A.A. Krasnovsky

Photoluminescence of singlet oxygen in pigment solutions

Luminescence of 1O2 (1270 nm) accompanying energy transfer to oxygen from photoexcited (triplet) molecules of sensitizers in air saturated solutions has been investigated. The luminescence was observed in CC14, CS2 and freon with the use of porphyrins, chlorophylls, pheophytins and aromatic hydrocarbons as sensitizers. The lifetime and quantum yield of the luminescence depended on the nature of the solvents. pigments and their concentrations. The maximum values of these parameters were equal to 28 ± 5 ms and 5 ± 4 times 10‐‐5, respectively. The quantum yield of 1O2 generation by pigments has been measured and the results used for determining the quantum yields of intersystem crossing in the pigment molecules. The rate constants of 1O2 reaction with different substances have been determined with the aid of luminescence quenching. It has been shown that along with β‐carotene. Chls, pheophytins, and some porphyrins are also very active quenchers of 1O2, The quenching effect depends on their molecular structure and on the presence and nature of the central metal atom. Quenching 1O2 by the pigments is due mainly to a “physical” mechanism (without destruction of the pigments). The destructive “chemical” quenching is by 1–4 orders of magnitude less effective and is accompanied with photochemiluminescence of the pigments. The experiments on 1O2 generation and quenching indicate that energy of triplet states of bacteriochlorophyll and bacteriopheophytin is somewhat higher than that of 1Δg oxygen. The data demonstrate wide possibilities of the luminescence studied as a method for investigating 1O2 reactivity and photophysical properties of sensitizers.

Rise and decay kinetics of photosensitized singlet oxygen luminescence in water. Measurements with nanosecond time-correlated single photon counting technique

The time-correlated single photon counting technique was applied to the measurement of photosensitized singlet oxygen luminescence induced by flash laser excitation. Rise and decay kinetics of this luminescence has been measured with 10 ns resolution in aqueous solutions of tetra (p-sulfophenyl)porphyrin at various oxygen pressures. The rise kinetics are shown to be determined by the process of energy transfer to oxygen from the triplet state of the photosensitizer. The lifetime of singlet molecular oxygen in water was measured with high precision: τΔ = 3.09±0.06 μs. The rise and decay kinetics of the singlet oxygen luminescence in aqueous suspensions of porphyrin-containing yeast cells were detected.

EFFECT OF EXTRACTION AND RE-ADDITION OF MANGANESE ON LIGHT REACTIONS OF PHOTOSYSTEM-II PREPARATIONS

Manganese plays an important role in photosynthetic oxidation of H20 (reviews [l-3]). Reaction centers (RC) of photosystem II (PS II) carry out successive 4-step oxidation of a special (Mncontaining) enzymatic system which in turn oxidizes Hz0 [l-3]. The minimal quantity of Mn necessary for 02 evolution is 5-6 atoms/400 chlorophyll (chl) molecules or /l RC of PS II [l-4]. The greater part (-2/3rds) of this Mn is ‘loosely bound’ and can be easily extracted by alkaline Tris, NH20H, Triton X-100 or by heating, and the extraction leads to inhibition of 02 evolution and associated light reactions of PS II [l-lo]. ‘Firmly bound’ Mn ( 1/3rd of the pool) which remains in PS II after the extraction procedures seems not to be required for electron transport in PS II 141. However, up to 70% of Mn can be removed from chloroplasts without essential loss of their ability to evolve 02 [ll]. Reported characteristics of EPR spectra of Mn in chloroplasts [ 12151 may indicate participation of either 4 or 2 atoms of Mn in PS II reactions. of Mg2+ or any other divalent cation of metals, M2+). New results from a thorough investigation of these effects reported here show that activity of the Mn-containing system in the donor side of PS II requires 4 Mn atoms, 2 of which can be replaced by either Mg2+ or some other divalent metal ions (M2+).

Quantum yield of photosensitized luminescence and radiative lifetime of singlet (1Δg) molecular oxygen in solutions

Abstract The absolute quantum yield of the photosensitized luminescence from 1ΔgO2 has been measured in air-saturated solutions of tetraphenylporphin in CCI4, the value (4.4 ± 1.3) × 10−3 being obtained. The radiative lifetime of 1ΔgO2 is approximately constant in all solvents investigated and is 4.1 ± 2 s.

Photophysical studies of pheophorbide a and pheophytin a. Phosphorescence and photosensitized singlet oxygen luminescence

The triplet states of pheophorbide a and pheophytin a were studied in several environments by direct measurement of the phosphorescence of the pigments and photosensitized singlet oxygen (1O2) luminescence. The spectra, lifetimes and quantum yields of phosphorescence and the quantum yields of 1O2 generation were determined. These parameters are similar for monomeric molecules of both pigments in all the environments studied. Aggregation of the pigment molecules leads to a strong decrease in the phosphorescence and 1O2 luminescence intensities, which is probably due to a large decrease in the triplet lifetime and triplet quantum yield in the aggregates. The results obtained for pheophorbide a and pheophytin a are compared with those previously reported for chlorophyll alpha. The data suggest that the photodynamic activity of the pigments in living tissues is probably determined by the monomeric pigment molecules formed in hydrophobic cellular structures. Aggregated molecules seem to have a much lower activity.

Primary mechanisms of photoactivation of molecular oxygen. History of development and the modern status of research.

This review starts from a brief historical account devoted to the principles of the Bach-Engler peroxidation theory and experiments and ideas which led A. N. Bach to its creation. Then, the discovery of photodynamic action is described, which was shown to result from pigment photosensitized activation of molecular oxygen. The dramatic history of mechanistic studies of oxygen photoactivation is reviewed starting from the Bach-Engler peroxidation theory to the hypothesis of moloxide, discovery of singlet oxygen and free radicals and, then, to modern views on the primary photoactivation processes. The origin of widely used division of photodynamic processes into type I and type II and the relation of these processes to the nature of the primary photochemical reactions of photosensitizers are discussed. New definitions of these reactions are proposed on the basis of the mechanisms of oxygen photoactivation. Photographs of the scientists who greatly contributed to the development of this field of research are presented.

DELAYED FLUORESCENCE AND PHOSPHORESCENCE OF PLANT PIGMENTS

The term “delayed luminescence” is usually applied to photoinduced light emissions with lifetimes much longer than those of fluorescence of the objects. Under this term are included a wide variety of luminescence phenomena with essentially different mechanisms. A generally adopted terminology for different types of delayed luminescence based on their mechanisms is lacking so far. In biological literature all types of delayed luminescence of pigments are often divided into two large groups: delayed fluorescence and phosphorescence. The delayed fluorescence includes afterglows with spectra similar to those of pigment fluorescence: the phosphorescence corresponds to light emissions accompanying radiative electronic transitions from triplet to ground states of pigment molecules. The present paper deals mostly with these two groups of delayed luminescence. Moreover, a related phenomenon, the photosensitized luminescence of singlet oxygen, is also briefly reviewed. The review covers the literature from January, 1980, through June, 1982, as well as some earlier papers which escaped attention in previous reviews in this field.

Quenching of singlet oxygen luminescence by fatty acids and lipids

By the use of photosensitized luminescence of singlet oxygen (1O2) in CCl4, the rate constants for quenching 1O2 by saturated and unsaturated fatty acids were determined. The experimental data suggest that saturated fatty acids quench 1O2 via a physical mechanism, presumably as a result of energy transfer to the vibrational sublevels of CH‐ and COOH‐groups. Unsaturated fatty acids are predominantly chemical quenchers. The relative contribution of the physical quenching depends on the number of double bonds in fatty acid molecules. It was found that the quenching activity of egg phosphatidylcholine is approximately equal to the sum of quenching activities of the lipid fatty acids. The data obtained may be used for prediction of the efficiency of singlet oxygen quenching by any lipids whose fatty acid composition is known.

Activation of molecular oxygen by infrared laser radiation in pigment-free aerobic systems

With the goal of mimicking the mechanisms of the biological effects of low energy laser irradiation, we have shown that infrared low intensity laser radiation causes oxygenation of the chemical traps of singlet oxygen dissolved in organic media and water saturated by air at normal atmospheric pressure. The photooxygenation rate was directly proportional to the oxygen concentration and strongly inhibited by the singlet oxygen quenchers. The maximum of the photooxygenation action spectrum coincided with the maximum of the oxygen absorption band at 1270 nm. The data provide unambiguous evidence that photooxygenation is determined by the reactive singlet 1Δg state formed as a result of direct laser excitation of molecular oxygen. Hence, activation of oxygen caused by its direct photoexcitation may occur in natural systems.

Photosensitization and quenching of singlet oxygen by pigments and lipids of photoreceptor cells of the retina

Studies of ‘02 generation and quenching in solutions of pigments and lipids isolated from photoreceptor cells are important for understanding the molecular mechanisms of photodamage of the retina [l-S]. The first works in this field were published by Delmelle ([4] and references therein), who attempted to demonstrate the ability of retinal to produce ‘02, using indirect ‘chemical’ methods of ‘02 detection. Here we have employed a more accurate method, i.e., registration of oxygen luminescence observed in airsaturated solutions of various pigments ([5,6] and references therein). With the aid of this emission the following questions were studied: is retinal able to generate ‘02 and what is the efficiency of ‘02 interaction with retinal and photoreceptor lipids?