Artificial Control of Cell Signaling Using a Photocleavable Cobalt(III)–Nitrosyl Complex

Controlling Reactivity through Spin Manipulation: Steric Bulkiness of Peroxocobalt(III) Complexes

The intrinsic relationship between spin states and reactivity in peroxocobalt(III) complexes was investigated, specifically focusing on the influence of steric modulation on supporting ligands. Together with the previously reported [CoIII(TBDAP)(O2)]+ (2Tb), which exhibits spin crossover characteristics, two peroxocobalt(III) complexes, [CoIII(MDAP)(O2)]+ (2Me) and [CoIII(ADDAP)(O2)]+ (2Ad), bearing pyridinophane ligands with distinct N-substituents such as methyl and adamantyl groups, were synthesized and characterized. By manipulating the steric bulkiness of the N-substituents, control of spin states in peroxocobalt(III) complexes was demonstrated through various physicochemical analyses. Notably, 2Ad oxidized the nitriles to generate hydroximatocobalt(III) complexes, while 2Me displayed an inability for such oxidation reactions. Furthermore, both 2Ad and 2Tb exhibited similarities in spectroscopic and geometric features, demonstrating spin crossover behavior between S = 0 and S = 1. The steric bulkiness of the adamantyl and tert-butyl group on the axial amines was attributed to inducing a weak ligand field on the cobalt(III) center. Thus, 2Ad and 2Tb are an S = 1 state under the reaction conditions. In contrast, the less bulky methyl group on the amines of 2Me resulted in an S = 0 state. The redox potential of the peroxocobalt(III) complexes was also influenced by the ligand field arising from the steric bulkiness of the N-substituents in the order of 2Me (-0.01 V) < 2Tb (0.29 V) = 2Ad (0.29 V). Theoretical calculations using DFT supported the experimental observations, providing insights into the electronic structure and emphasizing the importance of the spin state of peroxocobalt(III) complexes in nitrile activation.

Effect of Electron-donating Group on NO Photolysis of {RuNO}6 Ruthenium Nitrosyl Complexes with N2 O2 Lgands Bearing π-Extended Rings

In this study, we introduced the electron-donating group (-OH) to the aromatic rings of Ru(salophen)(NO)Cl (0) (salophenH2 =N,N'-(1,2-phenylene)bis(salicylideneimine)) to investigate the influence of the substitution on NO photolysis and NO-releasing dynamics. Three derivative complexes, Ru((o-OH)2 -salophen)(NO)Cl (1), Ru((m-OH)2 -salophen)(NO)Cl (2), and Ru((p-OH)2 -salophen)(NO)Cl (3) were developed and their NO photolysis was monitored by using UV/Vis, EPR, NMR, and IR spectroscopies under white room light. Spectroscopic results indicated that the complexes were diamagnetic Ru(II)-NO+ species which were converted to low-spin Ru(III) species (d5 , S=1/2) and released NO radicals by photons. The conversion was also confirmed by determining the single-crystal structure of the photoproduct of 1. The photochemical quantum yields (ΦNO s) of the photolysis were determined to be 0>1, 2, 3 at both the visible and UV excitations. Femtosecond (fs) time-resolved mid-IR spectroscopy was employed for studying NO-releasing dynamics. The geminate rebinding (GR) rates of the photoreleased NO to the photolyzed complexes were estimated to be 0≃1, 2, 3. DFT and TDDFT computations found that the introduction of the hydroxyl groups elevated the ligand π-bonding orbitals (π (salophen)), resulting in decrease of the HOMO-LUMO gaps in 1-3. The theoretical calculations suggested that the Ru-NNO bond dissociations of the complexes were mostly initiated by the ligand-to-ligand charge transfer (LLCT) of π(salophen)→π*(Ru-NO) with both the visible and UV excitations and the decreasing ΦNO s could be explained by the changes of the electronic structures in which the photoactivable bands of 1-3 have relatively less contribution of transitions related with Ru-NO bond than those of 0.

Analysis of Phase Heterogeneity in Lipid Membranes Using Single-Molecule Tracking in Live Cells

In live cells, the plasma membrane is composed of lipid domains separated by hundreds of nanometers in dynamic equilibrium. Lipid phase separation regulates the trafficking and spatiotemporal organization of membrane molecules that promote signal transduction. However, visualizing domains with adequate spatiotemporal accuracy remains challenging because of their subdiffraction limit size and highly dynamic properties. Here, we present a single lipid-molecular motion analysis pipeline (lipid-MAP) for analyzing the phase heterogeneity of lipid membranes by detecting the instantaneous velocity change of a single lipid molecule using the excellent optical properties of nanoparticles, high spatial localization accuracy of single-molecule localization microscopy, and separation capability of the diffusion state of the hidden Markov model algorithm. Using lipid-MAP, individual lipid molecules were found to be in dynamic equilibrium between two statistically distinguishable phases, leading to the formation of small (∼170 nm), viscous (2.5× more viscous than surrounding areas), and transient domains in live cells. Moreover, our findings provide an understanding of how membrane compositional changes, i.e., cholesterol and phospholipids, affect domain formation. This imaging method can contribute to an improved understanding of spatiotemporal-controlled membrane dynamics at the molecular level.

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).

Promotion of S-nitrosation of cysteine by a {Co(NO)2}10 complex

S-Nitrosothiols (SNOs) serve as endogenous carriers and donors of NO within living cells, releasing nitrosonium ions (NO+), NO, or other nitroso derivatives. In this study, we present a bioinspired {Co(NO)2}10 complex 1 that achieved S-nitrosation towards Cys residues. The incorporation of a ferrocenyl group in 1 allowed for fine-tuning of the nitrosation reaction, taking advantage of the redox ability of Cys residues. Complex 1 was synthesized and characterized, demonstrating its NO translation reactivity. Furthermore, complex 1 successfully converted Cys into S-nitrosocysteine (Cys-SNO), as confirmed by UV-Vis, IR, and XAS spectroscopy. This study presents a promising approach for S-nitrosation of Cys residues for further exploration in the modification of Cys-containing peptides.

Single-Molecule Imaging of Membrane Proteins on Vascular Endothelial Cells

Transporting substances such as gases, nutrients, waste, and cells is the primary function of blood vessels. Vascular cells use membrane proteins to perform crucial endothelial functions, including molecular transport, immune cell infiltration, and angiogenesis. A thorough understanding of these membrane receptors from a clinical perspective is warranted to gain insights into the pathogenesis of vascular diseases and to develop effective methods for drug delivery through the vascular endothelium. This review summarizes state-of-the-art single-molecule imaging techniques, such as super-resolution microscopy, single-molecule tracking, and protein-protein interaction analysis, for observing and studying membrane proteins. Furthermore, recent single-molecule studies of membrane proteins such as cadherins, integrins, caveolins, transferrin receptors, vesicle-associated protein-1, and vascular endothelial growth factor receptor are discussed.

Visible-light NO photolysis of ruthenium nitrosyl complexes with N2O2 ligands bearing π-extended rings and their photorelease dynamics

NO photorelease and its dynamics for two {RuNO}⁶ complexes, Ru(salophen)(NO)Cl (1) and Ru(naphophen)(NO)Cl (2), with salen-type ligands bearing π-extended systems (salophenH₂ = N,N'-(1,2-phenylene)-bis(salicylideneimine) and naphophenH₂ = N,N'-1,2-phenylene-bis(2-hydroxy-1-naphthylmethyleneimine)) were investigated. NO photolysis was performed under white room light and monitored by UV/Vis, EPR, and NMR spectroscopies. NO photolysis was also performed under 459 and 489 nm irradiation for 1 and 2, respectively. The photochemical quantum yields of the NO photolysis (Φ_NO) of both 1 and 2 were determined to be 9% at the irradiation wavelengths. The structural and spectroscopic characteristics of the complexes before and after the photolysis confirmed the conversion of diamagnetic Ru(II)(L)(Cl)-NO⁺ to paramagnetic S = ½ Ru(III)(L)(Cl)-solvent by photons (L = salophen^2- and naphophen^2-). The photoreleased NO radicals were detected by spin-trapping EPR. DFT and TDDFT calculations found that the photoactive bands are configured as mostly the ligand-to-ligand charge transfer (LLCT) of π(L) → π*(Ru-NO), suggesting that the NO photorelease was initiated by the LLCT. Dynamics of NO photorelease from the complexes in DMSO under 320 nm excitation were investigated by femtosecond (fs) time-resolved mid-IR spectroscopy. The primary photorelease of NO occurred for less than 0.32 ps after the excitation. The rate constants (k_-1) of the geminate rebinding of NO to the photolyzed 1 and 2 were determined to be (15 ps)^-1 and (13 ps)^-1, respectively. The photochemical quantum yields of NO photolysis (Φ_NO, λ = 320 nm) were estimated to be no higher than 14% for 1 and 11% for 2, based on the analysis of the fs time-resolved IR data. The results of fs time-resolved IR spectroscopy and theoretical calculations provided some insight into the overall kinetic reaction pathway, localized electron pathway or resonance pathway, of the NO photolysis of 1 and 2. Overall, our study found that the investigated {RuNO}⁶ complexes, 1 and 2, with planar N₂O₂ ligands bearing π-extended rings effectively released NO under visible light.

The influence of trans-ligand to NO on the thermal stability of photoinduced side-bond coordinated linkage isomer

The influence of trans-to NO ligand (X) on the thermal stability of transient side-on coordinated nitrosyl linkage isomer (MS2) is investigated in a series of trans-[RuNOPy4X](PF6)2 (X = F- (RuF),...

A new photoactivable NO-releasing {Ru-NO}6 ruthenium nitrosyl complex with a tetradentate ligand containing aniline and pyridine moieties

A new type of photoactivable NO-releasing ruthenium nitrosyl complex, [Ru(EPBP)Cl(NO)], with a tetradentate ligand, N,N'-(ethane-1,2-diyldi-o-phenylene)-bis(pyridine-2-carboxamide) (= H₂ EPBP) was synthesized. Single crystal X-ray crystallography revealed that the complex has a distorted octahedral coordination geometry and NO is positioned at cis to Cl^- ion. NO-photolysis was observed under a white room light. The photodissociation of Ru-NO bond was identified by various techniques including X-ray crystallography, IR, UV/Vis absorption, electron paramagnetic resonance (EPR), and NMR spectroscopies. Quantum yields for the NO-photolysis of the complex in CH₃ OH, CHCl₃ , DMSO, CH₃ CN, and CH₃ NO₂ were measured to be 0.19-0.36 with 400 (±5) nm excitation. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations were performed to understand the details of the photodissociation of the complex. The calculations suggest that the NO photolysis is most likely initiated by the electronic transition from the aniline moiety π MOs (π (aniline)) of the EPBP^2- chelating ligand to the π-antibonding MO of Ru-NO (π*(Ru-NO)). Experimental and theoretical investigations indicate that the EPBP^2- ligand provides an effective platform forming ruthenium nitrosyl complexes useful for NO-photoreleasing.

Bent and Linear {CoNO}8 Entities: Structure and Bonding in a Prototypic Class of Nitrosyls

Among the isoelectronic ligands CN^-, CO, and NO⁺, an oblique bonding to the metal is well-established for the nitrosyl ligand, with M-N-O angles down to ≈120°. In the last decades, the nitrosyl community got into the habit of addressing a bent-bonded nitrosyl ligand as ¹NO^-. Thus, because various redox forms of a nitrosyl ligand seem to exist, the ligand is considered to be "noninnocent" because of the obvious ambiguity of an oxidation state (OS) assignment of the ligand and metal. Among the bent-bonded species, the low-spin {CoNO}⁸ class is prototypic. From this class, some 20 new nitrosyl compounds, the X-ray structure determinations of which comply with strict quality criteria, were analyzed with respect to the OS issue. As a result, the effective OS method shows a low-spin d⁸ Co^I-NO⁺ couple instead of a negative OS of the ligand at the BP86/def2-TZVP (+D3, +CPCM with infinite permittivity) level of theory. The same holds for some new members of the linear subclass of {CoNO}⁸ compounds. For all compounds, a largely invariable "real" charge of ≈ -0.3 e was obtained from population analyses. All of these electron-rich d⁸ species strive to manage Pauli repulsion between the metal electrons and the lone pair at the nitrosyl's nitrogen atom, with the bending of the CoNO unit as the most frequent escape.

Visible-to-NIR-Light Activated Release: From Small Molecules to Nanomaterials

Photoactivatable (alternatively, photoremovable, photoreleasable, or photocleavable) protecting groups (PPGs), also known as caged or photocaged compounds, are used to enable non-invasive spatiotemporal photochemical control over the release of species of interest. Recent years have seen the development of PPGs activatable by biologically and chemically benign visible and near-infrared (NIR) light. These long-wavelength-absorbing moieties expand the applicability of this powerful method and its accessibility to non-specialist users. This review comprehensively covers organic and transition metal-containing photoactivatable compounds (complexes) that absorb in the visible- and NIR-range to release various leaving groups and gasotransmitters (carbon monoxide, nitric oxide, and hydrogen sulfide). The text also covers visible- and NIR-light-induced photosensitized release using molecular sensitizers, quantum dots, and upconversion and second-harmonic nanoparticles, as well as release via photodynamic (photooxygenation by singlet oxygen) and photothermal effects. Release from photoactivatable polymers, micelles, vesicles, and photoswitches, along with the related emerging field of photopharmacology, is discussed at the end of the review.