The civilian, commercial, and defense sectors of most advanced industrialized nations are faced with a tremendous set of environmental problems related to the remediation of hazardous wastes, contaminated groundwaters, and the control of toxic air contaminants. For example, the slow pace of hazardous waste remediation at military installations around the world is causing a serious delay in conversion of many of these facilities to civilian uses. Over the last 10 years problems related to hazardous waste remediation have emerged as a high national and international priority.

Environmental Applications of Semiconductor Photocatalysis

Critical review of rate condtants for reaction of hydrated electrons, hydrogen atoms and hydroxyl radicals in aqueous solution

The phenomenon of growth, decline and death—aging—has been the source of considerable speculation (1, 8, 10). This cycle seems to be a more or less direct function of the metabolic rate and this in turn depends on the species (animal or plant) on which are superimposed the factors of heredity and the effects of the stresses and strains of life—which alter the metabolic activity. The universality of this phenomenon suggests that the reactions which cause it are basically the same in all living things. Viewing this process in the light of present day free radical and radiation chemistry and of radiobiology, it seems possible that one factor in aging may be related to deleterious side attacks of free radicals (which are normally produced in the course of cellular metabolism) on cell constituents.* Irradiation of living things induces mutation, cancer, and aging (9). Inasmuch as these also arise spontaneously in nature, it is natural to inquire if the processes might not be similar. It is believed that one mechanism of irradiation effect is through liberation of OH and HO 2 radicals (12). There is evidence, although indirect, that these two highly active free radicals are produced normally in living systems. In the first place, free radicals are present in living cells; this was recently demonstrated in vivo by a paramagnetic resonance absorption method (3). Further, it was shown that the concentration of free radicals increased with increasing metabolic activity in conformity with the postulates set forth some years ago that free radicals were involved in biologic oxidation-reduction reactions (11, 13). Are some of these free radicals OH and/or HO2, or radicals of a similar high order of reactivity, and where might they arise in the cell? The most likely source of OH and HO2 radicals, at least in the animal cell, would be the interaction of the respiratory enzymes involved

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Aging: A Theory Based on Free Radical and Radiation Chemistry

The interest in heterogeneous photocatalysis is intense and increasing, as shown by the number of publications on this theme which regularly appear in this journal, and the fact that over 2000 papers have been published on this topic since 1981. This article is an overview of the field of semiconductor photocatalysis : a brief examination of its roots, achievements and possible future. The semiconductor titanium dioxide (TiO 2 ) features predominantly in past and present work on semiconductor photocatalysis; as a result, in the most of the examples selected in this overview to illustrate various points the semiconductor is TiO 2 .

An overview of semiconductor photocatalysis

Photosynthesis uses light energy to drive the oxidation of water at an oxygen-evolving catalytic site within photosystem II (PSII). We report the structure of PSII of the cyanobacterium Thermosynechococcus elongatus at 3.5 angstrom resolution. We have assigned most of the amino acid residues of this 650-kilodalton dimeric multisubunit complex and refined the structure to reveal its molecular architecture. Consequently, we are able to describe details of the binding sites for cofactors and propose a structure of the oxygen-evolving center (OEC). The data strongly suggest that the OEC contains a cubane-like Mn 3 CaO 4 cluster linked to a fourth Mn by a mono-μ-oxo bridge. The details of the surrounding coordination sphere of the metal cluster and the implications for a possible oxygen-evolving mechanism are discussed.

/pdf/architecture-of-the-photosynthetic-oxygen-evolving-center-1kavwn4g0h.pdf

Architecture of the Photosynthetic Oxygen-Evolving Center

Metal phthalocyanines and porphyrins as photosensitizers for reduction of water to hydrogen

IT has been a matter of general experience that photochemical reactions are not much altered in their courses by change of light intensity The range of light intensity hitherto available, however, has been limited by that obtainable from the sun and from such sources as high-pressure mercury vapour lamps, the total usable output of which in the region between 2,000 and 5,000 A does not exceed 1020 quanta/second There are many cases, particularly those photochemical reactions where free radicals are involved, where it would be desirable to extend investigations to much higher intensities into the region where the concentration of intermediates is comparable with that of the reactants

Chemical Reactions Produced by Very High Light Intensities

Photochemistry provides us with one of the most generally useful methods of studying the reactions of free radicals and atoms, but the concentration of these intermediates in the usual photochemical systems is too low to allow the use of direct physical methods of investigation such as absorption spectroscopy. To overcome this difficulty a new technique of flash photolysis and spectroscopy has been developed, using gas-filled flash discharge tubes of very high power. The properties of these lamps as spectroscopic and photochemical sources have been studied and details are given of their construction, spectra, duration of flash, and luminous efficiency in the photochemicaliy useful region. An apparatus is described which produces a very great photochemical change, in some cases over 80%, in one-thousandth of a second and in a gas at several cm. pressure contained in an absorption tube 1 m. long, and which photographs the absorption spectrum at high resolution in one twenty-thousandth of a second at short intervals afterwards. Examples of the rapidly changing spectra of substances undergoing reaction, including the spectra of some of the intermediate radicals involved, are shown. These include the recombination of chlorine atoms, the absorption Spectra of S 2 and CS obtained during the photochemical decomposition of carbon disulphide and new spectra attributed to the CIO and CH 3 CO radicals.

Flash photolysis and spectroscopy. A new method for the study of free radical reactions

We consider a model of the photosystem II (PS II) reaction center in which its spectral properties result from weak (approximately 100 cm-1) excitonic interactions between the majority of reaction center chlorins. Such a model is consistent with a structure similar to that of the reaction center of purple bacteria but with a reduced coupling of the chlorophyll special pair. We find that this model is consistent with many experimental studies of PS II. The similarity in magnitude of the exciton coupling and energetic disorder in PS II results in the exciton states being structurally highly heterogeneous. This model suggests that P680, the primary electron donor of PS II, should not be considered a dimer but a multimer of several weakly coupled pigments, including the pheophytin electron acceptor. We thus conclude that even if the reaction center of PS II is structurally similar to that of purple bacteria, its spectroscopy and primary photochemistry may be very different.

https://www.pnas.org/content/pnas/92/11/4798.full.pdf

A multimer model for P680, the primary electron donor of photosystem II.

IN the primary process of plant photosynthesis it is generally accepted that efficient energy migration occurs between about 300 molecules of chlorophyll a, with subsequent light collection by a chemical trap. Solutions of chlorophyll in vitro, whether in fluid solvents1, monolayers2,3, multilayers4,5 or, as shown recently in our laboratory, in rigid matrices of PMMA and in bilayer lipid vesicles, exhibit the phenomenon of concentration quenching of the excited state at concentrations much lower than those which are present in the chloroplast. At a chlorophyll concentration of 10−1 M, which is comparable with that in the chloroplast, none of the in vitro systems has a fluorescent yield as high as is found in vivo, especially when the photochemical traps are closed. To try to understand the apparent absence of concentration quenching in vivo, we have re-examined its mechanism in vitro, and conclude that each chlorophyll molecule in the light-collecting system must be separated from other chlorophyll molecules, so as to prevent trap formation by orbital overlap and so that the minimum distance, averaged over all orientations in a random array, is 10 A.

George Porter

Papers

Metal phthalocyanines and porphyrins as photosensitizers for reduction of water to hydrogen

Chemical Reactions Produced by Very High Light Intensities

Flash photolysis and spectroscopy. A new method for the study of free radical reactions

A multimer model for P680, the primary electron donor of photosystem II.

Concentration quenching in chlorophyll