitumor activity.

Albumin as a prospective carrier of the nitrosyl iron complex with thiourea and thiosulfate ligands under aerobic conditions

High-molecular-weight dinitrosyl iron complexes (DNICs) are formed in living systems and are a stable depot of nitrogen monoxide (NO). In this work, using experimental and theoretical methods, we investigated the interaction of their synthetic analog, a promising cardiotropic complex of the composition [Fe(SC(NH₂)₂)₂(NO)₂]₂[Fe₂(S₂O₃)₂(NO)₄], with bovine serum albumin (BSA) in aqueous aerobic solutions. We suggested that, under these conditions, the decomposition product of the initial complex with oxygen, the [Fe(NO)(NO₂)]⁺ fragment, can bind in the hydrophobic pocket of the protein. As a result of this interaction, high-molecular-weight Fe(Cys34)(His39)(NO)(NO₂) is formed. The binding constant of the complex with protein measured by the quenching of intrinsic fluorescence of BSA is 7.2 × 10⁵ M^-1. According to EPR and UV-spectroscopy data, the interaction of the complex with the protein leads to its significant stabilization. In addition to coordination binding, the studied complex can be adsorbed onto the protein surface due to weak intermolecular interactions, resulting in the prolonged generation of NO.

Magnetic Responsive Release of Nitric Oxide from an MOF-Derived Fe3O4@PLGA Microsphere for the Treatment of Bacteria-Infected Cutaneous Wound

Nitric oxide (NO) is an essential endogenous signaling molecule regulating multifaceted physiological functions in the (cardio)vascular, neuronal, and immune systems. Due to the short half-life and location-/concentration-dependent physiological function of NO, translational application of NO as a novel therapeutic approach, however, awaits a strategy for spatiotemporal control on the delivery of NO. Inspired by the magnetic hyperthermia and magneto-triggered drug release featured by Fe₃O₄ conjugates, in this study, we aim to develop a magnetic responsive NO-release material (MagNORM) featuring dual NO-release phases, namely, burst and steady release, for the selective activation of NO-related physiology and treatment of bacteria-infected cutaneous wound. After conjugation of NO-delivery [Fe(μ-S-thioglycerol)(NO)₂]₂ with a metal-organic framework (MOF)-derived porous Fe₃O₄@C, encapsulation of obtained conjugates within the thermo-responsive poly(lactic-co-glycolic acid) (PLGA) microsphere completes the assembly of MagNORM. Through continuous/pulsatile/no application of the alternating magnetic field (AMF) to MagNORM, moreover, burst/intermittent/slow release of NO from MagNORM demonstrates the AMF as an ON/OFF switch for temporal control on the delivery of NO. Under continuous application of the AMF, in particular, burst release of NO from MagNORM triggers an effective anti-bacterial activity against both Gram-positive Staphylococcus aureus (S. aureus) and Gram-negative Escherichia coli (E. coli). In addition to the magneto-triggered bactericidal effect of MagNORM against E. coli-infected cutaneous wound in mice, of importance, steady release of NO from MagNORM without the AMF promotes the subsequent collagen formation and wound healing in mice.

Enhanced Oral NO Delivery through Bioinorganic Engineering of Acid-Sensitive Prodrug into a Transformer-like DNIC@MOF Microrod

Nitric oxide (NO) is an endogenous gasotransmitter regulating alternative physiological processes in the cardiovascular system. To achieve translational application of NO, continued efforts are made on the development of orally active NO prodrugs for long-term treatment of chronic cardiovascular diseases. Herein, immobilization of NO-delivery [Fe₂(μ-SCH₂CH₂COOH)₂(NO)₄] (DNIC-2) onto MIL-88B, a metal-organic framework (MOF) consisting of biocompatible Fe³⁺ and 1,4-benzenedicarboxylate (BDC), was performed to prepare a DNIC@MOF microrod for enhanced oral delivery of NO. In simulated gastric fluid, protonation of the BDC linker in DNIC@MOF initiates its transformation into a DNIC@tMOF microrod, which consisted of DNIC-2 well dispersed and confined within the BDC-based framework. Moreover, subsequent deprotonation of the BDC-based framework in DNIC@tMOF under simulated intestinal conditions promotes the release of DNIC-2 and NO. Of importance, this discovery of transformer-like DNIC@MOF provides a parallel insight into its stepwise transformation into DNIC@tMOF in the stomach followed by subsequent conversion into molecular DNIC-2 in the small intestine and release of NO in the bloodstream of mice. In comparison with acid-sensitive DNIC-2, oral administration of DNIC@MOF results in a 2.2-fold increase in the oral bioavailability of NO to 65.7% in mice and an effective reduction of systolic blood pressure (SBP) to a ΔSBP of 60.9 ± 4.7 mmHg in spontaneously hypertensive rats for 12 h.

Positive (Regulatory) and Negative (Cytotoxic) Effects of Dinitrosyl Iron Complexes on Living Organisms

The proposed in our studies mechanism of dinitrosyl iron complex (DNIC) formation through the main step of disproportionation of two NO molecules in complex with Fe2+ ion leads to emergence of the resonance structure of dinitrosyl-iron fragment of DNIC, [Fe2+(NO)(NO⁺)]. The latter allowed suggesting capacity of these complexes to function as donor of both neutral NO molecules as well as nitrosonium cations (NO⁺), which has been demonstrated in experiments. Analysis of biological activity of DNICs with thiol-containing ligands presented in this review demonstrates that NO molecules and nitrosonium cations released from the complexes exert respectively positive (regulatory) and negative (cytotoxic) effects on living organisms. It has been suggested to use dithiocarbamate derivatives to enhance selective release of nitrosonium cations from DNIC in living organisms followed by simultaneous incorporation of the released NO molecules into the biologically non-active mononitrosyl iron complexes with dithiocarbamate derivatives.

The Biologically Relevant Coordination Chemistry of Iron and Nitric Oxide: Electronic Structure and Reactivity

Nitric oxide (NO) is an important signaling molecule that is involved in a wide range of physiological and pathological events in biology. Metal coordination chemistry, especially with iron, is at the heart of many biological transformations involving NO. A series of heme proteins, nitric oxide synthases (NOS), soluble guanylate cyclase (sGC), and nitrophorins, are responsible for the biosynthesis, sensing, and transport of NO. Alternatively, NO can be generated from nitrite by heme- and copper-containing nitrite reductases (NIRs). The NO-bearing small molecules such as nitrosothiols and dinitrosyl iron complexes (DNICs) can serve as an alternative vehicle for NO storage and transport. Once NO is formed, the rich reaction chemistry of NO leads to a wide variety of biological activities including reduction of NO by heme or non-heme iron-containing NO reductases and protein post-translational modifications by DNICs. Much of our understanding of the reactivity of metal sites in biology with NO and the mechanisms of these transformations has come from the elucidation of the geometric and electronic structures and chemical reactivity of synthetic model systems, in synergy with biochemical and biophysical studies on the relevant proteins themselves. This review focuses on recent advancements from studies on proteins and model complexes that not only have improved our understanding of the biological roles of NO but also have provided foundations for biomedical research and for bio-inspired catalyst design in energy science.

Physico-Chemistry of Dinitrosyl Iron Complexes as a Determinant of Their Biological Activity

In this article we minutely discuss the so-called "oxidative" mechanism of mononuclear form of dinitrosyl iron complexes (M-DNICs) formations proposed by the author. M-DNICs are proposed to be formed from their building material-neutral NO molecules, Fe2+ ions and anionic non-thiol (L-) and thiol (RS-) ligands based on the disproportionation reaction of NO molecules binding with divalent ion irons in pairs. Then a protonated form of nitroxyl anion (NO-) appearing in the reaction is released from this group and a neutral NO molecule is included instead. As a result, M-DNICs are produced. Their resonance structure is described as [(L-)2Fe2+(NO)(NO+)], in which nitrosyl ligands are represented by NO molecules and nitrosonium cations in equal proportions. Binding of hydroxyl ions with the latter causes conversion of these cations into nitrite anions at neutral pH values and therefore transformation of DNICs into the corresponding high-spin mononitrosyl iron complexes (MNICs) with the resonance structure described as [(L-)2Fe2+(NO)]. In case of replacing L- by thiol-containing ligands, which are characterized by high π-donor activity, electron density transferred from sulfur atoms to iron-dinitrosyl groups neutralizes the positive charge on nitrosonium cations, which prevents their hydrolysis, ensuring relatively a high stability of the corresponding M-DNICs with the resonance structure [(RS-)2Fe2+ (NO, NO+)]. Therefore, M-DNICs with thiol-containing ligands, as well as their binuclear analogs (B-DNICs, respective resonance structure [(RS-)2Fe2+2 (NO, NO+)2]), can serve donors of both NO and NO+. Experiments with solutions of B-DNICs with glutathione or N-acetyl-L-cysteine (B-DNIC-GSH or B-DNIC-NAC) showed that these complexes release both NO and NO+ in case of decomposition in the presence of acid or after oxidation of thiol-containing ligands in them. The level of released NO was measured via optical absorption intensity of NO in the gaseous phase, while the number of released nitrosonium cations was determined based on their inclusion in S-nitrosothiols or their conversion into nitrite anions. Biomedical research showed the ability of DNICs with thiol-containing ligands to be donors of NO and NO+ and produce various biological effects on living organisms. At the same time, NO molecules released from DNICs usually have a positive and regulatory effect on organisms, while nitrosonium cations have a negative and cytotoxic effect.

Cell-Penetrating Delivery of Nitric Oxide by Biocompatible Dinitrosyl Iron Complex and Its Dermato-Physiological Implications

After the discovery of endogenous dinitrosyl iron complexes (DNICs) as a potential biological equivalent of nitric oxide (NO), bioinorganic engineering of [Fe(NO)₂] unit has emerged to develop biomimetic DNICs [(NO)₂Fe(L)₂] as a chemical biology tool for controlled delivery of NO. For example, water-soluble DNIC [Fe₂(μ-SCH₂CH₂OH)₂(NO)₄] (DNIC-1) was explored for oral delivery of NO to the brain and for the activation of hippocampal neurogenesis. However, the kinetics and mechanism for cellular uptake and intracellular release of NO, as well as the biocompatibility of synthetic DNICs, remain elusive. Prompted by the potential application of NO to dermato-physiological regulations, in this study, cellular uptake and intracellular delivery of DNIC [Fe₂(μ-SCH₂CH₂COOH)₂(NO)₄] (DNIC-2) and its regulatory effect/biocompatibility toward epidermal cells were investigated. Upon the treatment of DNIC-2 to human fibroblast cells, cellular uptake of DNIC-2 followed by transformation into protein-bound DNICs occur to trigger the intracellular release of NO with a half-life of 1.8 ± 0.2 h. As opposed to the burst release of extracellular NO from diethylamine NONOate (DEANO), the cell-penetrating nature of DNIC-2 rationalizes its overwhelming efficacy for intracellular delivery of NO. Moreover, NO-delivery DNIC-2 can regulate cell proliferation, accelerate wound healing, and enhance the deposition of collagen in human fibroblast cells. Based on the in vitro and in vivo biocompatibility evaluation, biocompatible DNIC-2 holds the potential to be a novel active ingredient for skincare products.

Dinitrosyl iron complexes (DNICs) as inhibitors of the SARS-CoV-2 main protease

By repurposing DNICs designed for other medicinal purposes, the possibility of protease inhibition was investigated in silico using AutoDock 4.2.6 (AD4) and in vitro via a FRET protease assay. AD4 was validated as a predictive computational tool for coordinatively unsaturated DNIC binding using the only known crystal structure of a protein-bound DNIC, PDB- (calculation RMSD = 1.77). From the in silico data the dimeric DNICs TGTA-RRE, [(μ-S-TGTA)Fe(NO)₂]₂ (TGTA = 1-thio-β-d-glucose tetraacetate) and TG-RRE, [(μ-S-TG)Fe(NO)₂]₂ (TG = 1-thio-β-d-glucose) were identified as promising leads for inhibition via coordinative inhibition at Cys-145 of the SARS-CoV-2 Main Protease (SC2M^pro). In vitro studies indicate inhibition of protease activity upon DNIC treatment, with an IC₅₀ of 38 ± 2 μM for TGTA-RRE and 33 ± 2 μM for TG-RRE. This study presents a simple computational method for predicting DNIC-protein interactions; the in vitro study is consistent with in silico leads.