Manganese oxides, photochemical

Manganese oxides are also subject to reductive dissolution by light in surface seawaters. This process produces Mn(II), which is kinetically stable to oxidation in the absence of bacteria that are subject to photoinhibition. These two effects lead to the formation of a surface maximum in soluble Mn(II), in contrast to most metals which are depleted in surface waters due to biological removal processes. Other examples of the impact of photochemical reactions on trace metal chemistry are provided in Further Reading. 

In photosystem II an intermediate tyrosyl radical is formed which then repetitively oxidizes an adjacent manganese cluster leading to a four-electron oxidation of two water molecules to dioxygen. In broad detail, the model compounds" described above were demonstrated to undergo similar reactions on photochemical excitation of the respective ruthenium centers. 

An interesting variation of this system involves embedding surface active zinc and manganese(III) porphyrins in lecithin membranes. Provided that the porphyrin is negatively charged, and with zwitterionic PSV in the bulk aqueous phase, the Mn111 can be photochemically oxidized to Mn,v. No electron transfer from PSV" to MuIV occurs, perhaps because the MnIV is deeply embedded in the membrane.310... 

Nowadays, not only Fe but other trace metals as well, for example, Mn, Co, or Cu, are thought to limit primary production. It is thus a real challenge for oceanographers not just to assess correctly the very low levels of Fe and Mn in the oceans but also to carry out the speciation of these elements (total dissolved concentrations are at the nM level, labile forms oxidation states in natural aquatic systems Fe(II), which is readily soluble, and Fe(III), which is almost insoluble. Flowever, both Fe ions can form diverse complexes with organic ligands with different labilities and solubilities, and colloidal particles, which are also considered part of the dissolved phase. Manganese also exists in two oxidation states in aquatic systems soluble Mn(II) and insoluble Mn(IV) both are present in a dynamic cycle in seawater. The nonlabile Mn pool consists of oxidized Mn(IV) species, but these can be photochemically reduced and thus solubilized.23... 

Han G, Li J, Chen G, et al. Reconstruction of the water-oxidizing complex in manganese-depleted Photosystem II using synthetic manganese complexes. J Photochem Photobiol B Biol 2005 81 114-20. 

There has been recent interest in a somewhat different aspect of adsorption and reaction on metal oxides photocatalysis. The interest stems partially from that role that some transition-metal oxides can play in photochemical reactions in the atmosphere. Atmospheric aerosol particles can act as substrates to catalyze heterogeneous photochemical reactions in the troposphere. Most tropospheric aerosols are silicates, aluminosilicates and salts whose bandgaps are larger than the cutoff of solar radiation in the troposphere (about 4.3 eV) they are thus unable to participate directly in photoexcited reactions. However, transition-metal oxides that have much smaller bandgaps also occur as aerosols — the most prevalent ones are the oxides of iron and manganese — and these materials may thus undergo charge-transfer excitations (discussed above) in the pres-... 

Several new natural products based on the ionone skeleton have been isolated. Damescenone (56), jS-damascone (57) and 3-keto-a-ionol (58) were isolated from tobacco, while 4-keto-/S-ionone (59) was isolated from black tea. Treatment of dehydro-/l-ionol [alcohol from (53)] with diborane gave 3-hydroxy- -ionol (60). Photochemical oxidation gave loliolide (67) and its C-3 epimer, and the allene (61). Manganese dioxide oxidation gave an isomer of grasshopper... 

This concept was consistent with the results of other investigations. First, reaction-center preparations free of iron or in which iron is replaced by another metal ion such as manganese, remain photochemi-cally active in other words, the presence of iron for electron transfer is not obligatory. Second, examination of T e-enriched reaction-center preparations by Mdssbauer spectroscopy showed that the iron remains in the high-spin state irrespective of whether ubiquinone is oxidized or reduced. On the other hand, ifubiquinone is removed, the reaction center loses its photochemical activity but when the ubiquinone is restored, the original level of photochemical activity is also restored. 

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