Photochemistry and Photobiology

Abstract— Using isolated chloroplasts and techniques as described by Joliot and Joliot[6] we studied the evolution of O₂ in weak light and light flashes to analyze the interactions between light induced O₂ precursors and their decay in darkness. The following observations and conclusions are reported: 1. Light flashes always produce the same number of oxidizing equivalents either as precursor or as O₂. 2. The number of unstable precursor equivalents present during steady state photosynthesis is ∼ 1.2 per photochemical trapping center. 3. The cooperation of the four photochemically formed oxidizing equivalents occurs essentially in the individual reaction centers and the final O₂ evolution step is a one quantum process. 4. The data are compatible with a linear four step mechanism in which a trapping center, or an associated catalyst, (S) successively accumulates four + charges. The S⁴⁺ state produces O₂ and returns to the ground state S₀. 5. Besides S₀ also the first oxidized state S⁺ is stable in the dark, the two higher states, S²⁺ and S³⁺ are not. 6. The relaxation times of some of the photooxidation steps were estimated. The fastest reaction, presumably S*₁←S₂, has a (first) half time ≤ 200 μsec. The S*₂ state and probably also the S*₀ state are processed somewhat more slowly (˜ 300–400 μsec).

Abstract— The variable chlorophyll (Chl) a fluorescence yield is known to be related to the photochemical activity of photosystem II (PSII) of oxygen‐evolving organisms. The kinetics of the fluorescence rise from the minimum yield, F₀, to the maximum yield, F_m, is a monitor of the accumulation of net reduced primary bound plastoquinone (Q_A) with time in all the PSII centers. Using a shutter‐less system (Plant Efficiency Analyzer, Hansatech, UK), which allows data accumulation over several orders of magnitude of time (40 μs to 120 s), we have measured on a logarithmic time scale, for the first time, the complete polyphasic fluorescence rise for a variety of oxygenic plants and cyanobacteria at different light intensities. With increasing light intensity, the fluorescence rise is changed from a typical O‐I‐P characteristic to curves with two intermediate levels J and I, both of which show saturation at high light intensity but different intensity dependence. Under physiological conditions, Chl a fluorescence transients of all the organisms examined follow the sequence of O‐J‐I‐P. The characteristics of the kinetics with respect to light intensity and temperature suggest that the O‐J phase is the photochemical phase, leading to the reduction of Q_A to Q_A^‐. The intermediate level I is suggested to be related to a heterogeneity in the filling up of the plastoquinone pool. The P is reached when all the plastoquinone (PQ) molecules are reduced to PQH₂. The addition of 3‐(3–4‐dichlorophenyl)‐1,1‐dimethylurea leads to a transformation of the O‐J‐I‐P rise into an O‐J rise. The kinetics of O‐J‐I‐P observed here was found to be similar to that of O‐I₁‐I₂‐P, reported by Neubauer and Schreiber (Z. Naturforsch.42c, 1246–1254, 1987). The biochemical significance of the fluorescence steps O‐J‐I‐P with respect to the filling up of the plastoquinone pool by PSII reactions is discussed.

NHIK 3025 cells were incubated with Photofrin II (PII) and/or tetra (3‐hydroxyphenyl)porphyrin (3THPP) and exposed to light at either 400 or 420 nm, i. e. at the wavelengths of the maxima of the fluorescence excitation spectra of the two dyes. The kinetics of the photodegradation of the dyes were studied. When present separately in the cells the two dyes are photodegraded with a similar quantum yield. 3THPP is degraded 3–6 times more efficiently by light quanta absorbed by the fluorescent fraction of 3THPP than by light quanta absorbed by the fluorescent fraction of PII present in the same cells. The distance diffused by the reactive intermediate, supposedly mainly ¹O₂, causing the photodegradation was estimated to be on the order of 0.01–0.02 μm, which corresponds to a lifetime of 0.01–0.04 μs of the intermediate in the cells. PII has binding sites at proteins in the cells as shown by an energy transfer band in the fluorescence excitation spectrum at 290 nm. During light exposure this band decays faster than the Soret band of PII under the present conditions. Photoproducts (¹O₂ etc.) generated at one binding site contribute significantly in the destruction of remote binding sites.

Abstract. Advances in the use of photosensitizers for detection and treatment of malignant tumors during the previous reviews in this journal have been mainly in the areas of development of potentially useful new photosensitizers (e.g. phthalocyanines and chlorins), further understanding of the mechanisms of photodynamic therapy (PDT), and in dosimetry of light delivery in tissue. Perhaps the most significant advance during the past year has been the initiation of Phase III clinical trials of PDT vs standard therapy in treatment of superficial bladder cancer and obstructive endobronchial tumors. In the final analysis, only such clinical trials can determine the future of this new modality for cancer treatment.

Photobiomodulation (PBM) involves the use of red or near‐infrared light at low power densities to produce a beneficial effect on cells or tissues. PBM therapy is used to reduce pain, inflammation, edema, and to regenerate damaged tissues such as wounds, bones, and tendons. The primary site of light absorption in mammalian cells has been identified as the mitochondria and, more specifically, cytochrome c oxidase (CCO). It is hypothesized that inhibitory nitric oxide can be dissociated from CCO, thus restoring electron transport and increasing mitochondrial membrane potential. Another mechanism involves activation of light or heat‐gated ion channels. This review will cover the redox signaling that occurs in PBM and examine the difference between healthy and stressed cells, where PBM can have apparently opposite effects. PBM has a marked effect on stem cells, and this is proposed to operate via mitochondrial redox signaling. PBM can act as a preconditioning regimen and can interact with exercise on muscles.

Abstract— In experiments on the interception of reactive intermediates of strongly oxidizing character in dye (S) sensitized photooxidations using p‐nitrosodimethylaniline (RNO) as a selective scavenger, it has been observed that some substrates (A) or ¹O₂ acceptors (like imidazole derivatives) induce the bleaching of RNO as followed spectrophotometrically at 440 nm. Since singlet oxygen (¹O₂) does not react chemically with RNO, this bleaching is a consequence of ¹O₂ capture by the imidazole ring which results in the formation of a trans‐annular peroxide intermediate [¹O₂] capable of inducing the bleaching of RNO (‐RNO). In the absence of RNO, [¹O₂] decomposes or rearranges into the final oxygenation product ¹O₂: ¹Δ_g Thus, the system imidazole plus RNO can be used as a sensitive and selective test for the presence of ¹O₂ in aqueous solutions. The method can also be applied in the presence of sensitizing dyes which, under visible irradiation, can partially bleach RNO even in the absence of imidazole derivatives. In such a case, the bleaching of RNO is strongly increased by the presence of imidazoles with a characteristic dependence on their concentration. The separation of the product of RNO bleaching by thin layer chromatography can serve as additional proof of the presence of ¹O₂ in the system. The imidazole plus RNO method has been applied to a number of sensitizing and non‐sensitizing dyes.