Singlet oxygen: there is indeed something new under the sun

Targeted oxidation of Torpedo californica acetylcholinesterase by singlet oxygen: identification of N-formylkynurenine tryptophan derivatives within the active-site gorge of its complex with the photosensitizer methylene blue

The principal role of AChE (acetylcholinesterase) is termination of impulse transmission at cholinergic synapses by rapid hydrolysis of the neurotransmitter acetylcholine. The active site of AChE is near the bottom of a long and narrow gorge lined with aromatic residues. It contains a CAS (catalytic 'anionic' subsite) and a second PAS (peripheral 'anionic' site), the gorge mouth, both of which bind acetylcholine via π-cation interactions, primarily with two conserved tryptophan residues. It was shown previously that generation of (1)O(2) by illumination of MB (Methylene Blue) causes irreversible inactivation of TcAChE (Torpedo californica AChE), and suggested that photo-oxidation of tryptophan residues might be responsible. In the present study, structural modification of the TcAChE tryptophan residues induced by MB-sensitized oxidation was investigated using anti-N-formylkynurenine antibodies and MS. From these analyses, we determined that N-formylkynurenine derivatives were specifically produced from Trp(84) and Trp(279), present at the CAS and PAS respectively. Peptides containing these two oxidized tryptophan residues were not detected when the competitive inhibitors, edrophonium and propidium (which should displace MB from the gorge) were present during illumination, in agreement with their efficient protection against the MB-induced photo-inactivation. Thus the bound MB elicited selective action of (1)O(2) on the tryptophan residues facing on to the water-filled active-site gorge. The findings of the present study thus demonstrate the localized action and high specificity of MB-sensitized photo-oxidation of TcAChE, as well as the value of this enzyme as a model system for studying the mechanism of action and specificity of photosensitizing agents.

Role of surface exposed tryptophan as substrate generators for the antibody catalyzed water oxidation pathway

The reaction of singlet oxygen with water to form hydrogen peroxide was catalyzed by antibodies and has been termed as the antibody catalyzed water oxidation pathway (ACWOP) (Nieva and Wentworth, Trends Biochem. Sci. 2004, 29, 274-278; Nieva et al. Immunol. Lett. 2006, 103, 33-38). While conserved and buried tryptophans in the antibody are thought to play a major role in this pathway, our studies with a monoclonal antibody, mAb-1 and its mutant W53A, clearly demonstrate the role of surface-exposed tryptophans in production of hydrogen peroxide, via the photo-oxidation pathway. Reactive oxygen species (ROS) such as singlet oxygen and superoxide were detected and site-specific tryptophan (Trp53) oxidation was observed under these conditions using RP-HPLC and mass spectrometry. The single mutant of the surface exposed Trp53 to Ala53 (W53A) results in a 50% reduction in hydrogen peroxide generated under these conditions, indicating that surface exposed tryptophans are highly efficient in transferring light energy to oxygen and contribute significantly to ROS generation. ACWOP potentially leads to the chemical instability of mAb-1 via the generation of ROS and is important to consider during clinical and pharmaceutical development of mAbs.

Structural and functional characterization of the interaction of the photosensitizing probe methylene blue with Torpedo californica acetylcholinesterase

The photosensitizer, methylene blue (MB), generates singlet oxygen that irreversibly inhibits Torpedo californica acetylcholinesterase (TcAChE). In the dark, it inhibits reversibly. Binding is accompanied by a bathochromic absorption shift, used to demonstrate displacement by other acetylcholinesterase inhibitors interacting with the catalytic "anionic" subsite (CAS), the peripheral "anionic" subsite (PAS), or bridging them. MB is a noncompetitive inhibitor of TcAChE, competing with reversible inhibitors directed at both "anionic" subsites, but a single site is involved in inhibition. MB also quenches TcAChE's intrinsic fluorescence. It binds to TcAChE covalently inhibited by a small organophosphate (OP), but not an OP containing a bulky pyrene. Differential scanning calorimetry shows an ~8° increase in the denaturation temperature of the MB/TcAChE complex relative to native TcAChE, and a less than twofold increase in cooperativity of the transition. The crystal structure reveals a single MB stacked against Trp279 in the PAS, oriented down the gorge toward the CAS; it is plausible that irreversible inhibition is associated with photooxidation of this residue and others within the active-site gorge. The kinetic and spectroscopic data showing that inhibitors binding at the CAS can impede binding of MB are reconciled by docking studies showing that the conformation adopted by Phe330, midway down the gorge, in the MB/TcAChE crystal structure, precludes simultaneous binding of a second MB at the CAS. Conversely, binding of ligands at the CAS dislodges MB from its preferred locus at the PAS. The data presented demonstrate that TcAChE is a valuable model for understanding the molecular basis of local photooxidative damage.

Separation and atropisomer isolation of ortho-halogenated tetraarylporphyrins by HPLC: Full characterization using 1D and 2D NMR

The separation and isolation of the four atropisomers of ortho-halogenated tetraarylporphyrins by semi-preparative HPLC is described. Full characterization and assignment of all 1 H and 13 C resonances of 5,10,15,20-tetrakis(2-fluoro or 2-chlorophenyl)porphyrins and 5,10,15,20-tetrakis(2-fluoro or chloro-5-N-ethylsulfamoylphenyl)porphyrins by 1D and 2D NMR techniques is reported. The outcome is an unequivocal evidence of the chlorosulfonation of meso-tetra(2-haloaryl)porphyrins on the 5′-position.

Fluorine end-capped optical fibers for photosensitizer release and singlet oxygen production

The usefulness of a fiber optic technique for generating singlet oxygen and releasing the pheophorbide photosensitizer has been increased by the fluorination of the porous Vycor glass tip. Singlet oxygen emerges through the fiber tip with 669-nm light and oxygen, releasing the sensitizer molecules upon a [2 + 2] addition of singlet oxygen with the ethene spacer and scission of a dioxetane intermediate. Switching from a nonfluorinated to a fluorinated glass tip led to a clear reduction of the adsorbtive affinity of the departing sensitizer with improved release into homogeneous toluene solution and bovine tissue, but no difference was found in water since the sensitizer was insoluble. High surface coverage of the nonafluorohexylsilane enhanced the cleavage efficiency by 15% at the ethene site. The fluorosilane groups also caused crowding and seemed to reduce access of (1)O(2) to the ethene site, which attenuated the total quenching rate constant k(T), although there was less wasted (1)O(2) (from surface physical quenching) at the fluorosilane-coated than the native SiOH silica. The observations support a quenching mechanism that the replacement of the SiOH groups for the fluorosilane C-H and C-F groups enhanced the (1)O(2) lifetime at the fiber tip interface due to less efficient electronic-to-vibronic energy transfer.

Small molecule signaling agents: the integrated chemistry and biochemistry of nitrogen oxides, oxides of carbon, dioxygen, hydrogen sulfide, and their derived species

Several small molecule species formally known primarily as toxic gases have, over the past 20 years, been shown to be endogenously generated signaling molecules. The biological signaling associated with the small molecules NO, CO, H₂S (and the nonendogenously generated O₂), and their derived species have become a topic of extreme interest. It has become increasingly clear that these small molecule signaling agents form an integrated signaling web that affects/regulates numerous physiological processes. The chemical interactions between these species and each other or biological targets is an important factor in their roles as signaling agents. Thus, a fundamental understanding of the chemistry of these molecules is essential to understanding their biological/physiological utility. This review focuses on this chemistry and attempts to establish the chemical basis for their signaling functions.

Selective oxidation with nanoporous silica supported sensitizers: an environment friendly process using air and visible light

Transparent and porous silica xerogels containing various grafted photosensitizers (PSs) such as anthraquinone derivatives, Neutral Red, Acridine Yellow and a laboratory-made dicyano aromatics (DBTP) were prepared. In most cases, the xerogels were shown to be mainly microporous by porosimetry. The PSs were characterized in the powdered monoliths (form, aggregation, concentration) by electronic spectroscopy which also proved to be a useful tool for monitoring the material evolution after irradiation. These nanoporous xerogels were used as microreactors for gas/solid solvent-free photo-oxygenation of dimethylsulfide (DMS) using visible light and air as the sole reactant. All these PSs containing monoliths were efficient for gas-solid DMS oxidation, leading to sulfoxide and sulfone in varying ratios. As these polar oxidation products remained strongly adsorbed on the silica matrix, the gaseous flow at the outlet of the reactor was totally free of sulfide and odorless. The best results in term of yield and initial rate of degradation of DMS were obtained with DBTP containing xerogels. Moreover, as these materials were reusable without loss of efficiency and sensitizer photobleaching after a washing regeneration step, the concept of recyclable sensitizing materials was approved, opening the way to green process.

Atmospheric photosensitized heterogeneous and multiphase reactions: from outdoors to indoors

This proposal involves direct photolysis processes occurring in the troposphere incorporating photochemical excitation and intermolecular energy transfer. The study of such processes could provide a better understanding of ·OH radical formation pathways in the atmosphere and in consequence, of a more accurate prediction of the oxidative capacity of the atmosphere. Compounds that readily absorb in the tropospheric actinic window (ionic organic complexes, PAHs, aromatic carbonyl compounds) acting as potential photosensitizers of atmospheric relevant processes are explored. The impact of hotosensitation on relevant systems which could act as powerful atmospheric reactors,that is, interface ocean-atmosphere, urban and forest surfaces and indoor air environments is also discussed.

Small change in structure leads to large difference in protein photocleavage: two porphyrins bearing rhodanine-based pendants

Two 5,10,15,20-tetraphenylporphyrins with one phenyl group anchored to a rhodanine-terminated side chain, RhD-TPP and RhDCOOH-TPP, were designed and synthesized, and their protein photocleavage activities were investigated using bovine serum albumin (BSA) as a model protein. Both porphyrins exhibit similar absorption spectra, fluorescence spectra, fluorescence quantum yields, and singlet oxygen ((1)O(2)) quantum yields in organic solvents due to their structure similarity. They also show similar binding affinities and binding sites toward BSA. However, RhD-TPP is nearly inactive in protein photocleavage while RhDCOOH-TPP can lead to distinct photocleavage of BSA under the same experimental conditions. Such a difference may be attributed to the different binding modes of the two porphyrin derivatives toward BSA, though the apparent binding affinities and the binding sites are similar, and consequently a great difference in the (1)O(2) quantum yields of the two porphyrins bound on BSA. The presence of the COOH group in RhDCOOH is proposed to play an important role, leading to less hydrophobic character and additional interactions towards BSA.

Irradiation- and sensitizer-dependent changes in the lifetime of intracellular singlet oxygen produced in a photosensitized process

Singlet oxygen, O(2)(a(1)Δ(g)), was produced upon pulsed-laser irradiation of an intracellular photosensitizer and detected by its 1275 nm O(2)(a(1)Δ(g)) → O(2)(X(3)Σ(g)(-)) phosphorescence in time-resolved experiments using (1) individual mammalian cells on the stage of a microscope and (2) suspensions of mammalian cells in a 1 cm cuvette. Data were recorded using hydrophilic and, independently, hydrophobic sensitizers. The microscope-based single cell results are consistent with a model in which the behavior of singlet oxygen reflects the environment in which it is produced; nevertheless, the data also indicate that a significant fraction of a given singlet oxygen population readily crosses barriers between phase-separated intracellular domains. The singlet oxygen phosphorescence signals reflect the effects of singlet-oxygen-mediated damage on cell components which, at the limit, mean that data were collected from dead cells and, in some cases, reflect contributions from both intracellular and extracellular populations of singlet oxygen. Despite the irradiation-induced changes in the environment to which singlet oxygen is exposed, the "inherent" intracellular lifetime of singlet oxygen does not appear to change appreciably as the cell progresses toward death. The results obtained from cell suspensions reflect key features that differentiate cell ensemble from single cell experiments (e.g., the ensemble experiment is more susceptible to the effects of sensitizer that has leaked out of the cell). Overall, the data clearly indicate that measuring the intracellular lifetime of singlet oxygen in a O(2)(a(1)Δ(g)) → O(2)(X(3)Σ(g)(-)) phosphorescence experiment is a challenging endeavor that involves working with a dynamic system that is perturbed during the measurement. The most important aspect of this study is that it establishes a useful framework through which future singlet oxygen data from cells can be interpreted.

Two new "protected" oxyphors for biological oximetry: properties and application in tumor imaging

We report the synthesis, calibration, and examples of application of two new phosphorescent probes, Oxyphor R4 and Oxyphor G4, optimized specifically for in vivo oxygen imaging by phosphorescence quenching. These "protected" dendritic probes can operate in either albumin-rich (blood plasma) or albumin-free (interstitial space) environments at all physiological oxygen concentrations, from normoxic to deep hypoxic conditions. Oxyphors R4 and G4 are derived from phosphorescent Pd-meso-tetra-(3,5-dicarboxyphenyl)-porphyrin (PdP) or Pd-meso-tetra-(3,5-dicarboxyphenyl)-tetrabenzoporphyrin (PdTBP), respectively, and possess features common for protected dendritic probes, i.e., hydrophobic dendritic encapsulation of phosphorescent metalloporphyrins and hydrophilic PEGylated periphery. The new Oxyphors are highly soluble in aqueous environments and do not permeate biological membranes. The probes were calibrated under physiological conditions (pH 6.4-7.8) and temperatures (22-38 °C), showing high stability, reproducibility of signals, and lack of interactions with biological solutes. Oxyphor G4 was used to dynamically image intravascular and interstitial oxygenation in murine tumors in vivo. The physiological relevance of the measurements was demonstrated by dynamically recording changes in tissue oxygenation during application of anesthesia (isofluorane). These experiments revealed that changes in isofluorane concentration significantly affect tissue oxygenation.

Single cell responses to spatially controlled photosensitized production of extracellular singlet oxygen

The response of individual HeLa cells to extracellularly produced singlet oxygen was examined. The spatial domain of singlet oxygen production was controlled using the combination of a membrane-impermeable Pd porphyrin-dendrimer, which served as a photosensitizer, and a focused laser, which served to localize the sensitized production of singlet oxygen. Cells in close proximity to the domain of singlet oxygen production showed morphological changes commonly associated with necrotic cell death. The elapsed postirradiation "waiting period" before necrosis became apparent depended on: (1) the distance between the cell membrane and the domain irradiated, (2) the incident laser fluence and, as such, the initial concentration of singlet oxygen produced and (3) the lifetime of singlet oxygen. The data imply that singlet oxygen plays a key role in this process of light-induced cell death. The approach of using extracellularly generated singlet oxygen to induce cell death can provide a solution to a problem that often limits mechanistic studies of intracellularly photosensitized cell death: it can be difficult to quantify the effective light dose, and hence singlet oxygen concentration, when using an intracellular photosensitizer.

Photophysical and photochemical properties of the pharmaceutical compound salbutamol in aqueous solutions

Salbutamol is a potent β(2)-adrenergic receptor agonist widely used in the treatment of bronchial asthma and chronic obstructive pulmonary disease. An increasing number of studies have detected salbutamol in natural water systems worldwide. Studies have shown that sunlight degrades salbutamol resulting in the formation of products; some showing higher toxicity to bacteria Vibrio fischeri than the parent compound. In this contribution, steady-state absorption and emission techniques, high-performance liquid chromatography, and transient absorption spectroscopy are used to investigate the photochemistry of salbutamol in aqueous buffer solutions at controlled pH values. Ground- and excited-state calculations that include solvent effects are performed to guide the interpretation of the experimental results. Salbutamol is sensitive to UVB light absorption in the pH range from 3 to 12, forming products that absorb light at longer wavelengths than the parent compound. Quantum yields of degradation reveal that the deprotonated species is 10-fold more photo-active than the protonated species. In line with this result, the fluorescence quantum yield of the protonated species is more than an order of magnitude higher than that of the deprotonated species. Transient absorption spectroscopy shows that population of the triplet state occurs with a rate constant of 7.1×10(8)s(-1) in the protonated species, while a rate constant of 1.7×10(10)s(-1) is measured for the deprotonated species. While degradation of the deprotonated species is not affected by the presence of molecular oxygen, a twofold increase in the photodegradation yield of the protonated species in air-saturated conditions is observed.

Evaluation of phototoxicity of dendritic porphyrin-based phosphorescent oxygen probes: an in vitro study

Biological oxygen measurements by phosphorescence quenching make use of exogenous phosphorescent probes, which are introduced directly into the medium of interest (e.g. blood or interstitial fluid) where they serve as molecular sensors for oxygen. The byproduct of the quenching reaction is singlet oxygen, a highly reactive species capable of damaging biological tissue. Consequently, potential probe phototoxicity is a concern for biological applications. Herein, we compared the ability of polyethyleneglycol (PEG)-coated Pd tetrabenzoporphyrin (PdTBP)-based dendritic nanoprobes of three successive generations to sensitize singlet oxygen. It was found that the size of the dendrimer has practically no effect on the singlet oxygen sensitization efficiency in spite of the strong attenuation of the triplet quenching rate with an increase in the dendrimer generation. This unexpected result is due to the fact that the lifetime of the PdTBP triplet state in the absence of oxygen increases with dendritic generation, thus compensating for the concomitant decrease in the rate of quenching. Nevertheless, in spite of their ability to sensitize singlet oxygen, the phosphorescent probes were found to be non-phototoxic when compared with the commonly used photodynamic drug Photofrin in a standard cell-survival assay. The lack of phototoxicity is presumably due to the inability of PEGylated probes to associate with cell surfaces and/or penetrate cellular membranes. In contrast, conventional photosensitizers bind to cell components and act by generating singlet oxygen inside or in the immediate vicinity of cellular organelles. Therefore, PEGylated dendritic probes are safe to use for tissue oxygen measurements as long as the light doses are less than or equal to those commonly employed in photodynamic therapy.