Magnetic field effects on radical pair intermediates in bacterial photosynthesis

We have investigated the effects of magnetic fields on the formation and decay of excited states in the photochemical reaction centers of Rhodopseudomonas sphaeroides. In chemically reduced reaction centers, a magnetic field decreases the fraction of the transient state P^F that decays by way of the bacteriochlorophyll triplet state P^R. At room temperature, a 2-kG field decreases the quantum yield of P^R by about 40%. In carotenoid-containing reaction centers, the yield of the carotenoid triplet state which forms via P^R is reduced similarly. The effect of the field depends monotonically on field-strength, saturating at about 1 kG. The effect decreases at lower temperatures, when the yield of P^R is higher. Magnetic fields do not significantly affect the formation of the triplet state of bacteriochlorophyll in vitro, the photooxidation of P-870 in reaction centers at moderate redox potential, or the decay kinetics of states P^F and P^R.The effects of magnetic fields support the view that state P^F is a radical pair which is born in a singlet state but undergoes a rapid transformation into a mixture of singlet and triplet states. A simple kinetic model can account for the effects of the field and relate them to the temperature dependence of the yield of P^R.     

Antenna organization and evidence for the function of a new antenna pigment species in the green photosynthetic bacterium Chloroflexus aurantiacus

Whole cells and isolated chlorosomes (antenna complex) of the green photosynthetic bacterium Chloroflexus aurantiacus have been studied by absorption spectroscopy (77 K and room temperature), fluorescence spectroscopy, circular dichroism, linear dichroism and electron spin resonance spectroscopy. The chlorosome absorption spectrum has maxima at 450 (contributed by carotenoids and bacteriochlorophyll (BChl) a Soret), 742 (BChl c) and 792 nm (BChl a) with intensity ratios of 20:25. The fluorescence emission spectrum has peaks at 748 and 802 nm when excitation is into either the 742 or 450 nm absorption bands, respectively. Whole cells have fluorescence peaks identical to those in chlorosomes with the addition of a major peak observed at 867 nm. The CD spectrum of isolated chlorosomes has an asymmetric-derivative-shaped CD centered at 739 nm suggestive of exciton interaction at least on the level of dimers. Linear dichroism of oriented chlorosomes shows preferential absorption at 742 nm of light polarized parallel to the long axis of the chlorosome. This implies that the transition dipoles are also oriented more or less parallel to the long axis of the chlorosome. Treatment with ferricyanide results in the appearance of a 2.3 G wide ESR spectrum at g 2.002. Whole cells grown under different light conditions exhibit different fluorescence behavior when absorption is normalized at 742 nm. Cells grown under low light conditions have higher fluorescence intensity at 748 nm and lower intensity at 802 nm than cells grown under high light conditions. These results indicate that the BChl c in chlorosomes is highly organized, and transfers energy from BChl c (742 nm) to a connector of baseplate BChl B792 (BChl a) presumably located in the chlorosome baseplate adjacent to the cytoplasmic membrane.     

Origin and early evolution of photosynthesis

Photosynthesis was well-established on the earth at least 3.5 thousand million years ago, and it is widely believed that these ancient organisms had similar metabolic capabilities to modern cyanobacteria. This requires that development of two photosystems and the oxygen evolution capability occurred very early in the earth's history, and that a presumed phase of evolution involving non-oxygen evolving photosynthetic organisms took place even earlier. The evolutionary relationships of the reaction center complexes found in all the classes of currently existing organisms have been analyzed using sequence analysis and biophysical measurements. The results indicate that all reaction centers fall into two basic groups, those with pheophytin and a pair of quinones as early acceptors, and those with iron sulfur clusters as early acceptors. No simple linear branching evolutionary scheme can account for the distribution patterns of reaction centers in existing photosynthetic organisms, and lateral transfer of genetic information is considered as a likely possibility. Possible scenarios for the development of primitive reaction centers into the heterodimeric protein structures found in existing reaction centers and for the development of organisms with two linked photosystems are presented.Abbreviation Gyr gigayears     

Sulfate reduction and S-oxidation in a moorland pool sediment

In an oligotrophic moorland pool in The Netherlands, S cycling near the sediment/water boundary was investigated by measuring (1) SO₄ ^2– reduction rates in the sediment, (2) depletion of SO₄ ^2– in the overlying water column and (3) release of³⁵S from the sediment into the water column. Two locations differing in sediment type (highly organic and sandy) were compared, with respect to reduction rates and depletion of SO₄ ^2– in the overlying water.Sulfate reduction rates in sediments of an oligotrophic moorland pool were estimated by diagenetic modelling and whole core³⁵SO₄ ^2– injection. Rates of SO₄ ^2– consumption in the overlying water were estimated by changes in SO₄ ^2– concentration over time in in situ enclosures. Reduction rates ranged from 0.27–11.2 mmol m^–2 d^–1. Rates of SO₄ ^2– uptake from the enclosed water column varied from –0.5, –0.3 mmol m^–2 d^–1 (November) to 0.43–1.81 mmol m^–2 d^–1 (July, August and April). Maximum rates of oxidation to SO₄ ^2– in July 1990 estimated by combination of SO₄ ^2– reduction rates and rates of in situ SO₄ ^2– uptake in the enclosed water column were 10.3 and 10.5 mmol m^–2 d^–1 at an organic rich and at a sandy site respectively.Experiments with³⁵S^2– and³⁵SO₄ ^2– tracer suggested (1) a rapid formation of organically bound S from dissimilatory reduced SO₄ ^2– and (2) the presence of mainly non SO₄ ^2–-S derived from reduced S transported from the sediment into the overlying water. A³⁵S^2– tracer experiment showed that about 7% of³⁵S^2– injected at 1 cm depth in a sediment core was recovered in the overlying water column.Sulfate reduction rates in sediments with higher volumetric mass fraction of organic matter did not significantly differ from those in sediments with a lower mass fraction of organic matter.Corresponding author     

Deacetylation of N8-acetylspermidine by subcellular fractions of rat tissue

The in vitro deacetylation of N⁸-acetylspermidine by an enzyme activity in rat tissues is described. This deacetylase activity occurs as a soluble, cytoplasmic enzyme in rat liver and was detected in the 100,000g supernatant fraction of all tissues examined. The highest specific activity was found in liver. Spleen, kidney, and lung were found to contain 20–50% of the activity in liver, while heart, brain, and skeletal muscle exhibited from 2 to 10% of the activity in liver. Serum contained only barely detectable levels of activity, much lower than any of the tissues studied. The in vitro metabolism of N¹-acetylspermidine differed from that observed for N⁸-acetylspermidine and does not appear to involve a simple deacetylation reaction.     

Hepatic fatty cirrhosis in Texas white-tailed deer

Hepatic fatty cirrhosis (HFC) has been known to occur in certain domestic livestock species since 1931. Early studies in Texas indicated that HFC was restricted to five western counties. Recently HFC was identified in white-tailed deer (Odocoileus virginianus texanus) in South Texas for the first time. Of the deer examined, 25% were affected. The etiology of HFC was not determined.     

A flash photolysis ESR study of photosystem II signal IIvf, the physiological donor to P-680+.

In flash-illuminated, oxygen-evolving spinach chloroplasts and green algae, a free radical transient has been observed with spectral parameters similar to those of Signal II (g approximately 2.0045, deltaHpp approximately 19G). However, in contrast with ESR Signal II, the transient radical does not readily saturate even at microwave power levels of 200 mW. This species is formed most efficiently with "red" illumination (lambda less than 680 nm) and occurs stoichiometrically in a 1:1 ratio with P-700+. The Photosystem II transient is formed in less than 100 mus and decays via first-order kinetics with a halftime of 400-900 mus. Additionally, the t1/2 for radical decay is temperature independent between 20 and 4 degrees C; however, below 4 degrees C the transient signal exhibits Arrhenius behavior with an activation energy of approx. 10 kcal-mol-1. Inhibition of electron transport through Photosystem II by o-phenanthroline, 3-(3,4-dichlorophenyl)-1,1-dimethylurea or reduced 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone suppresses the formation of the light-induced transient. At low concentrations (0.2 mM), 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone partially inhibits the free radical formation, however, the decay kinetics are unaltered. High concentrations of 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (1-5 mM) restore both the transient signal and electron flow through Photosystem II. These findings suggest that this "quinoidal" type ESR transient functions as the physiological donor to the oxidized reaction center chlorophyll, P-680+.     

Diazocyclopentadiene (DACP), a light sensitive reagent for the ethylene receptor in plants

Diazocyclopentadiene (DACP) has been shown to be an effective reagent for the ethylene receptor. Treatment of mung bean sprouts or tobacco leaves with DACP in the light or in the dark inactivates much of the ethylene binding. In the light, inactivation seems to be permanent, while in the dark, the site becomes active again after the DACP diffuses away. The compound is 10 times more effective in the light than in the dark. DACP inhibits banana ripening indicating the physiological receptor is involved. It also overcomes the inhibitory effect of ethylene on mung bean seedling growth (Km = 0.09 µl/1 E) at low ethylene levels. At high ethylene levels, an apparent high ethylene level site becomes apparent (Km = 50 µl/1 E) and growth is inhibited.