Interaction between hypocrellin A and some biological substrates with emphasis on an electron transfer mechanism

Hypocrellin A (HA), a hydroxyperylenequinone derivative, is an efficient phototherapeutic agent. Laser flash photolysis is used to produce and investigate the properties of the lowest excited triplet state (T1) and semiquinone radical anion of hypocrellin A (HA˙–) at room temperature. In the presence of some biological substrates (such as ascorbic acid and cysteine), the formation and decay of HA radical anion at different pH, attributed to the electron transfer between triplet HA and substrates, are observed. Meanwhile, the superoxide radical anion (O2;˙–) production by photoactivated HA in the presence of biological substrates is examined by using the nitro blue tetrazolium (NBT) trapping method in order to elucidate the mechanism of formation of superoxide and to quantify this formation. Specifically, production of O2;˙– is demonstrated unequivocally to be by reaction with the superoxide dismutase. The rate of reduction of NBT is dependent not only on the concentration of NBT but also on the pH of the system. The relative quantum yield of superoxide anion radicals increases considerably in alkaline solution: ϕO2;˙– = 6.97 × 10–3 at pH 9.0 as compared with ϕO2;˙– = 2.49 × 10–4 at pH 5.8 in the presence of ascorbic acid. Based on the experimental results, electron transfer (Type I) mechanism may play a hitherto unrecognized role in the photodynamic interaction between HA and some biological substrates.

Hypocrellin A (HA), a peryloquinone derivative, has recently been isolated from a fungus Hypocrella bambusae. This lipid soluble pigment, in combination with phototherapy, has been used to treat many skin diseases including the keloids caused by scalding and burns. We have studied the effects of photosensitized HA on biomembranes using pig heart microsomes. Photosensitization of HA was found to peroxidize the membrane lipids in the cardiac microsomes. The photodamage imposed by HA depended not only on the concentration of HA but also on the time of irradiation and pH of the system. Superoxide dismutase (SOD), ascorbic acid, beta-carotene and 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) inhibited the lipid peroxidation approximately 50, approximately 50, approximately 30 and approximately 97%, respectively. Spin trapping in combination with EPR spectroscopic techniques was used to identify the reactive free radicals during the photoreaction. Formation of superoxide anion radical, (O2-.), was identified by the SOD-inhibitable DMPO-O2- EPR spectrum. Both SOD and ascorbic acid inhibited the EPR signal intensity in a dose-dependent manner with rate constants of 6.78 x 10(8) M-1 s-1 and 1.82 x 10(4) M-1 s-1, respectively. The lifetime of O2-., under these conditions, was found to be 1.1 s. Photoirradiation of HA yielded a HA free radical with a g = 2.002 which was not suppressed by SOD but in the presence of reductants such as ascorbic acid and catechol the septum was completely suppressed. The increase of the EPR signal intensity and malondialdehyde formation with increasing pH may be due, in part, to the production of predominant *HA- species at high pH which would be more reactive with oxygen to yield O2-.. These results indicate that the lipid peroxidation of the cardiac membranes observed during photooxidation of HA may arise, in part, from the interaction of membrane lipids with reactive species of oxygen and HA free radical produced during the photo-irradiation.

Electron transfer in colloidal TiO 2 semiconductors sensitized by hypocrellin A

Study of electron transfer interaction between hypocrellin and N, N-diethylaniline by UV-visible, fluorescence, electron spin resonance spectra and time-resolved transient absorption spectra

Construction and photophysical properties of hypocrellin A/fullerene C 70 supramolecular assembly

Characteristics of the reaction between semiquinone radical anion of hypocrellin A and oxygen in aprotic media

The transient phenomena in a laser flash photolysis study of two photosensitizing agents for photodynamic therapy, hypocrellin A(HA) and hypocrellin B(HB), are described in solvents of varying polarity at room temperature. The difference between the triplet- and ground-state absorption spectra of HA or HB shows peaks ca. 400, 530, 570 (sh) and 620 nm in Ar-saturated n-hexane solution. The absorption coefficients for the T–T absorption and triplet quantum yield of HA (or HB) have been determined. Electron transfer in the quenching of triplet HA or HB by ground-state molecules of the dye was investigated, and the bimolecular self-quenching rate constant for HA or HB in DMSO, which is 1.2×109 or 0.8×109 d mol-1 s-1 respectively, suggested a nearly diffusion-controlled quenching process of the triplet excited state. Meanwhile, the corresponding anion radicals HA-, HB-, which are produced by electron-transfer reactions from aromatic amines, have absorption maxima at 640 nm. Triplet-state lifetimes and electron-transfer kinetics are also reported.

Spectroscopic study on the photoreduction of hypocrellin A: generation of semiquinone radical anion and hydroquinone

Studies on the chelation of hypocrellin A with aluminium ion and the photodynamic action of the resulting complex

A TD-DFT study on photo-physicochemical properties of hypocrellin A and its implications for elucidating the photosensitizing mechanisms of the pigment

Photooxygenations of PhSMe and Bu2S sensitized by N-methylquinolinium (NMQ+) and 9,10-dicyanoanthracene (DCA) in O2-saturated MeCN have been investigated by laser and steady-state photolysis. Laser photolysis experiments showed that excited NMQ+ promotes the efficient formation of sulfide radical cations with both substrates either in the presence or in absence of a cosensitizer (toluene). In contrast, excited DCA promotes the formation of radical ions with PhSMe, but not with Bu2S. To observe radical ions with the latter substrate, the presence of a cosensitizer (biphenyl) was necessary. With Bu2S, only the dimeric form of the radical cation, (Bu2S)2+*, was observed, while the absorptions of both PhSMe+* and (PhSMe)2+* were present in the PhSMe time-resolved spectra. The decay of the radical cations followed second-order kinetics, which in the presence of O2, was attributed to the reaction of the radical cation (presumably in the monomeric form) with O2-* generated in the reaction between NMQ* or DCA-* and O2. The fluorescence quenching of both NMQ+ and DCA was also investigated, and it was found that the fluorescence of the two sensitizers is efficiently quenched by both sulfides (rates controlled by diffusion) as well by O2 (kq = 5.9 x 10(9) M(-1) s(-1) with NMQ+ and 6.8 x 10(9) M(-1) s(-1) with DCA). It was also found that quenching of 1NMQ* by O2 led to the production of 1O2 in significant yield (PhiDelta = 0.86 in O2-saturated solutions) as already observed for 1DCA*. The steady-state photolysis experiments showed that the NMQ+- and DCA-sensitized photooxygenation of PhSMe afford exclusively the corresponding sulfoxide. A different situation holds for Bu2S: with NMQ+, the formation of Bu2SO was accompanied by that of small amounts of Bu2S2; with DCA, the formation of Bu2SO2 was also observed. It was conclusively shown that with both sensitizers, the photooxygenations of PhSMe occur by an electron transfer (ET) mechanism, as no sulfoxidation was observed in the presence of benzoquinone (BQ), which is a trap for O2-*, NMQ*, and DCA-*. BQ also suppressed the NMQ+-sensitized photooxygenation of Bu2S, but not that sensitized by DCA, indicating that the former is an ET process, whereas the second proceeds via singlet oxygen. In agreement with the latter conclusion, it was also found that the relative rate of the DCA-induced photooxygenation of Bu2S decreases by increasing the initial concentration of the substrate and is slowed by DABCO (an efficient singlet oxygen quencher). To shed light on the actual role of a persulfoxide intermediate also in ET photooxygenations, experiments in the presence of Ph2SO (a trap for the persulfoxide) were carried out. Cooxidation of Ph2SO to form Ph2SO2 was, however, observed only in the DCA-induced photooxygenation of Bu2S, in line with the singlet oxygen mechanism suggested for this reaction. No detectable amounts of Ph2SO2 were formed in the ET photooxygenations of PhSMe with both DCA and NMQ+ and of Bu2S with NMQ+. This finding, coupled with the observation that 1O2 and ET photooxygenations lead to different product distributions, makes it unlikely that, as currently believed, the two processes involve the same intermediate, i.e., a nucleophilic persulfoxide. Furthermore, the cooxidation of Ph2SO observed in the DCA-induced photooxygenation of Bu2S was drastically reduced when the reaction was performed in the presence of 0.5 M biphenyl as a cosensitizer, that is, under conditions where an (indirect) ET mechanism should operate. This observation confirms that a persulfoxide is formed in singlet oxygen but not in ET photosulfoxidations. The latter conclusion was further supported by the observation that also the intermediate formed in the reaction of thianthrene radical cation with KO2, a reaction which mimics step d (Scheme 2) in the ET mechanism of photooxygenation, is an electrophilic species, being able to oxidize Ph2S but not Ph2SO. It is thus proposed that the intermediate involved in ET sulfoxidations is a thiadioxirane, whose properties (it is an electrophilic species) seem more in line with the observed chemistry. Theoretical calculations concerning the reaction of a sulfide radical cation with O2-* provide a rationale for this proposal.

The mechanisms of electron transfer of association of chemoorganotrophic bacteria to the anode in microbial fuel cells are summarized in the survey. These mechanisms are not mutually exclusive and are divided into the mechanisms of mediator electron transfer, mechanisms of electron transfer with intermediate products of bacterial metabolism and mechanism of direct transfer of electrons from the cell surface. Thus, electron transfer mediators are artificial or synthesized by bacteria riboflavins and phenazine derivatives, which also determine the ability of bacteria to antagonism. The microorganisms with hydrolytic and exoelectrogenic activity are involved in electron transfer mechanisms that are mediated by intermediate metabolic products, which are low molecular carboxylic acids, alcohols, hydrogen etc. The direct transfer of electrons to insoluble anode is possible due to membrane structures (cytochromes, pili, etc.). Association of microorganisms, and thus the biochemical mechanisms of electron transfer depend on the origin of the inoculum, substrate composition, mass transfer, conditions of aeration, potentials and location of electrodes and others, that are defined by technological and design parameters.

UV-visible spectroscopy and fluorescence quenching measurements have been employed to study electron transfer from excited Hypocrellin A (HA) into the conduction band of a colloidal ZnO semiconductor. The adsorption of HA on the surface of ZnO colloidal particles and the electron transfer process from its singlet excited state to the conduction band of colloidal ZnO semiconductor were examined under experimental conditions. Exciting HA with visible light could induce electron transfer from its singlet excited state into conduction band of colloidal ZnO semiconductor. Adsorption of HA onto the surface of colloidal ZnO semiconductor extended its absorption spectrum further into the visible region, the quenching behavior were observed as the colloidal ZnO semiconductor was added and determined by fluorescence quenching method.

1. Mechanistic criteria, based on the side-chain fragmentation reactions of aromatic cation radicals, involving the cleavage of a beta bond (i.e. C-H, C-Si and C-S) have been developed for the detection of electron transfer mechanisms in oxidative processes of alkylbenzenes and aromatic sulphides. 2. For benzylic oxidations, the distinction between electron transfer (ET) and hydrogen atom transfer mechanism (HAT) has been based: (a) on studies of intramolecular selectivity, which, with appropriate substrates (5-Z-1,2,3,-trimethylbenzenes and 4-Z-1,2-dimethylbenzenes, where Z = OMe, alkyl), turns out to be much higher in ET than in HAT processes; and (b) on products studies concerning the reactions of bicumyl and benzyltrimethylsilanes since in these systems, the nature of products can be significantly different for ET and HAT mechanisms. 3. These criteria have been applied to the reactions of alkylbenzenes with an NO3 radical (shown to be an ET process) as well as to the microsomal and biomimetic (by iron porphyrins in the presence of PhIO) side-chain oxidation of the same compounds, where the mechanistic probes have suggested a HAT mechanism, with the exception of the biomimetic oxidation of 4-methoxybenzyltrimethylsilane in CH2Cl2-H2O-MeOH, which probably occurs by an ET mechanism. 4. For the enzymatic and biomimetic oxidation of aromatic sulphides an oxygen transfer is suggested, since, with cumyl phenyl sulphide and 4-methoxybenzyl phenyl sulphide, these reactions lead exclusively to the corresponding sulphoxides and sulphones, whereas the same substrates, in genuine ET reactions, form cation radicals which undergo C-H and C-S bond cleavage. 5. An oxygen transfer mechanism is also likely in the biomimetic and enzymatic oxidations of sulphoxides since in these reactions 4-methoxybenzyl phenyl sulphoxide is exclusively converted to sulphone, whereas in ET reactions it forms only C-S bond cleavage products.

Surface binding and improved photodamage of the lanthanum ion complex of hypocrellin A to calf thymus DNA