Synthesis and properties of umbelliferone-nitroxide radical hybrid compounds as fluorescence and spin-label probes

Fluorescence activation, patterning and enhancement with photogenerated radicals, a prefluorescent probe and silver nanostructures

We designed a switchable fluorophore activated by UVA light and a radical initiator, for optical lithography with concomitant metal-enhanced fluorescence by silver nanoparticles.

Fluorescent probes for the detection of nitroxyl (HNO)

Nitroxyl (HNO), which according to the IUPAC recommended nomenclature should be named azanone, is the protonated one-electron reduction product of nitric oxide. Recently, it has gained a considerable attention due to the interesting pharmacological effects of its donors. Although there has been great progress in the understanding of HNO chemistry and chemical biology, it still remains the most elusive reactive nitrogen species, and its selective detection is a real challenge. The development of reliable methodologies for the direct detection of azanone is essential for the understanding of important signaling properties of this reactive intermediate and its pharmacological potential. Over the last decade, there has been considerable progress in the development of low-molecular-weight fluorogenic probes for the detection of HNO, and therefore, in this review, we have focused on the challenges and limitations of and perspectives on nitroxyl detection based on the use of such probes.

Synthesis and fluorescence properties of six fluorescein-nitroxide radical hybrid-compounds

Six fluorescein-nitroxide radical hybrid-compounds (2ab, 3ab, 4, and 5) were synthesized by the condensation of 5- or 6-carboxy-fluorescein and 4-amino-TEMPO (2ab), 5- or 6-aminofluorescein and 4-carboxy-TEMPO (3ab), and fluorescein and 4-carboxy-TEMPO (4), or by reaction of the 3-hydroxyl group of fluorescein with DPROXYL-3-ylmethyl methanesulfonate (5). Fluorescence intensities (around 520nm) after reduction of the radical increased to 1.43-, 1.38-, and 1.61-folds for 2a, 2b and 3b respectively; 3a alone exhibited a decrease in intensity on reduction. Since 4 was readily solvolyzed in PBS or even methanol to afford fluorescein and 4-carboxy-TEMPO, its fluorescence change could not be measured. Hybrid compound 5 containing an ether-linkage between the fluorescein phenol and 3-hydroxymethyl-DPROXYL hydroxyl centers, was stable and on reduction, showed a maximum increase (3.21-fold) in relative fluorescence intensity in PBS (pH5.0), despite its remarkably low absolute fluorescence intensity.

(6-Bromo-2-oxo-2H-chromen-4-yl)methyl diethylcarbamodithioate

In the title compound, C15H16BrNO2S2, the 2H-chromene ring system is nearly planar, with a maximum deviation of 0.0182 (22) Å. In the crystal, π–π interactions between pyran and benzene rings of chromene [shortest centroid–centroid distance = 3.7588 (14) Å] occur.

X-ray structures of precursors of styrylpyridine-derivatives used to obtain 4-((E)-2-(pyridin-2-yl)vinyl)benzamido-TEMPO: synthesis and characterization

The synthesis and characterization of the precursor isomers trans-4-(2-(pyridin-2-yl)vinylbenzaldehyde (I), trans-4-(2-(pyridin-4-yl)vinylbenzaldehyde (II), trans-4-(2-(pyridin-2-yl)vinylbenzoic acid (III) and (E)-4-(2-(pydridin-4-yl)vinylbenzoic acid (IV) are reported. These compounds were prepared in order to obtain trans-4-((E)-2-(pyridin-2-yl)vinyl)benzamide-TEMPO (V). Compounds I and II were obtained by using a Knoevenagel reaction in the absence of a condensing agent and solvent. Oxidation of the aldehyde group using the Jones reagent afforded the corresponding acid forms III and IV. A condensation reaction with 4-amino-TEMPO using oxalyl chloride/DMF/CH2Cl2 provided the 4-((E)-2-(pyridin-2-yl)vinyl)benzamide-TEMPO. Single crystals of compounds I, II and III were obtained and characterized by X-ray diffraction. Compound I belongs to space group P2(1)/c, a = 12.6674(19) Å, b = 7.2173(11) Å, c = 11.5877(14) Å, b = 97.203(13)° and the asymmetric unit was Z = 4, whereas compound II was in the space group P2(1), with a = 3.85728(9) Å, b = 10.62375(19) Å, c = 12.8625(2) Å, b = 91.722 (2)° and the asymmetric unit was Z = 2. Compound III crystallized as single colorless needle crystals, belonging to the monoclinic system with space group P2(1), with Z = 2, with a = 3.89359(7) Å, b = 17.7014(3) Å, c = 8.04530(12) Å, b = 94.4030 (16)°. All compounds were completely characterized by IR, (1)H-NMR, EI-MS and UV-Vis.

Scavenging and characterization of short-lived radicals using a novel stable nitroxide radical with a characteristic UV-vis absorption spectrum

A stable tert-butyl(10-phenyl-9-anthryl)nitroxide (BPAN) radical was newly synthesized and used for the capture/characterization of reactive radicals. Adducts obtained from the reactions of BPAN with in situ generated reactive radicals showed excellent stability, assuring complete isolation and purification. The structures of the adducts were established by LC-MS and NMR analyses.

1-Nitronyl nitroxide pyrene as a new off–on fluorescent chemosensor for Cu2+

A highly selective and sensitive colorimetric chemosensor based on nitronyl nitroxide was developed for detection of Cu²⁺.

Two-photon fluorescence microscopy imaging of cellular oxidative stress using profluorescent nitroxides

A range of varying chromophore nitroxide free radicals and their nonradical methoxyamine analogues were synthesized and their linear photophysical properties examined. The presence of the proximate free radical masks the chromophore's usual fluorescence emission, and these species are described as profluorescent. Two nitroxides incorporating anthracene and fluorescein chromophores (compounds 7 and 19, respectively) exhibited two-photon absorption (2PA) cross sections of approximately 400 G.M. when excited at wavelengths greater than 800 nm. Both of these profluorescent nitroxides demonstrated low cytotoxicity toward Chinese hamster ovary (CHO) cells. Imaging colocalization experiments with the commercially available CellROX Deep Red oxidative stress monitor demonstrated good cellular uptake of the nitroxide probes. Sensitivity of the nitroxide probes to H(2)O(2)-induced damage was also demonstrated by both one- and two-photon fluorescence microscopy. These profluorescent nitroxide probes are potentially powerful tools for imaging oxidative stress in biological systems, and they essentially "light up" in the presence of certain species generated from oxidative stress. The high ratio of the fluorescence quantum yield between the profluorescent nitroxide species and their nonradical adducts provides the sensitivity required for measuring a range of cellular redox environments. Furthermore, their reasonable 2PA cross sections provide for the option of using two-photon fluorescence microscopy, which circumvents commonly encountered disadvantages associated with one-photon imaging such as photobleaching and poor tissue penetration.

Profluorescent Nitroxides as Sensitive Probes of Oxidative Change and Free Radical Reactions

This paper presents a review on the use of tethered nitroxide–fluorophore molecules as probes of oxidative change and free radical generation and reaction. The proximity of the nitroxide free radical to the fluorophore suppresses the normal fluorescence emission process. Nitroxide free radical scavenging, metabolism or redox chemistry return the system to its natural fluorescent state and so these tethered nitroxide–fluorophore molecules are described as being profluorescent. A survey of profluorescent nitroxides found in the literature is provided as well as background on the mechanism of action and applications of these compounds as fluorometric probes within the fields of biological, materials and environmental sciences.

Synthesis and spectral properties of polymethine-cyanine dye-nitroxide radical hybrid compounds for use as fluorescence probes to monitor reducing species and radicals

Various hybrid compounds comprised of two types of nitroxide radicals and either a pentamethine (Cy5) or trimethine cyanine (Cy3) were synthesized. The nitroxide radicals were linked either via an ester-bond to one or two N-alkyl carboxyl-terminated groups of Cy5, or via two amido-bonds (aminocarbonyl or carbonylamino group) to the 5-position of the indolenine moieties of Cy5 and Cy3. Changes in fluorescence and ESR intensities of the hybrid compounds were measured before and after addition of Na ascorbate in PBS (pH 7.0) to reduce the radicals. Among the hybrid compounds synthesized, those that linked the nitroxide radicals via an aminocarbonyl residue at the 5-position of the indolenine moieties on Cy5 and Cy3 exhibited a 1.8- and 5.1-fold increase in fluorescence intensity with the reduction of the nitroxide segment by the addition of Na ascorbate, respectively. In contrast, fluorescence intensity was not enhanced in the other hybrid compounds. Thus, the hybrid compounds which exhibited an increase in fluorescent intensity with radical reduction can be used in the quantitative measurement of reducing species such as Fe(2+) and ascorbic acid, and hydroxyl radicals. Because these hybrid compounds have the advantage of fluorescing at longer wavelengths-661 (Cy5) or 568 (Cy3)nm, respectively, they can be used to measure radical-reducing species or radicals either in solution or in vivo.