Dana M. Wagnerová

Reduction of dioxygen to superoxide photosensitized by anthraquinone-2-sulphonate

Abstract When irradiated by UV light, solutions of anthraquinone-2-sulphonate in dimethyl sulphoxide containing dissolved oxygen are a source of a considerable amount of the superoxide anion O 2 − . In aqueous solutions at least 70% of OH* radicals observed after irradiation originate from primarily formed O 2 − . The superoxide formation is due to photo-initiated electron transfer from the sensitizer to dioxygen. The activity of various 9,10-anthraquinone derivatives as photosensitizers of superoxide production is correlated with the nature of the substituents.

Photophysical Properties and Photoinduced Electron Transfer Within Host–Guest Complexes of 5,10,15,20‐Tetrakis(4‐N‐methylpyridyl)porphyrin with Water‐soluble Calixarenes and Cyclodextrins¶

Abstract We report the formation of host–guest complexes between water-soluble calix[n]arene-p-tetrasulfonates (n = 4, 6, 8) or 2-hydroxypropyl-cyclodextrins (α-, β-, γ-) and the tetratosylate salt of 5,10,15,20-tetrakis(4-N-methylpyridyl)porphyrin (TMPyP). The binding constants ranging between 102 and 105 M−1 were calculated from the absorption and fluorescence changes. Calix[4]arene-p-tetrasulfonate has a high binding affinity and forms with TMPyP a 1:1 complex, whereas other calixarenes bind two molecules of TMPyP. Electrostatic attraction is the dominating binding mode. Binding to calixarenes leads to a considerable decrease of the quantum yields of the triplet and excited singlet states and to shortening of the singlet and triplet lifetimes of TMPyP. The quenching mechanism is attributed to electron transfer between calixarene phenolates and excited TMPyP. Photoinduced electron transfer within a novel supramolecular complex calixarene/TMPyP (electron donor)/methyl viologen (electron acceptor) has been proven by absorption and fluorescence measurements. Electrostatic attraction between the cationic donor and cationic acceptor, on the one hand, and the anionic host, on the other, overcomes the electrostatic repulsion forces. In contrast, the interaction of cyclodextrin with TMPyP is hydrophobic in nature and only slightly influences the photophysical properties of TMPyP. The different behavior of TMPyP bound to either of the hosts has been assigned to the specific effects of the dominant binding modes, viz. the electrostatic attraction for calixarenes and the hydrophobic interactions for inclusion complexes with cyclodextrins.

The role of excited states in the photosensitized oxidation of substrates with dioxygen

Laser flash photolysis was employed to study the kinetics of the interaction of sulphonated aluminium and zinc phthalocyanines in excited states with substrates and dioxygen. The substrates (phenol, 4-chlorophenol, salicylic acid etc.) quench 1π-π* singlet states of the sensitizer whereas the 3π-π*, triplet states are quenched solely by dioxygen. Electron transfer occurs inside the charge-transfer complex in the singlet state and the liberated ion radicals react further with dioxygen. The reaction products were identified by high performance liquid chromatography after continuous irradiation of the reaction mixture.

Influence of protonation on the reactions of triplet-state sulfonated chloro-aluminium(III) phthalocyanine with dioxygen

The kinetics of the reactions of sulfonated chloro-aluminium(III) phthalocyanine (AISPC) in the excited state have been studied by laser flash photolysis. Two different protonated forms of this substance were identified in the triplet state 3ππ*, AISPC and AISPCH+, differing in their lifetimes, τ≈ 290 and 90 µs, respectively, and rate of quenching by dioxygen [ko2f=(1.6 ± 0.2)× 109 dm3 mol–1 s–1, ko2s=(6.6 ± 0.7)× 107 dm3 mol–1 s–1, respectively]. It follows from the reaction kinetics that the establishment of equilibrium between the two forms AISPC and AISPCH+ in the 3ππ* state is insufficiently mobile compared with the rate of quenching by dioxygen.

Quenching of the Triplet State of Metallophthalocyanines by Dioxygen in the Presence of Bovine Serum Albumin

Sulfonated chloro-aluminium phthalocyanine (A1SPC), excited by laser flash to the 3 * state, is quenched by dioxygen in a reaction with the rate constant k, = 1.7Xl09dm3 mol 1 s-1. In the presence of bovine serum albumin (BSA), the kinetics become biphasic: The faster process is identical with that in the absence of albumin, the slower one (k2 = 1.9Xl08dm3 mol1 s-1) corresponds to quenching of the excited phthalocyanine-BSA adduct by dioxygen. In the absence of oxygen, BSA increases the lifetime of the 3 * state from 440 ps to 1160 //s. The calculated A1SPC : BSA ratio in the adduct is 2:1. Analogous behaviour was observed with sulfonated zinc phthalocyanine and sulfonated free ligand.

Demethylation of tertiary amines by a reconstituted cytochrome P-450 enzyme system: kinetics of oxygen consumption and hydrogen peroxide formation.

Initial reaction rates of oxygen consumption and hydrogen peroxide formation in a cytochrome P-450 catalyzed reaction are practically independent of the nature of tertiary amines that were used as substrates. From the kinetic studies and the substrate conversion results that the amount of water formed in a side reaction is determined by the substrate specificity. Both hydrogen peroxide and water formation lower the efficiency of the monooxygenatic activity of cytochrome P-450.