Direct spectroscopic observation of singlet oxygen emission at 1268 nm excited by sensitizing dyes of biological interest in liquid solution

Optical detection of singlet oxygen from single cells

The lowest excited electronic state of molecular oxygen, singlet molecular oxygen, O(2)(a (1)Delta(g)), is a reactive species involved in many chemical and biological processes. To better understand the roles played by singlet oxygen in biological systems, particularly at the sub-cellular level, optical tools have been developed to create and directly detect this transient state in time- and spatially-resolved experiments from single cells. Data obtained indicate that, contrary to common perception, this reactive species can be quite long-lived in a cell and, as such, can diffuse over appreciable distances including across the cell membrane into the extracellular environment. On one hand, these results demonstrate that the behavior of singlet oxygen in an intact cell can be significantly different from that inferred from model bulk studies. More generally, these results provide a new perspective for mechanistic studies of intra- and inter-cellular signaling and events that ultimately lead to photo-induced cell death.

Chemically Induced Dynamic Electron Polarization Generated through the Interaction between Singlet Molecular Oxygen and Nitroxide Radicals

An absorptive chemically induced dynamic electron polarization (CIDEP) was generated by the quenching of singlet oxygen by nitroxide radicals (TEMPO derivatives). The spin polarization decay time of the nitroxide (measured by time-resolved EPR) correlates with the lifetime of singlet oxygen (measured by singlet oxygen phosphorescence spectroscopy). In addition, a deuterium isotope effect on the spin polarization decay time was observed, a signature of singlet oxygen involvement. With use of isotope labeled nitroxides (15N, 14N), the relative spin polarization efficiencies of TEMPO, 4-oxo-TEMPO, and 4-hydroxy-TEMPO by singlet oxygen were determined. The relative spin polarization efficiencies (per quenching event) decrease in the order 4-hydroxy-TEMPO > TEMPO > 4-oxo-TEMPO, whereas an opposite trend was observed for the total quenching rate constants of singlet oxygen by the nitroxides where the order is 4-hydroxy-TEMPO < TEMPO < 4-oxo-TEMPO.

Direct measurement of the singlet oxygen lifetime in zeolites by near-IR phosphorescence

Time-resolved near-IR phosphorescence spectroscopy was employed to determine the lifetime of singlet oxygen in Y-zeolites and porous silica and it was found to depend strongly on the alumina content of the zeolite.

Oxygen, oxysterols, ouabain, and ozone: a cautionary tale

The recent account of the oxidation of tissue cholesterol by ozone created in human arterial plaques by the oxidation of water by electronically excited (singlet) dioxygen depends on the identification of the oxysterols formed and on the presumption that they are formed uniquely by ozone action. The chief oxysterols found, 3beta-hydroxy-5-oxo-5,6-secocholestan-6-al and 3beta,5-dihydroxy-5beta-B-norcholestane-6beta-carboxaldehyde, were identified as their 2,4-dinitrophenylhydrazones by chromatographic properties and a single mass spectral ion m/z 597 interpreted as [M-H](-). Conventional identification procedures for oxysterols were not conducted. Accordingly, absent other evidence, error may exist; such errors are known in the literature. Moreover, the assertion that ozone be the only oxidant that could form the 5,6-secosterol aldehyde from cholesterol is unproven. Other equally novel unproven processes can be posed. The account of biological ozone mimics prior 30-year-old reports of singlet oxygen itself in biological systems. Lest a similar history develop for biological ozone three topics of steroid oxidation are here reviewed to aid in understanding the current matter. Caution in evaluating the account of biological ozone is warranted.

Stereochemical Features of the Physical and Chemical Interactions of Singlet Oxygen with Enecarbamates

Oxazolidinone-substituted enecarbamates represent a mechanistically rich system for the study of stereoelectronic, steric, and conformational effects on stereoselectivity and mode selectivity in (1)O(2) [2 + 2] and ene reactions. Photooxygenation of these enecarbamates with (1)O(2) leads to diastereomerically pure dioxetanes that decompose to yield an oxazolidinone carbaldehyde and one of the two enantiomers of methyldesoxybenzoin in enantiomeric excess. Stereoselectivity originates at the allylic stereocenter, a result supported by quenching studies, computational analysis, and deuterium solvent isotope effects. [reaction: see text]

Singlet molecular oxygen generated from lipid hydroperoxides by the russell mechanism: studies using 18(O)-labeled linoleic acid hydroperoxide and monomol light emission measurements

The decomposition of lipid hydroperoxides into peroxyl radicals is a potential source of singlet oxygen ((1)O(2)) in biological systems. We report herein on evidence of the generation of (1)O(2) from lipid hydroperoxides involving a cyclic mechanism from a linear tetraoxide intermediate proposed by Russell. Using (18)O-labeled linoleic acid hydroperoxide (LA(18)O(18)OH) in the presence of Ce(4+) or Fe(2+), we observed the formation of (18)O-labeled (1)O(2) ((18)[(1)O(2)]) by chemical trapping of (1)O(2) with 9,10-diphenylanthracene (DPA) and detected the corresponding (18)O-labeled DPA endoperoxide (DPA(18)O(18)O) by high-performance liquid chromatography coupled to tandem mass spectrometry. Spectroscopic evidence for the generation of (1)O(2) was obtained by measuring (i) the dimol light emission in the red spectral region (lambda > 570 nm); (ii) the monomol light emission in the near-infrared (IR) region (lambda = 1270 nm); and (iii) the quenching effect of sodium azide. Moreover, the presence of (1)O(2) was unequivocally demonstrated by the direct spectral characterization of the near-IR light emission. For the sake of comparison, (1)O(2) deriving from the H(2)O(2)/OCl(-) and H(2)O(2)/MoO(4)(2)(-) systems or from the thermolysis of the endoperoxide of 1,4-dimethylnaphthalene was also monitored. These chemical trapping and photoemission properties clearly demonstrate that the decomposition of LA(18)O(18)OH generates (18)[(1)O(2)], consistent with the Russell mechanism and pointing to the involvement of (1)O(2) in lipid hydroperoxide mediated cytotoxicity.

Formation of active oxygen species and lipid peroxidation induced by hypochlorite

Hypochlorite or its acid, hypochlorous acid, may exert both beneficial and toxic effects in vivo. In order to understand the role and action of hypochlorite, the formation of active oxygen species and its kinetics were studied in the reactions of hypochlorite with peroxides and amino acids. It was found that tert-butyl hydroperoxide and methyl linoleate hydroperoxide reacted with hypochlorite to give peroxyl and/or alkoxyl radicals with little formation of singlet oxygen in contrast to hydrogen peroxide, which gave singlet oxygen exclusively. Amino acids and ascorbate reacted with hypochlorite much faster than peroxides. Free radical-mediated lipid peroxidation of micelles and membranes in aqueous suspensions was induced by hypochlorite, the chain initiation being the decomposition of hydroperoxides by hypochlorite. It was suppressed efficiently by ebselen which reduced hydroperoxides and by alpha-tocopherol, which broke chain propagation, but less effectively by hydrophilic antioxidants present in the aqueous phase. Cysteine suppressed the oxidation, but it was poorer antioxidant than alpha-tocopherol. Ascorbate also exerted moderate antioxidant capacity, but it acted as a synergist with alpha-tocopherol. Taken together, it was suggested that the primary target of hypochlorite must be sulfhydryl and amino groups in proteins and that the lipid peroxidation may proceed as the secondary reaction, which is induced by radicals generated from sulfenyl chlorides and chloramines.

Reactive oxygen species contribute to oxygen-related lung injury after acid aspiration

Hyperoxia increases pulmonary damage after acid aspiration. We hypothesize that free radicals play a role in acute lung injury. To examine this hypothesis, we injured rats by intratracheal instillation of acidic isotonic sodium chloride solution (NS) (pH 1.25); NS + gastric particles (particle pH 5.3); or acid + particles (pH 1.25). Animals were exposed to 98% oxygen or air for 5 h. Superoxide (HO2) generation was measured in either an aliquot of white blood cells (WBCs) recovered from bronchoalveolar lavage (BAL) or from blood. Lungs were analyzed for thiobarbituric acid-reactive substances (TBARS) and carbonylated proteins. The antioxidant capacity was measured using a 2-2'-azo-bis-amidinopropane hydrochloride neutralizing assay. Generation of HO2 by WBCs in peripheral blood was greater in animals exposed to 98% O2 (89.8 +/- 12.5 U. min-1.10(5) neutrophils) compared with air exposure (37.5 +/- 9.2 U.min-1.10(5) neutrophils) after combined injury (P < 0.05). Similarly, HO2 generation by WBCs retrieved from BAL was higher in oxygen-exposed rats (987.74 +/- 128 U.min-1.10(5) WBC) compared with air-exposed animals after an identical injury (348 +/- 9.2 U. min-1.10(5) WBC) (P < 0.05). TBARS and carbonylated protein levels in the lungs of oxygen-exposed animals (587.9 +/- 58.6 and 55.8 +/- 3.1 pmol/mg of protein, respectively) were higher than those in air-exposed rats after combined injury (342.8 +/- 15.1 and 28.6 +/- 4.6 pmol/mg of protein, respectively) and compared with air-exposed uninjured rats (340.6 +/- 9.8 and 18.3 +/- 2.8 pmol/mg of protein, respectively; P < 0.01). Antioxidant capacity decreased in acid and combined injury groups (2.41 +/- 0.13 min and 1.94 +/- 0.15 min, respectively) compared with the uninjured group after 5 h of exposure to 98% oxygen (4.85 +/- 0.19 min; P < 0.01). We demonstrated evidence of increased oxidant activity on lipids and proteins in injured lungs after oxygen exposure. The decrease in antioxidant capacity after low pH aspiration with exposure to hyperoxia may contribute to this increase.

IMPLICATIONS

Oxygen administration results in a lung pathology known as oxygen toxicity. This effect is usually not significant if the duration of exposure is limited to < 24 h. In the presence of acute inflammatory lung injury, exposure to hyperoxia results in lung damage in a shorter time. We demonstrate that sufficiently decreased lung antioxidant reserve capacity may be accountable for this early toxicity.

Phototoxicity of some bromine-substituted rhodamine dyes: synthesis, photophysical properties and application as photosensitizers

The synthesis of some bromine-substituted rhodamine derivatives viz., 4,5-dibromorhodamine methyl ester (dye 2) and 4,5-dibromorhodamine n-butyl ester (dye 3) are reported. These dyes were synthesized to promote a more efficient cancer cell photosensitizer for potential use in in vitro bone marrow purging in preparation for autologous bone marrow transplantation. Spectroscopic and photophysical characterization of these dyes together with rhodamine 123 (dye 1) are reported in water, methanol, ethanol and also in a microheterogeneous system, sodium dodecyl sulfate. The possible mechanism of photosensitization is characterized in terms of singlet oxygen efficiency of these dyes. Singlet oxygen quantum yields for bromine-substituted dyes are in the range of 0.3-0.5 depending on the solvent. For dye 1 no singlet oxygen production is found. The photodynamic actions of these dyes in different cell lines are tested. It was found that dye 2 and dye 3 are efficient photosensitizers and mediate eradication of K562, EM2, myeloid cell lines (CML) and the SMF-AI rhabdomyosarcoma line.

Singlet molecular oxygen evolution upon simple acidification of aqueous hypochlorite: application to studies on the deleterious health effects of chlorinated drinking water

A study of the pH profile of the decomposition of aqueous hypochlorite has revealed the evolution (onset at pH 8) of single (1 delta g) molecular oxygen (singlet spin state dioxygen) detected spectroscopically (1268 nm), prior to the appearance of chlorine (onset at pH 5.5). The possible mechanism of the singlet state dioxygen evolution is presented, and the origin of its chloride ion dependence is discussed, especially in reference to chloride ion dependence of singlet molecular oxygen evolution in biological systems. Recent epidemiological analyses of the correlation of human cancer with chlorinated water supplies focus attention on the singlet oxygen mechanisms of DNA lesion formation.

Current perspectives of singlet oxygen detection in biological environments

There is widespread acceptance that singlet oxygen is a key intermediate on one of the pathways leading to the phenomenon of photodynamic action. However, the identification of this moiety within a particular biological system and the determination of a direct link between its presence and a particular photodynamic effect is a goal which photobiologists have hitherto failed to achieve. The aim of this review is to assess the problems associated with such a goal and methods whereby they might be overcome. Initially the general photochemical and environmental factors which govern the ability of a photosensitizer to promote photodynamic action via the intermediacy of singlet oxygen are introduced and the fundamental parameters defining the formation, decay and reactivity of this species summarized. The experimental requirements for relating a particular photodynamic effect to singlet oxygen intermediacy are then analysed and the intrinsic properties of singlet oxygen which will influence this goal are discussed. Having concluded that the singlet oxygen detection method of choice for this purpose is that in which the IR emission at 1269 nm of this molecule is monitored, the advantages and disadvantages of pulsed and continuous wave photoexcitation of cellular systems are analysed. It becomes evident that, no matter what the future improvements in instrumentation are likely to be, the inherent natures of singlet oxygen and the biological system lead to a kinetic situation which will preclude a successful time-resolved solution to this problem. In contrast, experimentation with continuous wave systems holds out significant hope for the future. In particular, the use of phase modulation techniques to overcome background emission problems, the enhancement of photosensitizer optical densities as a consequence of higher extinction coefficients and/or improved photosensitizer delivery systems and the use of high power lasers and/or improved light delivery systems can, at least in principle, lead to the solution of the problem addressed herein.

Measurement of rate constants for quenching singlet oxygen with a Cypridina luciferin analog (2-methyl-6-[p-methoxyphenyl]-3,7-dihydroimidazo [1,2-a]pyrazin-3-one) and sodium azide

The rate constants for [1O2] [MCLA] and [1O2] [NaN3] were measured by quenching the near-infrared emission (1 delta g----3 sigma g) in steady state with MCLA and NaN3, respectively. 1O2 was constantly generated by energy transfer to O2 from Ar laser-excited Rose Bengal. The Stern-Volmer plots yielded the second-order rate constants of 2.94 x 10(9) M-1 S-1 and 3.83 x 10(8) M-1 S-1 for quenching 1O2 with MCLA and NaN3 in water at pH 5.4, respectively. The 1O2 + MCLA reaction emitted light with maximum at 465 nm at pD 4.5 identical to the O(2-) + MCLA reaction.

Energy transfer, charge transfer, and proton transfer in molecular composite systems

Models for the fundamental mechanisms of excitation energy transfer, including cases involving singlet oxygen states, twisting-intramolecular-charge-transfer (TICT) states, and intramolecular proton transfer, are described in terms of elementary concepts and energy diagrams. Three limiting cases of energy transfer are distinguished, Davydov free excitons (Simpson and Peterson strong coupling) and localized excitons (weak coupling), and the Förster mechanism of vibrational-relaxation energy transfer. The prominent rôle of the singlet molecular oxygen states is described, together with the rôle of simultaneous transitions for molecular oxygen pairs. The origin of sudden polarization via the TICT-state potential is discussed and the generality of this phenomenon emphasized. The intramolecular proton-transfer phenomenon is outlined, and its role in molecular excitation transient phenomena is described. The complex interaction of all of these excitation mechanisms in determining photochemical and radiation chemical pathways is suggested.

Experimental tests of the feasibility of singlet oxygen luminescence monitoring in vivo during photodynamic therapy

Singlet oxygen (1O2) is thought to be the cytotoxic agent in photodynamic therapy (PDT) with current photosensitizers. Direct monitoring of 1O2 concentration in vivo would be a valuable tool in studying biological response. Attempts were made to measure 1O2 IR luminescence during PDT of cell suspensions and two murine tumour models using the photosensitizers Photofrin II and aluminium chlorosulphonated phthalocyanine. Instrumentation was virtually identical to that devised by Parker in the one positive report of in vivo luminescence detection in the literature. Despite the fact that our treatments caused cell killing and tissue necrosis, we were unable to observe 1O2 emission under any conditions. We attribute this negative result to a reduction in 1O2 lifetime in the cellular environment. Quantitative calibration of our system allowed us to estimate that the singlet oxygen lifetime in tissue is less than 0.5 microsecond. Some technical improvements are suggested which would improve detector performance and perhaps make such measurements feasible.

The role of compound III in reversible and irreversible inactivation of lactoperoxidase

In the presence of iodide (I-, 10 mM) and hydrogen peroxide in a large excess (H2O2, 0.1-10 mM) catalytic amounts of lactoperoxidase (2 nM) are very rapidly irreversibly inactivated without forming compound III (cpd III). In contrast, in the absence of I- cpd III is formed and inactivation proceeds very slowly. Increasing the enzyme concentration up to the micromolar range significantly accelerates the rate of inactivation. The present data reveal that irreversible inactivation of the enzyme involves cleavage of the prosthetic group and liberation of heme iron. The rate of enzyme destruction is well correlated with the production of molecular oxygen (O2), which originates from the oxidation of excess H2O2. Since H2O2 and O2 per se do not affect the heme moiety of the peroxidase, we suggest that the damaging species may be a primary intermediate of the H2O2 oxidation, such as oxygen in its excited singlet state (1 delta gO2), superoxide radicals (O-.2), or consequently formed hydroxyl radicals (OH.).

Phototoxicity of the tetracyclines: photosensitized emission of singlet delta dioxygen

The spectroscopic observation of 1268-nm emission of singlet oxygen photosensitized by tetracyclines in oxygenated solutions at room temperature is reported. In the series demeclocycline, tetracycline, and minocycline, the efficiency of singlet oxygen generation is found to parallel the clinical observation of relative frequency of phototoxicity of these antibiotics, suggesting singlet oxygen generation as the origin of their phototoxicity.

Digital imaging fluorescence microscopy: spatial heterogeneity of photobleaching rate constants in individual cells

Photobleaching and related photochemical processes are recognized experimental barriers to quantification of fluorescence by microscopy. We have measured the kinetics of photobleaching of fluorophores in living and fixed cells and in microemulsions, and have demonstrated the spatial variability of these processes within individual cells. An inverted fluorescence microscope and a high-sensitivity camera, together with high-speed data acquisition by a computer-controlled image processor, have been used to control precisely exposure time to excitation light and to record images. To improve the signal-to-noise ratio, 32 digital images were integrated. After correction for spatial variations in camera sensitivity and background fluorescence, the images of the relative fluorescence intensities for 0.065 micron2 areas in the object plane were obtained. To evaluate photobleaching objectively, an algorithm was developed to fit a three-parameter exponential equation to 20 images recorded from the same microscope field as a function of illumination time. The results of this analysis demonstrated that the photobleaching process followed first-order reaction kinetics with rate constants that were spatially heterogeneous and varied, within the same cell, between 2- and 65-fold, depending on the fluorophore. The photobleaching rate constants increased proportionally with increasing excitation intensity and, for benzo(a)pyrene, were independent of probe concentration over three orders of magnitude (1.25 microM to 1.25 mM). The propensity to photobleach was different with each fluorophore. Under the cellular conditions used in these studies, the average rates of photobleaching decreased in this order: N-(7-nitrobenz-2-oxa-1,3-diazole)-23,24-dinor-5-cholen-22-amine-3 beta-ol greater than acridine orange greater than rhodamine-123 greater than benzo(a)pyrene greater than fluorescein greater than tetramethylrhodamine greater than 1,1'dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine. The photobleaching appears to be an oxidation reaction, in that the addition of saturated solutions of Na2S2O5 to mineral oil microemulsions eliminated photobleaching of N-(7-nitrobenz-2-oxa-1,3-diazole)-23,24-dinor-5-cholen-22-amine-3 beta-ol or benzo(a)pyrene. We identified experimental conditions to observe, without detectable photobleaching, fluorophores in living cells, which can not be studied anaerobically. Useful images were obtained when excitation light was reduced to eliminate photobleaching, as determined from zero-time images calculated from the exponential fit routine.(ABSTRACT TRUNCATED AT 400 WORDS)

Fluorescence photobleaching recovery in solutions of labeled actin

We have demonstrated that the technique of fluorescence photobleaching recovery (FPR) can be used to examine the state of a single component in complex self-assembling macromolecular systems. Polymerization of actin, initiated by addition of salt or Mg+2 to a low-ionic-strength solution of G-actin, has been observed by sequential measurement of FPR with the aid of fluorescein-labeled actin. Solutions of actin which had been labeled using 5-iodoacetamido fluorescein (5-IAF) showed anomalous recovery of fluorescence above the initial value, which indicates a photoinduced increase in local polymerization. No such anomaly was observed with actin that had been labeled with fluorescein isothiocyanate (FITC). The FPR data are directly interpretable in terms of the fraction of labeled protein that is immobilized in the supramolecular assembly and in terms of the average diffusion coefficient of the mobile fraction. Our data are consistent with the "treadmill" model of actin polymerization, in that they show that actin is present under polymerizing conditions either as a high polymer or as monomer or low oligomer. We believe that the FPR technique can be applied to the study of many types of reconstituted motile or cytoskeletal systems in vitro or in vivo.