Mechanistic Aspects of the Oxidative and Reductive Fragmentation of N-Nitrosoamines: A New Method for Generating Nitrenium Cations, Amide Anions, and Aminyl Radicals

Sunlight-triggered prebiotic nanomotors for inhibition and elimination of pathogen and biofilm in aquatic environment

Pathogen contamination in drinking water sources causes waterborne infectious diseases, seriously threatening human health. Nowadays, stimuli-responsive self-propelled nanomotors are appealing therapeutic agents for antibacterial therapy in vivo. However, achieving water disinfection using these nanobots is still a great challenge. Herein, we report on prebiotic galactooligosaccharide-based nanomotors for sunlight-regulated water disinfection. The nanomotors can utilize galactooligosaccharide-based N-nitrosamines as sunlight-responsive fuels for the spontaneous production of antibacterial nitric oxide. Such a solar-to-chemical energy conversion would power the nanomotors for self-diffusiophoresis, which could promote the diffusion of the nanomotors in water and their penetration in the biofilm, significantly enhancing the inhibition and elimination of the pathogens and their biofilms in aquatic environments. After water treatments, the prebiotic-based residual disinfectants can be selectively utilized by beneficial bacteria to effectively relieve safety risks to the environment and human health. The low-energy-cost, green and potent antibacterial nanobots show promising potential in water disinfection.

Fluorescent small molecule donors

Small molecule donors (SMDs) play subtle roles in the signaling mechanism and disease treatments. While many excellent SMDs have been developed, dosage control, targeted delivery, spatiotemporal feedback, as well as the efficiency evaluation of small molecules are still key challenges. Accordingly, fluorescent small molecule donors (FSMDs) have emerged to meet these challenges. FSMDs enable controllable release and non-invasive real-time monitoring, providing significant advantages for drug development and clinical diagnosis. Integration of FSMDs with chemotherapeutic, photodynamic or photothermal properties can take full advantage of each mode to enhance therapeutic efficacy. Given the remarkable properties and the thriving development of FSMDs, we believe a review is needed to summarize the design, triggering strategies and tracking mechanisms of FSMDs. With this review, we compiled FSMDs for most small molecules (nitric oxide, carbon monoxide, hydrogen sulfide, sulfur dioxide, reactive oxygen species and formaldehyde), and discuss recent progress concerning their molecular design, structural classification, mechanisms of generation, triggered release, structure-activity relationships, and the fluorescence response mechanism. Firstly, from the large number of fluorescent small molecular donors available, we have organized the common structures for producing different types of small molecules, providing a general strategy for the development of FSMDs. Secondly, we have classified FSMDs in terms of the respective donor types and fluorophore structures. Thirdly, we discuss the mechanisms and factors associated with the controlled release of small molecules and the regulation of the fluorescence responses, from which universal guidelines for optical properties and structure rearrangement were established, mainly involving light-controlled, enzyme-activated, reactive oxygen species-triggered, biothiol-triggered, single-electron reduction, click chemistry, and other triggering mechanisms. Fourthly, representative applications of FSMDs for trackable release, and evaluation monitoring, as well as for visible in vivo treatment are outlined, to illustrate the potential of FSMDs in drug screening and precision medicine. Finally, we discuss the opportunities and remaining challenges for the development of FSMDs for practical and clinical applications, which we anticipate will stimulate the attention of researchers in the diverse fields of chemistry, pharmacology, chemical biology and clinical chemistry. With this review, we hope to impart new understanding thereby enabling the rapid development of the next generation of FSMDs.

Shedding light on cellular dynamics: the progress in developing photoactivated fluorophores

Photoactivated fluorophores (PAFs) are highly effective imaging tools that exhibit a removal of caging groups upon light excitation, resulting in the restoration of their bright fluorescence. This unique property allows for precise control over the spatiotemporal aspects of small molecule substances, making them indispensable for studying protein labeling and small molecule signaling within live cells. In this comprehensive review, we explore the historical background of this field and emphasize recent advancements based on various reaction mechanisms. Additionally, we discuss the structures and applications of the PAFs. We firmly believe that the development of more novel PAFs will provide powerful tools to dynamically investigate cells and expand the applications of these techniques into new domains.

Near Infrared Light-Activatable Platelet-Mimicking NIR-II NO Nano-Prodrug for Precise Atherosclerosis Theranostics

Atherosclerosis is a chronic inflammatory disease that affects arteries and is the main cause of cardiovascular disease. Atherosclerotic plaque formation is usually asymptomatic and does not manifest until the occurrence of clinical events. Therefore, early diagnosis and treatment of atherosclerotic plaques is particularly important. Here, a series of NIR-II fluorescent dyes (RBT-NH) are developed for three photoresponsive NO prodrugs (RBT-NO), which can be controllably triggered by 808 nm laser to release NO and turn on the NIR-II emission in the clinical medicine "therapeutic window". Notably, RBT3-NO is selected for its exhibited high NO releasing efficiency and superior fluorescence signal enhancement. Subsequently, a platelet-mimicking nano-prodrug system (RBT3-NO-PEG@PM) is constructed by DSPE-mPEG5k and platelet membrane (PM) for effectively targeted diagnosis and therapy of atherosclerosis in mice. The results indicate that this platelet-mimicking NO nano-prodrug system can reduce the accumulation of lipids at the sites of atherosclerotic plaques, improve the inflammatory response at the lesion sites, and promote endothelial cell migration, thereby slowing the progression of plaques.

Visible-Light-Promoted Photoaddition of N-Nitrosopiperidines to Alkynes: Continuous Flow Chemistry Approach to Tetrahydroimidazo[1,2-a]pyridine 1-Oxides

The photoaddition of N-nitrosopiperidines to terminal alkynes was effected under visible-light irradiation, in which a novel synthetic access to tetrahydroimidazo[1,2-a]pyridine 1-oxides was achieved via the dehydrogenative cycloisomerization of β-nitroso enamine intermediates. The decomposition pathways of N-nitrosamines, alkynes, and β-nitroso enamine intermediates were better handled in a continuous flow setting through the diffusion control of chemical species that negatively affected the formation of tetrahydroimidazo[1,2-a]pyridine 1-oxides under batch reaction conditions.

Visible-Light-Induced Photoaddition of N-Nitrosoalkylamines to Alkenes: One-Pot Tandem Approach to 1,2-Diamination of Alkenes from Secondary Amines

The generation of aminium radical cation species from N-nitrosoamines is disclosed for the first time through visible-light excitation at 453 nm. The developed visible-light-promoted photoaddition reaction of N-nitrosoamines to alkenes was combined with the o-NQ-catalyzed aerobic oxidation protocol of amines to telescope the direct handling of harmful N-nitroso compounds, where the desired α-amino oxime derivatives were obtained in a one-pot tandem N-nitrosation and photoaddition sequence.

Controllable Z/E-selective synthesis of α-amino-ketoximes from N-nitrososulfonamides and aryl alkenes under neutral conditions

An amidoximation of alkenes with N-nitrososulfonamides enabled by triplet energy transfer under neutral conditions is presented. Both (Z)- and (E)-α-amino-ketoximes are selectively accessible depending on the triplet energy of the photosensitizer.

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.

Reactivity of Diarylnitrenium Ions

Hydride abstraction from diarylamines with the trityl ion is explored in an attempt to generate a stable diarylnitrenium ion, Ar₂ N⁺ . Sequential H-atom abstraction reactions ensue. The first H-atom abstraction leads to intensely colored aminium radical cations, Ar₂ NH^.+ , some of which are quite stable. However, most undergo a second H-atom abstraction leading to ammonium ions, Ar₂ NH₂ ⁺ . In the absence of a ready source of H-atoms, a unique self-abstraction reaction occurs when Ar=Me₅ C₆ , leading to a novel iminium radical cation, Ar=N^.+ Ar, which decays via a second self H-atom abstraction reaction to give a stable iminium ion, Ar=N⁺ HAr. These products differ substantially from those derived via photochemically produced diarylnitrenium ions.

Light-triggered nitric oxide (NO) release from photoresponsive polymersomes for corneal wound healing

Polymersomes have been extensively used in the delivery of both small and macromolecular payloads. However, the controlled delivery of gaseous therapeutics (e.g., nitric oxide, NO) remains a grand challenge due to its difficulty in loading of gaseous payloads into polymersomes without premature leakage. Herein, NO-releasing vesicles could be fabricated via the self-assembly of NO-releasing amphiphiles, which were synthesized by the direct polymerization of photoresponsive NO monomers (abbreviated as oNBN, pNBN, and BN). These monomers were rationally designed through the integration of the photoresponsive behavior of N-nitrosoamine moieties and the self-immolative chemistry of 4-aminobenzyl alcohol derivatives, which outperformed conventional NO donors such as diazeniumdiolates (NONOates) and S-nitrosothiols (SNOs) in terms of ease of preparation, stability of storage, and controllability of NO release. The unique design made it possible to selectively release NO by a light stimulus and to regulate the NO release rates. Importantly, the photo-mediated NO release could be manipulated in living cells and showed promising applications in the treatment of corneal wounds. In addition to delivering NO, the current design enabled the synergistic delivery of NO and other therapeutic payloads by taking advantage of NO release-mediated traceless crosslinking of the vesicles.

A lysosome specific theranostic NO donor inhibits cancer cells by stimuli responsive molecular self-decomposition with an on-demand fluorescence pattern

The anticancer mechanism of NO is difficult to study owing to its short lifetime and high reactivity. Thus, a theranostic anticancer NO donor assembled with NO on-demand release abilities, accurate lysosome location capabilities and signal feedback behavior was developed. Profiting from the theranostic properties, the specific mechanism was comprehensively studied. Spectral and cell imaging studies revealed that the as prepared NO donors could release NO in solution or within cancer cells. Fluorescence co-dyeing experiments demonstrated that Mo-Nap-NO entered lysosomes specifically and disrupted them after being triggered by light. Upon irradiation with 460 nm visible light, both the donors demonstrated considerable in vitro anticancer effects. A further mechanistic study showed that after entering the lysosome and being triggered by 460 nm irradiation, NO ruptured the lysosome, resulting in the release of cathepsin D into the cytosol, which activated the caspase3 mediated apoptosis pathway.

A Photo-triggered and photo-calibrated nitric oxide donor: Rational design, spectral characterizations, and biological applications

Nitric oxide (NO) donors are valuable tools to probe the profound implications of NO in health and disease. The elusive nature of NO bio-relevance has largely limited the use of spontaneous NO donors and promoted the development of next generation NO donors, whose NO release is not only stimulated by a trigger, but also readily monitored via a judiciously built-in self-calibration mechanism. Light is without a doubt the most sensitive, versatile and biocompatible method of choice for both triggering and monitoring, for applications in complex biological matrices. Herein, we designed and synthesized an N-nitroso rhodamine derivative (NOD560) as a photo-triggered and photo-calibrated NO donor to address this need. NOD560 is essentially non-fluorescent. Upon irradiation by green light (532 nm), it efficiently release NO and a rhodamine dye, the dramatic fluorescence turn-on from which could be harnessed to conveniently monitor the localization, flux, and dose of NO release. The potentials of NOD560 for in vitro biological applications were also exemplified in in vitro biological models, i.e. mesenchymal stem cell (MSC) migration suppression. NOD560 is expected to complement the existing NO donors and find widespread applications in chemical biological studies.

A two-photon excitable and ratiometric fluorogenic nitric oxide photoreleaser and its biological applications

We report a two-photon excitable nitric oxide photoreleaser with ratiometric fluorescence variation, its spatiotemporally controlled release and biological applications.

Biotic nitrosation of diclofenac in a soil aquifer system (Katari watershed, Bolivia)

Up till now, the diclofenac (DCF) transformation into its nitrogen-derivatives, N-nitroso-DCF (NO-DCF) and 5-nitro-DCF (NO2-DCF), has been mainly investigated in wastewater treatment plant under nitrification or denitrification processes. This work reports, for the first time, an additional DCF microbial mediated nitrosation pathway of DCF in soil under strictly anoxic conditions probably involving codenitrification processes and fungal activities. This transformation pathway was investigated by using field observations data at a soil aquifer system (Katari watershed, Bolivia) and by carrying out soil slurry batch experiments. It was also observed for diphenylamine (DPA). Field measurements revealed the occurrence of NO-DCF, NO2-DCF and NO-DPA in groundwater samples at concentration levels in the 6-68s/L range. These concentration levels are more significant than those previously reported in wastewater treatment plant effluents taking into account dilution processes in soil. Interestingly, the p-benzoquinone imine of 5-OH-DCF was also found to be rather stable in surface water. In laboratory batch experiments under strictly anoxic conditions, the transformation of DCF and DPA into their corresponding N-nitroso derivatives was well correlated to denitrification processes. It was also observed that NO-DCF evolved into NO2-DCF while NO-DPA was stable. In vitro experiments showed that the Fisher-Hepp rearrangement could not account for NO2-DCF formation. One possible mechanism might be that NO-DCF underwent spontaneous NO loss to give the resulting intermediates diphenylaminyl radical or nitrenium cation which might evolve into NO2-DCF in presence of NO2 radical or nitrite ion, respectively.

Time-resolved resonance Raman investigation of the 2-fluorenylnitrenium ion reactions with C8 guanosine derivatives

A nanosecond time-resolved resonance Raman (ns-TR3) spectroscopic study of the reactions of the 2-fluorenylnitrenium ion with several C8-substituted guanosine derivatives is reported. The TR3 spectra show that the 2-fluorenylnitrenium ion reacts with the C8-substituted guanosine derivatives (C8-methylguanosine and C8-bromoguanosine) to produce C8 intermediates with the methyl and bromine moieties still attached to the intermediate species at the C8 position. The C8-bromoguanosine species was observed to be less reactive toward the 2-fluorenylnitrenium ion compared to the guanosine and C8-methylguanosine species. Comparison of the TR3 spectra to the results obtained from density functional theory calculations was used to characterize the C8 intermediates observed to learn more about their structure and properties. The implications of these results for the chemical reactivity of arylnitrenium ions toward substituted guanosine derivatives are briefly discussed.

Disproportionation of clozapine radical: a link between one-electron oxidation of clozapine and formation of its nitrenium cation

The primary products of one-electron oxidation of clozapine and olanzapine, very effective atypical antipsychotic drugs, have been spectroscopically characterized. The oxidation process has been studied under glassy matrix conditions and by a pulse radiolysis technique in aqueous solution. The rate constants for the oxidation of clozapine with dibromide radical anion ( k = 2 x 10 (9) M (-1) s (-1)) and azide radical ( k = 2.3 x 10 (9) M (-1) s (-1)) in aqueous solution were measured. The computational DFT results support the identification of the transient species. The mechanistic aspects of reactivity of radical cations, radicals, and nitrenium cations have been investigated. A disproportionation reaction ( k > or = 1 x 10 (8) M (-1) s (-1)) was proposed as a link between the products of one-electron oxidation and formation of the nitrenium cations of clozapine and olanzapine, products likely responsible for the pathogenesis of adverse drug reactions. The rate constants for the reactions of nitrenium cation of clozapine with glutathione ( k = 3.4 x 10 (4) M (-1) s (-1)) and cysteine ( k = 9.8 x 10 (4) M (-1) s (-1)) were determined.