Christopher S. Foote

DEFINITION OF TYPE I and TYPE II PHOTOSENSITIZED OXIDATION

Photosensitized oxidations are the basis for photodynamic action. There has come to be some confusion about the definition of Type I and Type I1 photosensitized oxidation reactions. It is generally true that experimentally-based definitions are superior to mechanistic ones because mechanisms change but experimental observations are subject to vertification. For this reason, I suggest that the definition given below should prove the most useful. This editorial briefly reviews the history and mechanism of photosensitized oxidations in order to provide the background for the definition. The first step of these reactions is absorption of light by a sensitizer (Sens) to produce an excited state (Sens*). In the presence of oxygen, two competing reactions of the excited sensitizer can occur, as originally noted by Schenck and Gollnick (Gollnick, 1968; Schenck, 1963; Schenck and Koch, 1960). These processes are called Type I and Type I1 reactions.

Active oxygen in chemistry

Preface. Overview of energetics and reactivity of oxygen. Autoxidation. Superoxide and hydroxyl radical chemistry in aqueous solution. Properties and reactions of singlet dioxygen. Reactions of hydroperoxides and peroxides. Catalytic oxidations with oxygen: an industrial perspective. Reactions of oxygen species in the atmosphere. Reactive oxygen species in natural waters. References. Index.

CHEMISTRY OF SINGLET OXYGEN—XXVI. PHOTOOXYGENATION OF PHENOLSy

Abstract— The aerobic dye‐sensitized photooxygenation of monohydric phenols proceeds by way of singlet oxygen under the conditions studied. Various phenols give different proportions of reaction with and quenching of singlet oxygen. Para‐substituted 2,6‐di‐t‐butylphenols show a linear correlation between the log of the total rate of singlet oxygen removal and their halfwave oxidation potentials; the same correlation is given for certain phenol methyl ethers. A Hammett plot using s̀+ gives ρ ‐ 1.72 ± 0.12, consistent with development of some charge in the quenching step. Reaction of photo‐chemically generated singlet oxygen with 2,4,6‐triphenylphenol gives 2,4,6‐triphenylphenoxy radical as an intermediate in singlet oxygen quenching, although no overall reaction occurs. Kinetic analysis indicates that the radical is derived exclusively from the interaction of 2,4,6‐triphenylphenol with singlet oxygen. A charge‐transfer mechanism for quenching of singlet oxygen by phenols is proposed.

Photophysical and photochemical properties of fullerenes

On irradiation with visible and UV light, fullerenes C60 and C70 give high yields of their triplet states. Fluorescence is very weak from both, and phosphorescence has been observed only from C70. The triplet states are also formed efficiently by energy transfer from triplet sensitizers with energies above 35 kcal/mol. Triplet fullerenes transfer energy efficiently to oxygen, giving singlet molecular oxygen, but they react and quench singlet oxygen very inefficiently, making them excellent photosensitizers. The fullerenes C60 and C70 are easily reduced but very difficult to oxidize. Electron transfer to triplet C60 and C70 occurs readily from electron donors to produce the radical anions. C60 can also be oxidized by excited high-potential photosensitizers to the radical cation. Some dihydrofullerenes also give high yields of the triplet state and singlet oxygen on irradiation. Photoreaction of C60 with electron-rich compounds appears to be an efficient route to difunctionalized adducts.

Activated oxygen species and oxidation of food constituents.

Activated oxygen species which may be important in initiating oxidative changes in foods include singlet oxygen, hydroxyl radical, ozone, superoxide anion (perhydroxyl radical at low pH), and hydrogen peroxide. Chemical and enzymic reactions known to occur in biological materials can generate singlet oxygen, hydroxyl radical, superoxide anion, and hydrogen peroxide. Ozone is primarily a product of photoreactions in polluted air. Reactions involving singlet oxygen, hydroxyl radical, and ozone with food constituents can ultimately yield peroxides which decompose to initiate oxidative chain reactions. Superoxide anion and hydrogen peroxide are relatively inert toward organic molecules but can decompose to produce the more reactive singlet oxygen and hydroxyl radical. Inhibition of reactions initiated by reactive oxygen species in foods should be very important in preserving the oxidative stability of foods. The generation, detection, measurement, reaction, and inhibition of reactions of active oxygen species are surveyed in this review.

Photosensitized oxygenation of alkyl-substituted furans☆

Abstract Photooxidation in methanol of 2-methyl-, 2,5-dimethyl-, dicyclohexano- and menthofuran, sensitized by Rose Bengal, produces crystalline 2-methoxy-5-hydroxyperoxy-2,5-dihydrofuran derivatives by addition of solvent to intermediate ozonide-like peroxides.

Kinetics and Yield of Singlet Oxygen Photosensitized by Hypericin in Organic and Biological Media

The spectroscopy and photophysics of the photosensitizer hypericin when in homogeneous solutions and when bound to liposomes were studied. Hypericin was found to partition efficiently into DMPC liposomes, with a binding constant of 58 (mg lipid/mL)−1. In these liposomes the singlet oxygen production quantum yield was 0.43 ± 0.09. To determine the deactivation constant of singlet oxygen in lipid bilayers for the first time, we calculated extrapolated values from its quenching by DMPC and lecithin in homogeneous solutions and obtained decay times of 36.4 and 12.2 μs, respectively. We also measured the quenching of singlet oxygen, sensitized by hypericin in DMPC liposomes, by NaN3, diphenyl isobenzofuran and H2,O: D2O mixtures and explained the results on the basis of singlet oxygen diffusing rapidly out of the lipid bilayer into the aqueous medium. The observed temperature effect on the lifetime of singlet oxygen of about 50% over a 15°C range in liposome suspension contrasts with a 3% change in a homogeneous solution in 1‐nonanol and is explained by the temperature effect on the diffusion out of the liposome. A strong pH effect was observed, indicating that the deprotonated species formed above about pH 10 is a much weaker photosensitizer of singlet oxygen than the native, protonated species.

CHEMISTRY OF SINGLET OXYGEN—XXV. PHOTOOXYGENATION OF METHIONINE

Abstract— Methionine (Met) photooxidation sensitized by rose bengal has been studied as a function of pH and other variables. At pH ≤ 6, the reaction is a simple one, 2 Met + O2→ 2 Methionine sulfoxide (MetO). At pH 6–10, another mechanism becomes important, leading to dehydromethionine; the structure of this compound was correctly assigned by Lavine (1945) as the heterocyclic N‐S compound 2. One mole of H2O2 is also produced in this process. Dehydromethionine hydrolyzes slowly to MetO. Above pH9, a process leading directly to MetO + H2O2 becomes important. The stoichiometry of the latter two processes are Met + O2+ H2O → MetO + H2O2; competition among these three processes accounts for the puzzling variations in O2 uptake. N‐Formylated derivatives of methionine undergo only the first and third processes. Substantial catalytic effects of buffers complicate the picture. All the reactions appear to involve singlet oxygen, since there is the predicted effect of D2O vs H2O on the rate of reaction, although the situation is complicated by apparent aggregation of Met above 5 mM.

Active oxygen in biochemistry

Biological reactions of dioxygen - an introduction - Raymond Y.N. Ho, Joel F. Liebman, Joan Selverstone Valentine Oxygen activation by flavins and pterins - Bruce A. Palfey, David P. Ballou, Vincent Massey Reactions of dioxgen and its reduced forms with heme proteins and model porphyrin complexes - Teddy G. Traylor, Patricia S. Traylor Dioxygen reactivity in copper proteins and complexes - Stephen Fox, Kenneth Karlin Oxygen activation at nonheme iron centers - Lawrence Que Jr. The mechanism of lipoxygenases - Mark J. Nelson, Steven P. Scitz The biological significance of oxygen-derivative species - Barry Halliwell Metal-complex-catalyzed cleavage of biopolymers - Claude F. Meares, Rosemary A. Marusak Exploration of selected pathways for metabolic oxidative ring opening of benzene based upon estimates of molecular energetics - Arthur Greenberg The role of oxidized lipids in cardiovascular disease - Judith A. Berliner, Andrew D. Watson.

Sequence-specific modification of guanosine in DNA by a C60-linked deoxyoligonucleotide: Evidence for a non-singlet oxygen mechanism

Abstract A C60-linked deoxyoligonucleotide (C60-DON-1) was prepared from bromoacetate 3. This C60-oligonucleotide conjugate was hybridized to a complementary single-stranded DNA. This system reacted with light and oxygen to damage only guanosines in the single-stranded region which are closest to C60. The damage did not involve 1O2 as the active species but rather resulted from a single electron-transfer mechanism between guanosine and 3C60, as shown by comparison experiments with eosin-attached DON-1 and by the use of singlet oxygen quenchers.