Stoichiometric Nitroxyl Photorelease Using the (6-Hydroxy-2-naphthalenyl)methyl Phototrigger

Developing Photoactive Coumarin-Caged N-Hydroxysulfonamides for Generation of Nitroxyl (HNO)

Photoactive N-hydroxysulfonamides photocaged with the (6-bromo-7-hydroxycoumarin-4-yl)methyl chromophore have been successfully synthesized, and the mechanisms of photodecomposition investigated for two of the compounds. Upon irradiation up to 97% of a diagnostic marker for (H)NO release, sulfinate was observed for the trifluoromethanesulfonamide system. In the absence of a species that reacts rapidly with (H)NO, (H)NO instead reacts with the carbocation intermediate to ultimately generate (E)-BHC-oxime and (Z)-BHC-oxime. Alternatively, the carbocation intermediate reacts with solvent water to give a diol. Deprotonation of the N(H) proton is required for HNO generation via concerted C-O/N-S bond cleavage, whereas the protonation state of the O(H) does not affect the observed photoproducts. If the N(H) is protonated, C-O bond cleavage to generate the parent N-hydroxysulfonamide will occur, and/or O-N bond cleavage to generate a sulfonamide. The undesired competing O-N bond cleavage pathway increases when the volume percentage of water in acetonitrile/water solvent mixtures is increased.

Blue or Near-Infrared Light-Triggered Release of Halogens via Blebbistatin Photocage

Photocages can provide spatial and temporal control to accurately release the various chemicals and bioactive groups when excited by light. Although the absorption spectra of most photocages are in the ultraviolet absorption region, only a few absorb in the visible or near-infrared region. Blebbistatin (Bleb) would release a hydroxyl radical under blue one-photon or two-photon near-infrared light (800 nm) irradiation. In this work, typical chlorine and bromine as leaving groups substituted hydroxyl compounds (Bleb-Cl, Bleb-Br) are synthesized to evaluate the photocage's capability of Bleb's platform. Driven by the excited-state charge transfer, Bleb-Cl and Bleb-Br show good photolysis quantum yield to uncage the halogen anion and the uncaging process would be accelerated in water solution. The photochemical reaction, final product's analysis, and femtosecond transient absorption studies on Bleb-Cl/Bleb-Br demonstrate that Bleb can act as a photocage platform to release the halogen ion via heterolytic reaction when irradiated by blue or near-infrared light. Therefore, Bleb can be a new generation of visible or near-infrared light-triggered photocage.

Exploring the Potential of 2-(2-Nitrophenyl)ethyl-Caged N-Hydroxysulfonamides for the Photoactivated Release of Nitroxyl (HNO)

The emergence of nitroxyl (HNO) as a biological signaling molecule is attracting increasing attention. HNO-based prodrugs show considerable potential in treating congestive heart failure, with HNO reacting rapidly with metal centers and protein-bound and free thiols. A new class of 2-(2-nitrophenyl)ethyl (2-NPE)-photocaged N-hydroxysulfonamides has been developed, and the mechanisms of photodecomposition have been investigated. Three photodecomposition pathways are observed: the desired concomitant C-O/N-S bond cleavage to generate HNO, sulfinate, and 2-nitrostyrene, C-O bond cleavage to give the parent sulfohydroxamic acid and 2-nitrostyrene, and O-N bond cleavage to release a sulfonamide and 2-nitrophenylacetaldehyde. Laser flash photolysis experiments provide support for a Norrish type II mechanism involving 1,5-hydrogen atom abstraction to generate an aci-nitro species. A mechanism is proposed in which the (Z)-aci-nitro intermediate undergoes either C-O bond cleavage to release RSO₂NHO(H), concerted C-O/N-S bond cleavage to generate sulfinate and HNO, or isomerization to the (E)-isomer prior to O-N bond cleavage. The pK_a of the N(H) of the N-hydroxysulfonamide plays a key role in determining whether C-O or concerted C-O/N-S bond cleavage occurs. Deprotonating this site favors the desired C-O/N-S bond cleavage at the expense of an increased level of undesired O-N bond cleavage. Triplet state quenchers have no effect on the observed photoproducts.

Azanone (HNO): generation, stabilization and detection

HNO (nitroxyl, azanone), joined the 'biologically relevant reactive nitrogen species' family in the 2000s. Azanone is impossible to store due to its high reactivity and inherent low stability. Consequently, its chemistry and effects are studied using donor compounds, which release this molecule in solution and in the gas phase upon stimulation. Researchers have also tried to stabilize this elusive species and its conjugate base by coordination to metal centers using several ligands, like metalloporphyrins and pincer ligands. Given HNO's high reactivity and short lifetime, several different strategies have been proposed for its detection in chemical and biological systems, such as colorimetric methods, EPR, HPLC, mass spectrometry, fluorescent probes, and electrochemical analysis. These approaches are described and critically compared. Finally, in the last ten years, several advances regarding the possibility of endogenous HNO generation were made; some of them are also revised in the present work.

Mechanistic Insights into Rapid Generation of Nitroxyl from a Photocaged N-Hydroxysulfonamide Incorporating the (6-Hydroxynaphthalen-2-yl)methyl Chromophore

HNO is a highly reactive molecule that shows promise in treating heart failure. Molecules that rapidly release HNO with precise spatial and temporal control are needed to investigate the biology of this signaling molecule. (Hydroxynaphthalen-2-yl)methyl-photocaged N-hydroxysulfonamides are a new class of photoactive HNO generators. Recently, it was shown that a (6-hydroxynaphthalen-2-yl)methyl (6,2-HNM)-photocaged derivative of N-hydroxysulfonamide incorporating the trifluoromethanesulfonamidoxy group (1) quantitatively generates HNO. Mechanistic studies have now been carried out on this system and reveal that the ground state protonation state plays a key role in whether concerted heterolytic C-O/N-S bond cleavage to release HNO occurs versus undesired O-N bond cleavage. N-Deprotonation of 1 can be achieved by adding an aqueous buffer or a carboxylate salt to an aprotic solvent. Evidence is presented for C-O/N-S bond heterolysis occurring directly from the singlet excited state of the N-deprotonated parent molecule on the picosecond time scale, using femtosecond time-resolved transient absorption spectroscopy, to give a carbocation and ¹NO^-. This is consistent with the observation of significant fluorescence quenching when HNO is generated. The carbocation intermediate reacts rapidly with nucleophiles including water, MeOH, or even (H)NO in the absence of a molecule that reacts rapidly with (H)NO to give an oxime.

A turn-on and lysosome-targeted fluorescent NO releaser in water media and its application in living cells and zebrafishes

Due to the high activity and difficult to transport of nitric oxide, the controlled release of nitric oxide has been a new trend in the research on the biological effect of nitric oxide. In this paper, a water-soluble and turn-on fluorescent NO donor Rh-NO was synthesized. Upon 525 nm irradiation, the fluorescence of the Rh-NO at 568 nm enhanced with the quantum yield (Φ_F) of Rh-NO changing from 5.08% to 35.96%. The mechanism of NO releasing was proved by HRMS and the Dan. The releasing time of 6 min and the releasing yield of 0.61 proved the superiority of Rh-NO. Excellent cell activity above 80% of Rh-NO and Rh guaranteed that nitric oxide was released from Rh-NO in lysosome and zebrafishes successfully, which provided a good platform to understand the biological effects of nitric oxide in lysosomes.