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25 novembre 2011 5 25 /11 /novembre /2011 03:07

Biologically and thermochemically driven sulfate reduction

Sulfur can be reduced both biologically and thermochemically. Dissimilatory sulfate reduction has two different definitions:

1. the microbial process that converts sulfate to sulfide for energy gain, and

2. a set of forward and reverse pathways that progress from the uptake and release of sulfate by the cell to its conversion to various sulfur intermediates, and ultimately to sulfide which is released from the cell.Sony VAIO VGN-G11XN/B Battery

Sulfide and thiosulfate are the most abundant reduced inorganic sulfur species in the environments and are converted to sulfate, primarily by bacterial action, in the oxidative half of the sulfur cycle. Bacterial sulfate reduction (BSR) can only occur at temperature from 0 up to 60–80 °C because above that temperature almost all sulfate-reducing microbes can no longer metabolize. Sony VAIO VGN-G1AAPS Battery

Few microbes can form H2S at higher temperatures but appear to be very rare and do not metabolize in settings where normal bacterial sulfate reduction is occurring. BSR is geologically instantaneous happening on the order of hundreds to thousands of years.Sony VAIO VGN-G1AAPSA Battery

Thermochemical sulfate reduction (TSR) occurs at much higher temperatures (160–180 °C) and over longer time intervals, several tens of thousands to a few million years.

The main difference between these two reactions is obvious, one is organically driven and the other is chemically driven. Sony VAIO VGN-G1AAPSB Battery

Therefore the temperature for thermochemical sulfate reduction is much higher due to the activation energy required to reduce sulfate. Bacterial sulfate reductions requires lower temperatures because the sulfur reducing bacteria can only live at relatively low temperature (below 60 °C). Sony VAIO VGN-G1AAPSC Battery

BSR also requires a relatively open system; otherwise the bacteria will poison themselves when the sulfate levels rise above 5–10%.

The organic reactants involved in BSR are organic acids which are distinctive from the organic reactants needed for TSR. In both cases sulfate is usually derived from the dissolution of gypsum or taken directly out of the seawater. Sony VAIO VGN-G1ABNS Battery

The factors that control whether BSR or TSR will occur are temperature, which is generally a product of depth, with BSR occurring in shallower levels than TSR. Both can occur within the oil window. Their solid products are similar but can be distinguished from one another petrographically, due to their differing crystal sizes, shapes and reflectivity.Sony VAIO VGN-G1KAP Battery

?34S

Although 25 isotopes are known for sulfur, only four are stable and of geochemical importance. Of those four, two (32S, light and 34S, heavy) comprise (99.22%) of S on Earth. The vast majority (95.02%) of S occurs as 32S with only 4.21% in 34S. The ratio of these two isotopes is fixed in our solar system and has been since its formation. Sony VAIO VGN-G1KBN Battery

The bulk Earth sulfur isotopic ratio is thought to be the same as the ratio of 22.22 measured from the Canyon Diablo troilite (CDT), a meteorite. That ratio is accepted as the international standard and is therefore set at ?0.00. Deviation from 0.00 is expressed as the ?34S which is a ratio in per mill (‰).Sony VAIO VGN-G1KBNA Battery

Positive values correlate to increased levels of 34S, whereas negative values correlate with greater 32S in a sample.

Formation of sulfur minerals through non-biogenic processes does not substantially differentiate between the light and heavy isotopes, therefore sulfur values ingypsum or baryte should be the same as the overall ratio in the water column at their time of precipitation. Sony VAIO VGN-G1LAP Battery

Sulfate reduction through biologic activity strongly differentiates between the two isotopes because of the more rapid enzymic reaction with 32S. Sulfate metabolism results in an isotopic depletion of -18‰, and repeated cycles of oxidation and reduction can result in values up to -50 ‰. Average present day seawater values of ?34S are on the order of +21‰.Sony VAIO VGN-G1LBN Battery

Throughout geologic history the sulfur cycle and the isotopic ratios have coevolved with the biosphere becoming overall more negative with the increases in biologically driven sulfate reduction, but also show substantial positive excursion. In general positive excursions in the sulfur isotopes mean that there is an excess of pyrite deposition rather than oxidation of sulfide minerals exposed on land. Sony VAIO VGN-G218LN/T Battery

Evolution of the sulfur cycle

The isotopic composition of sedimentary sulfides provides primary information on the evolution of the sulfur cycle. The total inventory of sulfur compounds on the surface of the Earth (nearly 1022g S) represents the total outgassing of sulfur through geologic time.Sony VAIO VGN-G218N/B Battery

Rocks analyzed for sulfur content are generally organic-rich shales meaning they are likely controlled by biogenic sulfur reduction. Average seawater curves are generated from evaporites deposited throughout geologic time because again, since they do not discriminate between the heavy and light sulfur isotopes, they should mimic the ocean composition at the time of deposition.Sony VAIO VGN-G21XP/B Battery

4.6 billion years ago (Ga) the Earth formed and had a theoretical ?34S value of 0. Since there was no biologic activity on early Earth there would be no isotopic fractionation. All sulfur in the atmosphere would be released during volcanic eruptions. When the oceans condensed on Earth, the atmosphere was essentially swept clean of sulfur gases, owing to their high solubility in water. Sony VAIO VGN-G2AANS Battery

Throughout the majority of the Archean (4.6–2.5 Ga) most systems appeared to be sulfate-limited. Some small Archean evaporite deposits require that at least locally elevated concentrations (possibly due to local volcanic activity) of sulfate existed in order for them to be supersaturated and precipitate out of solution.Sony VAIO VGN-G2AAPS Battery

3.8–3.6 Ga marks the beginning of the exposed geologic record because this is the age of the oldest rocks on Earth. Metasedimentary rocks from this time still have an isotopic value of 0 because the biosphere was not developed enough (possibly at all) to fractionate sulfur.Sony VAIO VGN-G2AAPSA Battery

3.5 Ga anoxyogenic photosynthesis is established and provides a weak source of sulfate to the global ocean with sulfate concentrations incredibly low the ?34S is still basically 0. Shortly after, at 3.4 Ga the first evidence for minimal fractionation in evaporitic sulfate in association with magmatically derived sulfides can be seen in the rock record. Sony VAIO VGN-G2AAPSB Battery

This fractionation shows possible evidence for anoxygenic phototrophic bacteria.

2.8 Ga marks the first evidence for oxygen production through photosynthesis. This is important because there cannot be sulfur oxidation without oxygen in the atmosphere. Sony VAIO VGN-G2AAPSC Battery

This exemplifies the coevolution of the oxygen and sulfur cycles as well as the biosphere.

2.7–2.5 Ga is the age of the oldest sedimentary rocks to have a depleted ? 34S which provide the first compelling evidence for sulfate reduction. Sony VAIO VGN-G2KAN Battery

2.3 Ga sulfate increases to more than 1 mM; this increase in sulfate is coincident with the “Great Oxygenation Event", when redox conditions on Earth’s surface are thought by most workers to have shifted fundamentally from reducing to oxidizing.Sony VAIO VGN-G2KANA Battery

This shift would have led to an incredible increase in sulfate weathering which would have led to an increase in sulfate in the oceans. The large isotopic fractionations that would likely be associated with bacteria reduction are produced for the first time. Although there was a distinct rise in seawater sulfate at this time it was likely still only less than 5–15% of present-day levels.Sony VAIO VGN-G2KBNA Battery

At 1.8 Ga, Banded iron formations (BIF) are common sedimentary rocks throughout the Archean and Paleoproterozoic; their disappearance marks a distinct shift in the chemistry of ocean water. BIFs have alternating layers of iron oxides and chert. Sony VAIO VGN-NR160E/S Battery

BIFs only form if the water is be allowed to supersaturate in dissolved iron (Fe2+) meaning there cannot be free oxygen or sulfur in the water column because it would form Fe3+ (rust) or pyrite and precipitate out of solution.Sony VAIO VGN-NR160E/T Battery

Following this supersaturation, the water must become oxygenated in order for the ferric rich bands to precipitate it must still be sulfur poor otherwise pyrite would form instead of Fe3+. It has been hypothesized that BIFs formed during to the initial evolution of photosynthetic organisms that had phases of population growth, causing over production of oxygen. Sony VAIO VGN-NR160E/W Battery

Due to this over production they would poison themselves causing a mass die off, which would cut off the source of oxygen and produce a large amount of CO2 through the decomposition of their bodies, allowing for another bacterial bloom. After 1.8 Ga sulfate concentrations were sufficient to increase rates of sulfate reduction to greater than the delivery flux of iron to the oceans.Sony VAIO VGN-NR180E/S Battery

Along with the disappearance of BIF, the end of the Paleoproterozoic also marks the first large scale sedimentary exhalative deposits showing a link between mineralization and a likely increase in the amount of sulfate in sea water. In the Paleoproterozoic the sulfate in seawater had increased to an amount greater than in the Archean, but was still lower than present day values.Sony VAIO VGN-NR180E/T Battery

The sulfate levels in the Proterozoic also act as proxies for atmospheric oxygen because sulfate is produced mostly through weathering of the continents in the presence of oxygen. The low levels in the Proterozoic simply imply that levels of atmospheric oxygen fell between the abundances of the Phanerozoic and the deficiencies of the Archean.Sony VAIO VGN-NR290E/S Battery

750 million years ago (Ma) there is a renewed deposition of BIF which marks a significant change in ocean chemistry. This was likely due to snowball earth episodes where the entire globe including the oceans was covered in a layer of ice cutting off oxygenation.Sony VAIO VGN-NR290E/T Battery

In the late Neoproterozoic high carbon burial rates increased the atmospheric oxygen level to >10% of its present day value. In the Latest Neoproterozoic another major oxidizing event occurred on Earth’s surface that resulted in an oxic deep ocean and possibly allowed for the appearance of multicellular life.Sony VAIO VGN-NR298E/S Battery

During the last 600 million years, seawater SO4 has varied between +10 and +30‰ in ?34S, with an average value close to that of today. This coincides with atmospheric O levels reaching something close to modern values around the Precambrian–Cambrian boundary.Sony VAIO VGN-NR460E/L Battery

Over a shorter time scale (10 million years) changes in the sulfur cycle are easier to observe and can be even better constrained with oxygen isotopes. Oxygen is continually incorporated into the sulfur cycle through sulfate oxidation and then released when that sulfate is reduced once again.Sony VAIO VGN-NR460E/P Battery

Since different sulfate sources within the ocean have distinct oxygen isotopic values it may be possible to use oxygen to trace the sulfur cycle. Biological sulfate reduction preferentially selects lighter oxygen isotopes for the same reason that lighter sulfur isotopes are preferred. Sony VAIO VGN-NR498E/L Battery

By studying oxygen isotopes in ocean sediments over the last 10 million years   were able to better constrain the sulfur concentrations in sea water through that same time. They found that the sea level changes due to Plioceneand Pleistocene glacial cycles changed the area of continental shelves which then disrupted the sulfur processing, Sony VAIO VGN-NR498E/S Battery

lowering the concentration of sulfate in the sea water. This was a drastic change as compared to preglacial times before 2 million years ago.

Economic importance

Sulfur is intimately involved in production of fossil fuels and a majority of metal deposits because of its ability to act as an oxidizing or reducing agent. Sony VAIO VGN-NR498E/T Battery

The vast majority of the major mineral deposits on Earth contain a substantial amount of sulfur including, but not limited to: sedimentary exhalative deposits(SEDEX), Mississippi Valley-Type (MVT) and copper porphyry deposits. Iron sulfides, galena and sphalerite will form as by-products of hydrogen sulfide generation, Sony VAIO VGN-NR498E/W Battery

as long as the respective transition or base metals are present or transported to a sulfate reduction site. If the system runs out of reactive hydrocarbons economically viable elemental sulfur deposits may form. Sulfur also acts as a reducing agent in many natural gas reservoirs and generally ore forming fluids have a close relationship with ancient hydrocarbon seeps or vents.Sony VAIO VGN-NR50 Battery

Important sources of sulfur in ore deposits are generally deep-seated, but they can also come from local country rocks, sea water, or marine evaporites. The presence or absence of sulfur is one of the limiting factors on both the concentration of precious metals and its precipitation from solution.Sony VAIO VGN-NR50B Battery

pH, temperature and especially redox states determine whether sulfides will precipitate. Most sulfide brines will remain in concentration until they reach reducing conditions, a higher pH or lower temperatures.

Ore fluids are generally linked to metal rich waters that have been heated within a sedimentary basin under the elevated thermal conditions typically in extensional tectonic settings. Sony VAIO VGN-NR51 Battery

The redox conditions of the basin lithologies exert an important control on the redox state of the metal-transporting fluids and deposits can form from both oxidizing and reducing fluids. Metal-rich ore fluids tend to be by necessity comparatively sulfide deficient, so a substantial portion of the sulfide must be supplied from another source at the site of mineralization. Sony VAIO VGN-NR51B Battery

Bacterial reduction of seawater sulfate or a euxinic (anoxic and H2S-containing) water column is a necessary source of that sulfide. When present, the ?34S values of barite are generally consistent with a seawater sulfate source, suggesting barite formation by reaction between hydrothermal barium and sulfate in ambient seawater.Sony VAIO VGN-NR52 Battery

Once fossil fuels or precious metals are discovered and either burned or milled, the sulfur become a waste product which must be dealt with properly or it can become a pollutant. There has been a great increase in the amount of sulfur in our present day atmosphere because of the burning of fossil fuels. Sony VAIO VGN-NR52B Battery

Sulfur acts as a pollutant and an economic resource at the same time.

Human Impact

Human activities have had a major effect on the global sulfur cycle. The burning of coal, natural gas, and other fossil fuels has greatly increased the amount of S in the atmosphere and ocean and depleted the sedimentary rock sink.Sony VAIO VGN-NR71B Battery

Without human impact sulfur would stay tied up in rocks for millions of years until it was uplifted through tectonic events and then released through erosion and weathering processes. Instead it is being drilled, pumped and burned at a steadily increasing rate. Over the most polluted areas there has been a 30-fold increase in sulfate deposition.Sony VAIO VGN-NR71B2 Battery

Although the sulfur curve shows shifts between net sulfur oxidation and net sulfur reduction in the geologic past, the magnitude of the current human impact is probably unprecedented in the geologic record. Human activities greatly increase the flux of sulfur to the atmosphere, some of which is transported globally. Sony VAIO VGN-NR72B Battery

Humans are mining coal and extracting petroleum from the Earth’s crust at a rate that mobilizes 150 x 1012 gS/yr, which is more than double the rate of 100 years ago. The result of human impact on these processes is to increase the pool of oxidized sulfur (SO4) in the global cycle, at the expense of the storage of reduced sulfur in the Earth’s crust. Sony VAIO VGN-NS10E/S Battery

Therefore, human activities do not cause a major change in the global pools of S, but they do produce massive changes in the annual flux of S through the atmosphere.

When SO2 is emitted as an air pollutant, it forms sulfuric acid through reactions with water in the atmosphere. Sony VAIO VGN-NS10J/S Battery

Once the acid is completely dissociated in water thepH can drop to 4.3 or lower causing damage to both man-made and natural systems. According to the EPA acid rain is a broad term referring to a mixture of wet and dry deposition (deposited material) from the atmosphere containing higher than normal amounts of nitric and sulfuric acids. Sony VAIO VGN-NS10L/S Battery

Distilled water (water without any dissolved constituents), which contains no carbon dioxide, has a neutral pH of 7. Rain naturally has a slightly acidic pH of 5.6, because carbon dioxide and water in the air react together to form carbonic acid, a very weak acid. Around Washington, D.C., however, the average rain pH is between 4.2 and 4.4. Sony VAIO VGN-NS110E/L Battery

Since pH is on a log scale dropping by 1 (the difference between normal rain water and acid rain) has a dramatic effect on the strength of the acid. In the United States, roughly 2/3 of all SO2 and 1/4 of all NO3 come from electric power generation that relies on burning fossil fuels, like coal.Sony VAIO VGN-NS110E/S Battery

Stratospheric sulfur aerosols are sulfur-rich particles which exist in the stratosphere region of the Earth's atmosphere. The layer of the atmosphere in which they exist is known as the Junge layer, or simply the stratospheric aerosol layer. These particles consist of a mixture of sulfuric acid and water. Sony VAIO VGN-NS110E/W Battery

They are created naturally, such as by photochemical decomposition of sulfur-containing gases, e.g. carbonyl sulfide. When present in high levels, e.g after a strong volcanic eruption such as Mount Pinatubo, they produce a cooling effect, by reflecting sunlight, and by modifying clouds as they fall out of the stratosphere.Sony VAIO VGN-NS115N/S Battery

This cooling may persist for a few years before the particles fall out.

An aerosol is a suspension of fine solid particles or liquid droplets in a gas. The sulfate particles or sulfuric acid droplets in the atmosphere are about 0.1 to 1.0 micrometer (a millionth of a meter) in diameter.Sony VAIO VGN-NS11ER/S Battery

Sulfur aerosols are common in the troposphere as a result of pollution with sulfur dioxide from burning coal, and from natural processes. Volcanos are a major source of particles in the stratosphere as the force of the volcanic eruption propels sulfur-containing gases into the stratosphere.Sony VAIO VGN-NS11E/S Battery

The relative influence of volcanoes on the Junge layer varies considerably according to the number and size of eruptions in any given time period, and also of quantities of sulfur compounds released. Onlystratovolcanoes containing primarily granitic rocks are responsible for these fluxes, as basaltic rock erupted in shield volcanoes doesn't result in plumes which reach the stratosphere.Sony VAIO VGN-NS11J/S Battery

Creating stratospheric sulfur aerosols deliberately is a proposed geoengineering technique which offers a possible solution to some of the problems caused by global warming. However, this will not be without side effects and it has been suggested that the cure may be worse than the disease.Sony VAIO VGN-NS11L/S Battery

Natural sulfur aerosols are formed in vast quantities from the SO2 ejected by volcanoes, which may be injected directly into the stratosphere during very large (Volcanic Explosivity Index, VEI, of 4 or greater) eruptions. A comprehensive analysis, dealing largely with tropospheric sulfur compounds in the atmosphere, is provided by Bates et al.Sony VAIO VGN-NS11MR/S Battery

The IPCC AR4 says explosive volcanic events are episodic, but the stratospheric aerosols resulting from them yield substantial transitory perturbations to the radiative energy balance of the planet, with both shortwave and longwave effects sensitive to the microphysical characteristics of the aerosols.Sony VAIO VGN-NS11M/S Battery

As of 2009 there have been no explosive and climatically significant volcanic events since Mt. Pinatubo and consequently stratospheric aerosol concentrations are at the lowest concentrations since the satellite era and global coverage began in about 1980.Sony VAIO VGN-NS11SR/S Battery

During periods lacking volcanic activity (and thus direct injection of SO2 into the stratosphere), oxidation of COS (carbonyl sulfide) dominates the production of stratospheric sulfur aerosol.

Chemistry

The chemistry of stratospheric sulfur aerosols varies significantly according to their source.Sony VAIO VGN-NS11S/S Battery

Volcanic emissions vary significantly in composition, and have complex chemistry due to the presence of ash particulates and a wide variety of other elements in the plume.

The chemical reactions affecting both the formation and elimination of sulfur aerosols are not fully understood.Sony VAIO VGN-NS11ZR/S Battery

It is difficult to estimate accurately, for example, whether the presence of ash and water vapour is important for aerosol formation from volcanic products, and whether high or low atmospheric concentrations of precursor chemicals (such as SO2 and H2S) are optimal for aerosol formation. Sony VAIO VGN-NS11Z/S Battery

This uncertainty makes it difficult to determine a viable approach for geoengineering uses of sulfur aerosol formation.

Scientific study

Understanding of these aerosols comes in large part from the study of volcanic eruptions, notably Mount Pinatubo in thePhilippines, which erupted in 1991 when scientific techniques were sufficiently far advanced to study the effects carefully.Sony VAIO VGN-NS12M/S Battery

The formation of the aerosols and their effects on the atmosphere can also be studied in the lab. Samples of actual particles can be recovered from the stratosphere using balloons or aircraft.

Computer models can be used to understand the behaviour of aerosol particles, and are particularly useful in modelling their effect on global climate.Sony VAIO VGN-NS12M/W Battery

Biological experiments in the lab, and field/ocean measurements can establish the formation mechanisms of biologically derived volatile sulfurous gases.

Effect on climate

It has been established that emission of precursor gases for sulfur aerosols is the principle mechanism by which volcanoes cause episodic global cooling.Sony VAIO VGN-NS12S/S Battery

The Intergovernmental Panel on Climate Change AR4 regards stratospheric sulfate aerosols as having a low level of scientific understanding.

The aerosol particles form a whitish  haze in the sky. This creates a global dimming effect, where less of the sun's radiation is able to reach the surface of the Earth.Sony VAIO VGN-NS21M/P Battery

This leads to a global cooling effect. In essence, they act as the reverse of a greenhouse gas, which tends to allow visible light from the sun through, whilst blocking infra-red emitted from the Earth's surface and its atmosphere. The particles also radiate infra red energy directly, as they lose heat into space.Sony VAIO VGN-NS21M/W Battery

Effects on light transmission through the atmosphere

All aerosols both absorb and scatter solar and terrestrial radiation. This is quantified in the Single Scattering Albedo (SSA), the ratio of scattering alone to scattering plus absorption (extinction) of radiation by a particle. Sony VAIO VGN-NS21S/S Battery

The SSA tends to unity if scattering dominates, with relatively little absorption, and decreases as absorption increases, becoming zero for infinite absorption. For example, sea-salt aerosol has an SSA of 1, as a sea-salt particle only scatters, whereas soot has an SSA of 0.23, showing that it is a major atmospheric aerosol absorber.Sony VAIO VGN-NS21S/W Battery

Aerosols, natural and anthropogenic, can affect the climate by changing the way radiation is transmitted through the atmosphere. Direct observations of the effects of aerosols are quite limited so any attempt to estimate their global effect necessarily involves the use of computer models. Sony VAIO VGN-NS21Z/S Battery

The Intergovernmental Panel on Climate Change, IPCC, says: While the radiative forcing due to greenhouse gases may be determined to a reasonably high degree of accuracy... the uncertainties relating to aerosol radiative forcings remain large, and rely to a large extent on the estimates from global modelling studies that are difficult to verify at the present time.Sony VAIO VGN-NS31M/P Battery

However, they are mostly talking about tropospheric aerosol.

Effect on ozone

The aerosols have a role in the destruction of ozone due to surface chemistry effects. Destruction of ozone has in recent years created large holes in the ozone layer, initially over the Antarctic and then theArctic. Sony VAIO VGN-NS31M/W Battery

These holes in the ozone layer have the potential to expand to cover inhabited and vegetative regions of the planet, leading to catastrophic environmental damage.

Ozone destruction occurs principally in polar regions, but the formation of ozone occurs principally in the tropics.Sony VAIO VGN-NS31S/S Battery

Ozone is distributed around the planet by the Brewer-Dobson circulation.Therefore, the source and dispersal pattern of aerosols is critical in understanding their effect on the ozone layer.

Effect on sky appearance

Aerosols scatter light, which affects the appearance of the sky and of sunsets. Changing the concentration of aerosols in theatmosphere can dramatically affect the appearance of sunsets. Sony VAIO VGN-NS31Z/P Battery

A change in sky appearance during the year without a summer(attributed to the eruption of Mount Tambora) was the inspiration for the paintings of J. M. W. Turner. Further volcanic eruptions and geoengineering projects involving sulfur aerosols are likely to affect the appearance of sunsets significantly,and to create a haze in the sky.Sony VAIO VGN-NS31Z/S Battery

Effect on the biosphere

Aerosol particles are eventually deposited from the stratosphere onto land and ocean. Depending on the volume of particles descending, the effects may be significant to ecosystems, or may not be. Modelling of the quantities of aerosols used in likelygeoengineering scenarios suggest that effects on terrestrial ecosystems from deposition is not likely to be significantly harmful.Sony VAIO VGN-NS31Z/W Battery

Geoengineering

The ability of stratospheric sulfur aerosols to create this global dimming effect has made them a possible candidate for use in geoengineering projects to limit the effect and impact of climate change due to rising levels of greenhouse gases. Delivery of precursor gases such as H2S and SO2 by artillery, aircraft andballoons has been proposed.Sony VAIO VGN-NS38M/P Battery

Understanding of this proposed technique is partly based on the fact that it is the adaptation of an existing atmospheric process. The technique is therefore potentially better understood than are comparable (but purely speculative) geoengineering schemes. Sony VAIO VGN-NS38M/W Battery

It is also partly based on the speed of action of any such solution deployed, in contrast to carbon sequestration projects such as carbon dioxide air capture which would take longer to work. However, gaps in understanding of these processes exist, for example the effect on stratospheric climate and on rainfall patterns, and further research is needed.Sony VAIO VGN-NS130E/L Battery

This technique has been studied more extensively than most other geoengineering techniques, and is suggested by commentators including Tom Wigley.

In chemistry, a disulfide bond (Br.E. disulphide bond) is a covalent bond, usually derived by the coupling of two thiol groups.Sony VAIO VGN-NS130E/S Battery

The linkage is also called anSS-bond or disulfide bridge. The overall connectivity is therefore R-S-S-R. The terminology is widely used in biochemistry. In formal terms, the connection is a persulfide, in analogy to its congener, peroxide (R-O-O-R), but this terminology is obscure and is no longer used (except in reference to R-S-S-H or H-S-S-H compounds).Sony VAIO VGN-NS130E/W Battery

Properties

The disulfide bond is strong, with a typical bond dissociation energy of 60 kcal/mole. Being about 40% weaker than C-C and C-H bonds, the disulfide bond is thus often the "weak link" in many molecules.Sony VAIO VGN-NS140E/L Battery

Furthermore, reflecting the polarizability of divalent sulfur, the S-S bond is susceptible to scission by polar reagents, both electrophiles and especially nucleophiles:

RS-SR + Nu- ? RS-Nu + RS-

The disulfide bond is about 2.05 Å in length, about 0.5 Å longer than a C-C bond. Rotation about the S-S axis is subject to a low barrier.Sony VAIO VGN-NS140E/S Battery

Disulfides show a distinct preference for dihedral angles approaching 90°. When the angle approaches 0° or 180°, then the disulfide is a significantly better oxidant.

Disulfides where the two R groups are the same are called symmetric, examples being diphenyl disulfide and dimethyl disulfide. When the two R groups are not identical, the compound is said to be an unsymmetric or mixed disulfide.Sony VAIO VGN-NS140E/W Battery

Although the hydrogenation of disulfides is usually not practical, the equilibrium constant for the reaction provides a measure of the standard redox potential for disulfides:

RSSR + H2 ? 2 RSH

This value is about -250 mV vs NHE (pH = 7). By comparison, the standard reduction potential for ferrodoxins is about -430 mV.Sony VAIO VGN-NS190J/L Battery

Formation of disulfides

Disulfide bonds are usually formed from the oxidation of sulfhydryl (-SH) groups, especially in biological contexts. The transformation is depicted as follows:

2 RSH ? RS-SR + 2 H+ + 2 e-

A variety of oxidants promote this reaction including air and hydrogen peroxide. Sony VAIO VGN-NS190J/S Battery

Such reactions are thought to proceed via sulfenic acid intermediates. In the laboratory, iodine in the presence of base is commonly employed to oxidize thiols to disulfides. Several metals, such as copper(II) and iron(III) complexes affect this reaction. Alternatively, disulfide bonds in proteins often formed by thiol-disulfide exchange:Sony VAIO VGN-NS190J/W Battery

RS-SR + R'SH   R'S-SR + RSH

Such reactions are mediated by enzymes in some cases and in other cases are under equilibrium control, especially in the presence of a catalytic amount of base.

The alkylation of alkali metal di- and polysulfides gives disulfides. "Thiokol" polymers arise when sodium polysulfide is treated with an alkyl dihalide. Sony VAIO VGN-NS235J/L Battery

In the converse reaction, carbanionic reagents react with elemental sulfur to afford mixtures of the thioether, disulfide, and higher polysulfides. These reactions are often unselective but can be optimized for specific applications.

Many specialized methods have been developed for forming disulfides, usually for applications in organic synthesis. Reagents that deliver the equivalent of "RS+" react with thiols to give asymmetrical disulfidesSony VAIO VGN-NS235J/P Battery

RSH + R'SNR"2 ? RS-SR' + HNR"2, where R"2N = phthalimido

Scission of disulfides

The most important reaction of disulfide bonds is their cleavage, which occurs via reduction. A variety of reductants can be used.Sony VAIO VGN-NS235J/S Battery

In biochemistry, thiols such asmercaptoethanol (b-ME) or dithiothreitol (DTT) serve as reductants, the thiol reagents are used in excess to drive the equilibrium to the right:

RS-SR + HOCH2CH2SH   HOCH2CH2S-SCH2CH2OH + 2 RSH

The reductant Tris(2-carboxyethyl)phosphine (TCEP) is useful, beside being odorless compared to b-ME and DTT, Sony VAIO VGN-NS235J/W Battery

because it is selective, working at both alkaline and acidic conditions (unlike DTT), is more hydrophilic and more resistant to oxidation in air. Furthermore, it is often not needed to remove TCEP before modification of protein thiols .

In organic synthesis, hydride agents are typically employed for scission of disulfides, such as sodium borohydride. More aggressive, alkali metals will effect this reaction:Sony VAIO VGN-NS240E/L Battery

RS-SR + 2 Na ? 2 NaSR

These reactions are often followed by protonation of the resulting metal thiolate:

NaSR + HCl ? HSR + NaCl

Thiol-disulfide exchange is a chemical reaction in which a thiolate group -S? attacks a sulfur atom of a disulfide bond -S-S-. Sony VAIO VGN-NS290J/L Battery

The original disulfide bond is broken, and its other sulfur atom (green atom in Figure 1) is released as a new thiolate, carrying away the negative charge. Meanwhile, a new disulfide bond forms between the attacking thiolate (red atom in Figure 1) and the original sulfur atom (blue atom in Figure 1).Sony VAIO VGN-NS290J/S Battery

Thiolates, not thiols, attack disulfide bonds. Hence, thiol-disulfide exchange is inhibited at low pH (typically, below 8) where the protonated thiol form is favored relative to the deprotonated thiolate form. (The pKa of a typical thiol group is roughly 8.3, but can vary due to its environment.)Sony VAIO VGN-NS50B/L Battery

Thiol-disulfide exchange is the principal reaction by which disulfide bonds are formed and rearranged in a protein. The rearrangement of disulfide bonds within a protein generally occurs via intra-protein thiol-disulfide exchange reactions; a thiolate group of a cysteine residue attacks one of the protein's own disulfide bonds. Sony VAIO VGN-NS50B/W Battery

This process of disulfide rearrangement (known as disulfide shuffling) does not change the number of disulfide bonds within a protein, merely their location (i.e., which cysteines are bonded). Disulfide reshuffling is generally much faster than oxidation/reduction reactions, which change the number of disulfide bonds within a protein.Sony VAIO VGN-NS51B/L Battery

The oxidation and reduction of protein disulfide bonds in vitro also generally occurs via thiol-disulfide exchange reactions. Typically, the thiolate of a redox reagent such as glutathione or dithiothreitol attacks the disulfide bond on a protein forming a mixed disulfide bond between the protein and the reagent. Sony VAIO VGN-NS51B/P Battery

This mixed disulfide bond when attacked by another thiolate from the reagent, leaves the cysteine oxidised. In effect, the disulfide bond is transferred from the protein to the reagent in two steps, both thiol-disulfide exchange reactions.

The in vivo oxidation and reduction of protein disulfide bonds by thiol-disulfide exchange is facilitated by a protein called thioredoxin. Sony VAIO VGN-NS51B/W Battery

This small protein, essential in all known organisms, contains two cysteine amino acid residues in a vicinal arrangement (i.e., next to each other), which allows it to form an internal disulfide bond, or disulfide bonds with other proteins. As such, it can be used as a repository of reduced or oxidized disulfide bond moieties.Sony VAIO VGN-NS52JB/L Battery

Other reactions of disulfides

Many specialized organic reactions have been developed for disulfides, again mainly associated with the scission of the S-S bond, which is usually the weakest bond in a molecule. In the Zincke disulfide cleavage reactions, disulfides are cleaved to give the to a sulfenyl halide by reaction with bromine or chlorine.Sony VAIO VGN-NS52JB/P Battery

Occurrence in proteins

Disulfide bonds play an important role in the folding and stability of some proteins, usually proteins secreted to the extracellular medium. Since most cellular compartments are reducing environm

ents, in general, disulfide bonds are unstable in the cytosol, with some exceptions as noted below, unless a sulfhydryl oxidase is present.Sony VAIO VGN-NS52JB/W Battery

Disulfide bonds in proteins are formed between the thiol groups of cysteine residues. The other sulfur-containing amino acid, methionine, cannot form disulfide bonds. A disulfide bond is typically denoted by hyphenating the abbreviations for cysteine, e.g.,Sony VAIO VGN-NS70B/W Battery

when referring toRibonuclease A the "Cys26-Cys84 disulfide bond", or the "26-84 disulfide bond", or most simply as "C26-C84"[10]where the disulfide bond is understood and does not need to be mentioned. The prototype of a protein disulfide bond is the two-amino-acid peptide cystine, Sony VAIO VGN-NS71B/W Battery

which is composed of two cysteine amino acids joined by a disulfide bond (shown in Figure 2 in its unionized form). The structure of a disulfide bond can be described by its ?ss dihedral angle between the C? ? S? ? S? ? C? atoms, which is usually close to ±90°.

The disulfide bond stabilizes the folded form of a protein in several ways:Sony VAIO VGN-NS72JB/W Battery

1) It holds two portions of the protein together, biasing the protein towards the folded topology. That is, the disulfide bond destabilizes the unfolded form of the protein by lowering its entropy.

2) The disulfide bond may form the nucleus of a hydrophobic core of the folded protein, i.e., local hydrophobic residues may condense around the disulfide bond and onto each other through hydrophobic interactions.Sony VAIO VGN-NS90HS Battery

3) Related to #1 and #2, the disulfide bond link two segments of the protein chain, the disulfide bond increases the effective local concentration of protein residues and lowers the effective local concentration of water molecules. Since water molecules attack amide-amidehydrogen bonds and break up secondary structure, a disulfide bond stabilizes secondary structure in its vicinity.Sony VAIO VGN-NS92JS Battery

For example, researchers have identified several pairs of peptides that are unstructured in isolation, but adopt stable secondary and tertiary structure upon forming a disulfide bond between them.

A disulfide species is a particular pairing of cysteines in a disulfide-bonded protein and is usually depicted by listing the disulfide bonds in parentheses, e.g., the "(26-84, 58-110) disulfide species".Sony VAIO VGN-NS92XS Battery

A disulfide ensemble is a grouping of all disulfide species with the same number of disulfide bonds, and is usually denoted as the 1S ensemble, the 2S ensemble, etc. for disulfide species having one, two, etc. disulfide bonds. Thus, the (26-84) disulfide species belongs to the 1S ensemble, whereas the (26-84, 58-110) species belongs to the 2S ensemble. Sony VAIO VGN-NW11S/S Battery

The single species with no disulfide bonds is usually denoted as R for "fully reduced". Under typical conditions, disulfide reshuffling is much faster than the formation of new disulfide bonds or their reduction; hence, the disulfide species within an ensemble equilibrate more quickly than between ensembles.Sony VAIO VGN-NW11S/T Battery

The native form of a protein is usually a single disulfide species, although some proteins may cycle between a few disulfide states as part of their function, e.g.,thioredoxin. In proteins with more than two cysteines, non-native disulfide species may be formed, which are almost always unfolded.Sony VAIO VGN-NW11Z/S Battery

As the number of cysteines increases, the number of nonnative species increases factorially. The number of ways of forming pdisulfide bonds from n cysteine residues is given by the formula.

For example, an eight-cysteine protein such as ribonuclease A has 105 different four-disulfide species, only one of which is the native disulfide species. Sony VAIO VGN-NW11Z/T Battery

Isomerases have been identified that catalyze the interconversion of disulfide species, accelerating the formation of the native disulfide species.

Disulfide species that have only native disulfide bonds (but not all of them) are denoted by des followed by the lacking native disulfide bond(s) in square brackets. Sony VAIO VGN-NW21EF/S Battery

For example, the des[40-95] disulfide species has all the native disulfide bonds except that between cysteines 40 and 95. Disulfide species that lack one native disulfide bond are frequently folded, in particular, if the missing disulfide bond is exposed to solvent in the folded, native protein.Sony VAIO VGN-NW21JF Battery

In order to analyze the structure of proteins, it is often necessary to break disulfide bonds. This reduction of disulfide bonds can be accomplished by treatment with 2-mercaptoethanol, dithiothreitol, or tris(2-carboxyethyl)phosphine.

In prokaryotes and archaea

Disulfide bonds play an important protective role for bacteria as a reversible switch that turns a protein on or off when bacterial cells are exposed to oxidationreactions.Sony VAIO VGN-NW21MF Battery

Hydrogen peroxide (H2O2) in particular could severely damage DNA and kill the bacterium at low concentrations if not for the protective action of the SS-bond. Archaea typically have fewer disulfides than higher organisms.

In eukaryotes

In eukaryotic cells, in general, disulfide bonds are formed in the lumen of the RER (rough endoplasmic reticulum) but not in the cytosol.Sony VAIO VGN-NW21MF/W Battery

This is due to the oxidative environment of the ER and the reducing environment of the cytosol (see glutathione). Thus disulfide bonds are mostly found in secretory proteins, lysosomal proteins, and the exoplasmic domains of membrane proteins.Sony VAIO VGN-NW21ZF Battery

Notable exceptions to this rule include a number of cytosolic proteins which have cysteine residues in proximity to each other that function as oxidation sensors; when the reductive potential of the cell fails, they oxidize and trigger cellular response mechanisms.Sony VAIO VGN-NW31EF/W Battery

Vaccinia virus also produces cytosolic proteins and peptides that have many disulfide bonds; although the reason for this is unknown presumably they have protective effects against intracellular proteolysis machinery.

Disulfide bonds are also formed within and between protamines in the sperm chromatin of many mammalian species.Sony VAIO VGN-NW31JF Battery

Disulfides in regulatory proteins

As disulfide bonds can be reversibly reduced and re-oxidized, the redox state of these bonds has evolved into a signaling element. In chloroplasts, for example, the enzymatic reduction of disulfide bonds has been linked to the control of numerous metabolic pathways as well as gene expression. Sony VAIO VGN-NW320F/B Battery

The reductive signaling activity has been shown, thus far, to be carried by the ferredoxin thioredoxin system, channeling electrons from the light reactions of photosystem I to catalytically reduce disulfides in regulated proteins in a light dependent manner.Sony VAIO VGN-NW320F/TC Battery

In this way chloroplasts adjust the activity of key processes such as the Calvin-Benson cycle, starchdegradation, ATP production and gene expression according to light intensity.

In hair and feathers

Over 90% of the dry weight of hair comprises proteins called keratins, which have a high disulfide content, from the amino acid cysteine. Sony VAIO VGN-NW35E Battery

The robustness conferred in part by disulfide linkages is illustrated by the recovery of virtually intact hair from ancient Egyptian tombs. Feathers have similar keratins and are extremely resistant to protein digestive enzymes. Different parts of the hair and feather have different cysteine levels, leading to harder or softer material. Sony VAIO VGN-NW380F/S Battery

Manipulating disulfide bonds in hair is the basis for the permanent wave in hairstyling. Reagents that affect the making and breaking of S-S bonds are key, e.g., ammonium thioglycolate. The high disulfide content of feathers dictates the high sulfur content of bird eggs. Sony VAIO VGN-NW380F/T Battery

The high disulfide content of hair and feathers contributes to the disagreeable odor that results when they are burned.

In industry

Disulfide and (polysulfide) bonds are the crosslinking groups that result from the vulcanization of rubber. In analogy to the role of disulfides in proteins, the S-S linkages in rubber are crosslinkers, and strongly affect the rheology of the material.Sony VAIO VGN-NW50JB Battery

Related compounds

Thiosulfoxides are orthogonally isomeric with disulfides, having the second sulfur branching from the first and not partaking in a continuous chain. i.e. -S(=S)- rather than -S-S-.

Disulfide bonds are analogous but more common than related peroxides and diselenide bonds.Sony VAIO VGN-NW51FB/N Battery

Intermediate compounds of these also exist, for example, thioperoxides, also known as oxadisulfide bonds, have formula R1OSR2(equivalently R2SOR1). These are isomeric to sulfoxides in a similar manner to the above; i.e. -S(=O)- rather than -S-O-.Sony VAIO VGN-NW51FB/W Battery

Thiuram disulfides, with the formula (R2NC(S)S)2, are disulfides but they behave distinctly because of the thiocarbonyl group.

Compounds with three sulfur atoms, e.g., CH3S-S-SCH3, are called trisulfides, or trisulfide bonds.Sony VAIO VGN-NW70JB Battery,Sony VAIO VGN-NW71FB/N Battery,Sony VAIO VGN-NW71FB/W Battery,Sony VAIO VGN-NW91FS Battery,Sony VAIO VGN-NW91GS Battery,Sony VAIO VGN-NW91VS Battery

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