Intramolecular quenching of excited singlet states by stable nitroxyl radicals

Factors affecting light energy conversion in dual fluorophore-nitroxide molecules in solution and a protein

Donor-acceptor structures capable of retaining the charge-photoseparated state during the time long enough for secondary chemical reactions of these charges to occur attract special interest from the viewpoint of the problem of light energy utilization. We proposed dual fluorophore-nitroxide compounds (FNO•) as systems for the conversion of light energy to chemical energy. In these systems, the fluorophore segment in the excited singlet state serves as an electron donor, and the nitroxide segment is an electron acceptor. In FNO•, the photo- and chemical reduction of nitroxide results in the drastic decay of the electron spin resonance (ESR) signal from the nitroxide and the parallel enhancement of fluorescence. The same groups allow one to measure the factors affecting the electron transfer, namely, molecular dynamics and micropolarity of the medium in the vicinity of the donor (by fluorescence technique) and acceptor (by ESR) moieties. We demonstrate that in the dual probes the nitroxide segment is photoreduced to hydroxylamine in solution and in such nanoscale structures as serum albumins. The photoreduction occurs by very weak reducing agents (glycerol, ethanol, ethylene glycol, etc.) without a violation of the fluorophore structure. Therefore, photochemical reactions in the dual compounds with the formation of a reducing agent as hydroxyl amine can be considered as processes of light energy transfer. The nitroxide segment tethered to the donor-bridge-acceptor triad affects the photoseparated charge recombination via the mechanism of spin catalysis. Proficiency of the dual compounds for developing energy conversion systems can be extended by an optimal choice of the participants of the photochemical and -physical processes.

Dual chromophore-nitroxides: novel molecular probes, photochemical and photophysical models and magnetic materials

Over the last decades scientists have faced growing requirements in novel methods of fast and sensitive analysis of antioxidant status of biological systems, spin redox probing and spin trapping, investigation of molecular dynamics, and of convenient models for studies of photophysical and photochemical processes. In approaching this problem, methods based upon the use of dual chromophore-nitroxide (CN) compounds have been suggested and developed. A CN consists of two molecular sub-functionality (a chromophore and a stable nitroxide radical) tethered together by spacers. In the dual compound the nitroxide is a strong intramolecular quencher of the fluorescence from the chromophore fragment. Reduction to hydroxylamine, oxidation of the nitroxide fragment or addition of an active radical yield the fluorescence increase and the parallel decay of the fragment electron spin resonance (ESR) signal. At certain conditions the dual molecules undergo photomagnetic switching and form excited state multi-spin systems. These unique properties of CN were intensively exploited as the basis for several methodologies, which include molecular probing, modeling intramolecular photochemical and photophysical processes, and construction of new magnetic materials.

Spin coupling in the supramolecular structure of a new tetra(quinoline-TEMPO)yttrium(III) complex

The newly synthesized tetra(quinoline-TEMPO)yttrium(III) potassium salt shows interesting structural features at the molecular and supramolecular levels, revealed by the analysis of the X-ray diffraction data. The magnetic susceptibility and EPR data corroborated with structural considerations showed that the exchange and dipolar spin coupling interactions are taking place at the nodes assembling the supramolecular 2D structure. The Y(III) center shows antiprismatic octacoordination, close to the idealized D2 symmetry. The diamagnetic transition metal plays no role in mediating the radical interactions since the TEMPO-type fragments are remote from the chelating moieties of the ligand. In turn, significant interaction occurs on the nodes consisting in the quasi-rectangular coordination of potassium counterions by the spin-bearing TEMPO groups coming from four distinct complex units. The antiferromagnetic susceptibility was consistently modeled by a spin Hamiltonian based on the rectangle topology of four spins S = 1/2. The fitted exchange parameters are Ja = -5.1 cm-1 and Jb = -3.4 cm-1 for the edges, imposing Jd = 0 for the diagonal. These values are in excellent agreement with the ab initio results Ja = -4.83 cm-1, Jb = -3.44 cm-1, Jd = -0.07 cm-1 obtained in a CASSCF(12,8) calculation. Based on the reliability of the ab initio results we were able to select the presented J parameters among several versions of multiple solutions with acceptable goodness of the fit. A methodological caveat about the artifacts of the automatic use of best fit parameters, in the absence of supplementary criteria, in the context of relative blindness of magnetic susceptibility modeling, is raised. The details of the EPR spectrum at 10 K are also consistent, in the frame of dipolar approximation, with the model of four interacting spins at the nodes of the supramolecular assembling.

Photolysis of an alkoxyamine using intramolecular energy transfer from a quinoline antenna—towards photo-induced living radical polymerization

The photolysis of an alkoxyamine triad comprised of the 1-phenylethyl moiety, 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) and 2-methyl-3-hydroxy quinoline leads to energy transfer from the quinoline moiety (antenna) and cleavage of the C-O alkoxyamine bond, suggesting its possible applications in low temperature living free radical polymerization.

Peptide Folding Dynamics: A Time-Resolved Study from the Nanosecond to the Microsecond Time Regime

Time-resolved spectroscopies, spanning from the nanosecond to the microsecond time regime, coupled with molecular mechanics calculations, allowed us to assess the most populated conformations in solution of a series of analogues of trichogin GA IV, a natural undecapeptide showing significant antimicrobial activity. This peptide is characterized by a high content of the conformationally constrained alpha-aminoisobutyric acid and by a glycine-glycine motif in the central part of the sequence. Nanosecond time-resolved fluorescence experiments were performed to determine the conformational properties of the peptide analogues in solution, while transient absorption measurements allowed us to study the peptide dynamics on the microsecond time scale. Because the peptides examined were functionalized by a fluorescent probe at the N-terminus and a nitroxide quencher placed along the backbone at three different positions, the distance-dependent fluorophore-quencher interaction was exploited to obtain a deeper insight into their three-dimensional structural and dynamical properties. Further information on the conformational and dynamical features was obtained by photophysical experiments as a function of the viscosity and polarity of the medium. Taken together, the results revealed a transition from an elongated, helical conformation to a family of compact, folded structures mimicking a helix-turn-helix motif, which may represent a model of the early steps of the protein hydrophobic collapse.

Recent advances in fluorescent probes for the detection of reactive oxygen species

Reactive oxygen species (ROS) have captured the interest of many researchers in the chemical, biological, and medical fields since they are thought to be associated with various pathological conditions. Fluorescent probes for the detection of ROS are promising tools with which to enhance our understanding of the physiological roles of ROS, because they provide spatial and temporal information about target biomolecules in in vivo cellular systems. ROS probes, designed to detect specific ROS with a high selectivity, would be desirable, since it is now becoming clear that each ROS has its own unique physiological activity. However, dihydro-compounds such as 2',7'-dichlorodihydrofluorescein (DCFH), which have traditionally been used for detecting ROS, tend to react with a wide variety of ROS and are not completely photostable. Some attractive fluorescent probes that exhibit a high degree of selectivity toward specific ROS have recently been reported, and these selective probes are expected to have great potential for elucidating unknown physiological mechanisms associated with their target ROS. This review focuses on the design, detection mechanism, and performance of fluorescent probes for the detection of singlet oxygen ((1)O(2)), hydrogen peroxide (H(2)O(2)), hydroxyl radicals ((.)OH), or superoxide anion (O(2) (-.)), a field in which remarkable progress has been achieved in the last few years.

High selective determination iron(II) by its enhancement effect on the fluorescence of pyrene-tetramethylpiperidinyl (TEMPO) as a spin fluorescence probe

A novel fluorescence method determination for iron(II) with a high selectivity and sensitivity has been proposed, based on the enhancement of fluorescence signals resulting from specific redox reaction between synthesized spin fluorescence probe pyrene-tetramethylpiperidinyl (TEMPO) and iron(II). Under the experimental conditions, fluorescent probe displayed a rapid and linear response for iron(II) over the concentration range from 2.4 x 10(-7) to 3.6 x 10(-6) mol/L. The limit of detection was 4.0 x 10(-8) mol/L. The relative standard deviation of six replicate measurements was 1.90% for 3.0 x 10(-7) mol/L iron(II). Because of the specific redox reaction between developed spin fluorescence probe and iron(II), there are few interference by other ions, especially in the presence of relative high concentration iron(III). The method has been successfully applied for iron(II) determinations in two different kinds of real samples. Results determined by the proposed method agree favorably with those determined UV-vis spectrometry method with 1,10-phenanthroline.

Fluorescence probes used for detection of reactive oxygen species

Endogenously produced pro-oxidant reactive species are essential to life, being involved in several biological functions. However, when overproduced (e.g. due to exogenous stimulation), or when the levels of antioxidants become severely depleted, these reactive species become highly harmful, causing oxidative stress through the oxidation of biomolecules, leading to cellular damage that may become irreversible and cause cell death. The scientific research in the field of reactive oxygen species (ROS) associated biological functions and/or deleterious effects is continuously requiring new sensitive and specific tools in order to enable a deeper insight on its action mechanisms. However, reactive species present some characteristics that make them difficult to detect, namely their very short lifetime and the variety of antioxidants existing in vivo, capable of capturing these reactive species. It is, therefore, essential to develop methodologies capable of overcoming this type of obstacles. Fluorescent probes are excellent sensors of ROS due to their high sensitivity, simplicity in data collection, and high spatial resolution in microscopic imaging techniques. Hence, the main goal of the present paper is to review the fluorescence methodologies that have been used for detecting ROS in biological and non-biological media.

Structural properties and photophysical behavior of conformationally constrained hexapeptides functionalized with a new fluorescent analog of tryptophan and a nitroxide radical quencher

The influence of the conformational properties on the photophysics of two de novo designed hexapeptides was studied by spectroscopic measurements (ir, NMR, steady-state, and time resolved fluorescence) and molecular mechanics calculations. The peptide sequences comprise two nonproteinogenic residues: a beta-(1-azulenyl)-L-alanine (Aal) residue, obtained by formally functionalizing the Ala side chain with the azulene chromophore, and a Calpha-tetrasubstituted alpha-amino acid (TOAC), incorporating a nitroxide group in a cycloalkyl moiety. Aal represents a new fluorescent, quasi-isosteric Trp analog and TOAC a stable radical species, frequently used as a paramagnetic probe in biochemical studies. The peptide chains differ in the sequence position of the two probes and are heavily based on Aib (alpha-aminoisobutyric acid) residues to generate conformationally restricted helical structures, as confirmed by both spectroscopic and computational results. The conformationally controlled, excited state interactions, determining the photophysical relaxation of the Aal*/TOAC pair, are also discussed.

Synthesis of profluorescent isoindoline nitroxides via palladium-catalysed Heck alkenylation

The synthesis of a new structural class of isoindoline nitroxides (aminoxyls), accessible via the palladium-catalysed Heck reaction, is presented. Reaction of the aryl bromoamine, 5-bromo-1,1,3,3-tetramethylisoindoline (4) or dibromoamine, 5,6-dibromo-1,1,3,3-tetramethylisoindoline (5) or the analogous bromonitroxides 6 and 7 with methyl acrylate gives the acrylate substituted tetramethylisoindoline amines 8 and 10 and nitroxides 12 and 14. Similarly, the reaction of the aryl bromides and dibromides 4-7 with methyl 4-vinylbenzoate gives the carboxystyryl substituted tetramethylisoindoline amines 9 and 11 and the analogous nitroxides 13 and 15. The carboxystyryl tetramethylisoindoline nitroxides demonstrate strongly suppressed fluorescence, which is revealed upon removal of the free radical by reduction or radical coupling.

Preparation and Photophysics of 2-(1-Pyrenyl)acrylic Acid and Its Methyl and 2′,2′,6′,6′-Tetramethyl-4′-Piperidyl Esters

Novel probes represented connection of pyrene as chromophore and sterically hindered amine stabilizers (HAS) in the form of esters of 2-(1-pyrenyl)acrylic acid were synthesized. HAS was in the form of parent amine (PAP) as well as stable nitroxyl radical form (PAP-NO.). Photophysics of these probes were compared with their precursor as 2-(1-pyrenyl)acrylic acid (PAA) and its methyl ester (PAM). The fluorescence spectrum of PAA strongly depends on the acidity of the solution. The spectrum in neutral methanol indicates that it originates from the anionic form -COO(-). Changes of acidity or basicity of methanol solution resulted in the changes of shape, position as well as the intensity of fluorescence band. This is due to the presence of protolytic equilibria, either in the ground state or in the singlet excited state, leading to the formation of molecular form -COOH and the cationic form -COOH(2) (+). The ester analogues did not show any changes in various pH conditions. Fluorescence of all probes depends on the polarity of solvents and the presence of oxygen. Intermolecular quenching was studied with external quenchers TEMPO and oxygen and the data were compared with the intramolecular quenching using 1'-oxo-2',2',6',6'-tetramethyl-4'-piperidinyl-2-(1-pyrenyl)acrylate (PAP-NO.).

Monitoring photodecomposition of dibenzyl ketone within NaY zeolite with a pre-fluorescent nitroxide compound

Benzyl radicals derived from photodecomposition of dibenzyl ketone have been monitored in NaY zeolite using the pre-fluorescent probe 4-(3-hydroxy-2-methyl-4-quinolinoyloxy)-2,2,6,6-tetramethylpiperidine-1-oxyl free radical (QT). The steady-state and time-resolved fluorescence show a strong interaction of QT within the zeolite cavities. Furthermore, the increase in the fluorescence with the irradiation time reveals the efficient trapping of benzyl radicals by QT under vacuum. Dispersion of the zeolite composites in liquid polymers results in an improvement in the fluorescence measurement and faster kinetics in comparison with the solid samples. These results are interpreted in terms of partial refractive index matching as a consequence of polymer reptation into the zeolite cavities. Analysis of the fluorescence lifetime distribution shows the presence of different fluorescence ensembles with different fluorescence quantum yields, depending on the microenvironment within the zeolite particle. Pre-fluorescent probes offer a novel and useful methodology for studying radical processes in zeolites.

Probing the Hydroxyl Radical-Mediated Reactivity of Peroxynitrite by a Spin-Labeling Fluorophore

The decomposition of peroxynitrite at physiological pH yielded a hydroxyl radical, which reacted rapidly with dimethyl sulfoxide (DMSO) to produce a methyl radical (*CH3), which was then trapped by a spin-label fluorophore nitroxide-linked naphthalene (NTEMPO), a carbon-centered radical probe with a low fluorescence intensity, and transformed to a stable diamagnetic O-alkoxyamine, a high-fluorescence compound. The fluorescence increment was proportional to the concentration of the hydroxyl radical, and then to the concentration of peroxynitrite. NTEMPO therefore was demonstrated to be capable of detecting hydroxyl radicals generated from peroxynitrite, and the method was proved to be simple and sensitive. The hydroxyl radical-mediated reactivities of peroxynitrite to several amino acids, such as tyrosine, phenylalanine and histidine, were then evaluated by the spin-labeling fluorophore NTEMPO at pH 7.4 and, 37 degrees C. The obtained data were in good agreement with the reference values, respectively.

Detection of nitric oxide from pig trachea by a fluorescence method

A nitronyl nitroxide radical covalently linked to an organic fluorophore, pyrene, was used to detect nitric oxide (NO) from freshly excited tissues. This approach is based on the phenomenon of the intramolecular fluorescence quenching of the fluorophore fragment by the nitroxide. The pyrene-nitronyl (PN) reacts with NO to yield a pyrene-imino nitroxide radical (PI) and NO(2). Conversion of PN to PI is accompanied by changes in the electron paramagnetic resonance (EPR) spectrum from a five-line pattern (two equivalent N nuclei) into a seven-line pattern (two nonequivalent N nuclei). The transformation of the EPR signal is accompanied by an increase in the fluorescence intensity since the imino nitroxide radical is a weaker quencher than the nitronyl one. The results indicate that the fluorescence measurements enable detection of nanomolar concentrations of NO compared to a sensitivity threshold of only several micromolar for the EPR technique. The method was applied to the determination of NO and S-nitroso compounds in tissue from pig trachea epithelia. The measured basal flux of S-nitroso compounds obtained from the tissues was about 1.2 nmol/g x min, and NO-synthase stimulated by extracellular adenosine 5'-triphosphate produced NO flux of 0.9 nmol/g x min.

TiO2-promoted mineralization of organic sunscreens in water suspension and sodium dodecyl sulfate micelles

The photostability of the widely used UVB sunscreen agents 2'-ethylhexyl-2-cyano-3-phenylcinnamate (1), 2-hydroxy-4-methoxybenzophenone (2), octyl salicilate (3), and 2'-ethylhexyl-4-methoxycinnamate (4) has been investigated under UVA irradiation in the absence and presence of TiO2, an inorganic filter commonly employed in combination with organic filters in sunscreen preparations. In the absence of TiO2, 1-3 are photostable and 4 undergoes the expected E-Z isomerization; the presence of TiO2 caused mineralization of the organic filters and, surprisingly, the process is noticeably faster in the presence of surfactant than in sunscreen and water suspensions. The results indicate that in water suspension, mineralization is likely to occur on or near the TiO2 particle surface; when the organic sunscreens are segregated in the micelle core, reactive radicals, produced during TiO2-promoted degradation of the micellar system, may participate in sunscreen degradation. In addition, a pre-fluorescent probe for carbon-centered radical detection, 4-(3-hydroxy-2-methyl-4-quinolineoxy)-2,2,6,6-tetramethylpiperidine-1-oxyl free radical or QT (5), was employed to demonstrate that carbon-centered radicals are evolved during micelle degradation and may participate in the mineralization of sunscreens.

Microenvironmental polarity control of electron-transfer photochirogenesis. Enantiodifferentiating polar addition of 1,1-diphenyl-1-alkenes photosensitized by saccharide naphthalenecarboxylates

Enantiodifferentiating polar photoaddition of alcohol to 1,1-diphenylpropene and 1,1-diphenyl-1-butene sensitized by saccharide naphthalene(di)carboxylates was performed in nonpolar to polar solvents containing methanol, ethanol, or 2-propanol as the nucleophile to give the corresponding anti-Markovnikov alcohol adduct, that is, 1,1-diphenyl-2-alkoxy-propane and -butane in low-to-good chemical yields, depending on the sensitizer, chiral auxiliary, alcohol, solvent, and temperature employed. The excited state and intermediate involved, the reaction and enantiodifferentiation mechanism operating, and the factors controlling chemical and optical yields were elucidated from the photochemical and stereochemical outcomes under various conditions and also from the sensitizer and exciplex fluorescence quenching experiments and the molecular orbital calculations. A new strategy was developed to overcome the normally accepted tradeoff between the chemical and optical yields. This is made possible by employing protected saccharides as chiral auxiliaries and running the photoreactions not in a nonpolar but in a low-polarity solvent such as diethyl ether, which jointly enhance the "microenvironmental" polarity around the sensitizer to facilitate electron transfer, keeping the intimate interactions between the chiral sensitizer and substrate within the exciplex intermediate. By optimizing these factors, we obtained the photoadduct in enantiomeric excesses of up to 58%, which is the highest ever reported for a photosensitized bimolecular enantiodifferentiating reaction.

Mapping photogenerated radicals in thin polymer films: fluorescence imaging using a prefluorescent radical probe

Prefluorescent radical probes, in which fluorescence is activated by radical trapping, and photoinitiators were used to detect radical generation in polymer films using fluorescence spectroscopy and microscopy. Prefluorescent radical probes are the foundation of a fluorescence imaging system for polymer films, that may serve both as a mechanistic tool in the study of photoinitiated radical processes in polymer films and in the preparation of functional fluorescent images.

Detection of singlet oxygen and superoxide with fluorescent sensors in leaves under stress by photoinhibition or UV radiation

In order to understand the physiological functions of reactive oxygen species (ROS) generated in leaves, their direct measurement in vivo is of special importance. Here we report experiments with two dansyl-based ROS sensors, the singlet oxygen specific DanePy and HO-1889NH, which is reactive to both singlet oxygen and superoxide radicals. Here we report in vivo detection of (1)O(2) and O(2)(-*) by fluorescence quenching of two dansyl-based ROS sensors, the (1)O(2) specific DanePy and HO-1889NH, which was reactive with both (1)O(2) and O(2)(-*). The ROS sensors were administered to spinach leaves through a pinhole, and then the leaves were exposed to either excess photosynthetically active radiation or UV (280-360 nm) radiation. Microlocalization of the sensors' fluorescence and its ROS-induced quenching was followed with confocal laser scanning microscopy and with fluorescence imaging. These sensors were specifically localized in chloroplasts. Quenching analysis indicated that the leaves exposed to strong light produced (1)O(2), but hardly any O(2)(-*). On the other hand, the dominant ROS in UV-irradiated leaves was O(2)(-*), while (1)O(2) was minor.

Stilbene photochrome-fluorescence-spin molecules: covalent immobilization on silica plate and applications as redox and viscosity probes

We report herein on the development of a new photochrome-fluorescence-spin method for the quantitative analysis of the redox status and viscosity of a medium. The method of the viscosity measurement is based on the use of double fluorescence-nitroxide molecules. In such hybrid compounds the nitroxide moiety quenches the fluorescence of the fluorophore (stilbene moiety). The reduction of nitroxide by an antioxidant (ascorbic acid) causes a rise of fluorescence of the fluorophore. The rate constant of the stilbene fragment photoisomerization in such systems is dependent upon the viscosity of the media. The synthesized dual stilbene-nitroxide probe was covalently immobilized onto the surface of a quartz plate as an eventual fiber-optic sensor. The immobilization procedure included a cyanogen bromide surface activation followed by smoothing with a protein tether. The rate of fluorescence change was monitored in aqueous-glycerol solutions of different viscosities and content of ascorbic acid. Good correlation was found: (a) between the concentration of ascorbic acid in the sample and the rate of fluorescence increase due to the reduction of the nitroxide moiety, and (b) between the rate constant of photoisomerization and the viscosity of the media. Appropriate calibration would make the determination of the viscosity of a media possible (in a range 1-500 cP), as well as ascorbate content, in a range (1-9) x 10(-4) M, with fast single measurement.

Structural features of linear (alphaMe)Val-based peptides in solution by photophysical and theoretical conformational studies

In continuation of our studies on the determination of the structural features of functionalized peptides in solution by combining time-resolved fluorescence data and molecular mechanics results, the conformational features of a series of linear, L-(alphaMe)Val-based peptides have been investigated in methanol. These foldamers have the general formula F[(alphaMe)Val](r)-T-[(alphaMe)Val](2)NHtBu, where (alphaMe)Val = C(alpha)-methylvaline and r = 0-3, while F [= fluoren-9-ylmethoxycarbonyl (Fmoc)] and T [= 2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-carboxylic (Toac)] are a fluorophoric N(alpha)-protecting group and a nitroxide-based alpha-amino acid quencher, respectively. According to ir and CD spectra, the longest term of the series (r = 3) attains a 3(10)-helical structure, while the other peptides populate an intramolecularly H-bonded, 3(10)-helix-like conformation affected by dynamic helical distortions, which are enhanced by the shortness of the backbone chain. Such distortions are reflected in both the energy of the stretching mode and the molar extinction coefficient of the H-bonded N-H groups, the former being higher and the latter smaller than those of a stable 3(10)-helix. Steady-state and time-resolved fluorescence measurements in methanol show a strong quenching of Fmoc by the Toac residue, located at different helix positions, depending on the r value. Comparison of quenching efficiencies and lifetime preexponents with those theoretically obtained from the deepest energy minimum conformers, assuming a Förster mechanism, is satisfactory. The computed structures exhibit a rather compact arrangement, which accounts for the few sterically favored conformations for each peptide, in full agreement with the time-resolved fluorescence data. Orientational effects between the probes must be taken into account for a correct interpretation of the fluorescence decay results, implying that interconversion among conformational substates involving the probes is slower than the energy transfer rate.

Depth of immersion of fluorescent chromophores in biomembranes studied by quenching with nitroxide radical

A novel method has been developed for measuring depth of immersion of a fluorescent chromophore in biological matrices, such as biomembranes. The method is based on dynamic quenching of chromophore fluorescence by a nitroxide probe freely diffused in solution. Theoretical considerations and experimental evidences relating to the method are discussed. The proposed method was applied to investigation of lecithin liposomes and membranes from Bacillus subtilis modified by the photochrome-fluorescent probe 4,4'-dimethylaminocyanostilbene.