Arsenate: Difference between revisions

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| StdInChIKey_Ref = {{stdinchicite|changed|chemspider}}
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| Formula={{chem|AsO<sub>|4</sub><sup>|3−</sup>}}
| MolarMass=138.919
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|Section3={{Chembox Hazards
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The '''arsenate''' [[ion]] is {{chem|[[Arsenic|As]][[Oxygen|O]]<sub>|4</sub><sup>3&minus;</sup>|3−}}.
An '''arsenate''' (compound) is any [[chemical compound|compound]] that contains this ion. Arsenates are [[salt]]s or [[ester]]s of [[arsenic acid]].
The arsenic atom in arsenate has a [[valency (chemistry)|valency]] of 5 and is also known as '''pentavalent arsenic''' or '''As[(V])'''.
Arsenate resembles [[phosphate]] in many respects, since [[arsenic]] and [[phosphorus]] occur in the same group (column) of the [[periodic table]].
Arsenates are moderate oxidizers, with an [[electrode potential]] of +0.56&nbsp;[[Volt|V]] for reduction to [[arsenite]]s.
 
==Occurrence==
 
==Ions==
 
* In strongly [[acid]]ic conditions it exists as [[arsenic acid]], H<sub>3</sub>AsO<sub>4</sub>;
* in weakly acidic conditions it exists as '''dihydrogen arsenate''' ion, {{chem|H<sub>|2</sub>|AsO<sub>|4</sub><sup>&minus;</sup>|−}};
* in weakly basic conditions it exists as '''hydrogen arsenate''' ion {{chem|HAsO<sub>|4</sub><sup>2&minus;</sup>|2−}};
* and finally, in strongly basic conditions, it exists as the arsenate ion {{chem|AsO<sub>|4</sub><sup>3&minus;</sup>|3−}}.
 
== Arsenate poisoning ==
Arsenate can replace inorganic [[phosphate]] in the step of [[glycolysis]] that produces [[1,3-bisphosphoglycerate]] from [[glyceraldehyde 3-phosphate]]. This yields [[1-arseno-3-phosphoglycerate]] instead, which is unstable and quickly hydrolyzes, forming the next intermediate in the pathway, [[3-phosphoglycerate]]. Therefore glycolysis proceeds, but the [[Adenosine triphosphate|ATP]] molecule that would be generated from [[1,3-bisphosphoglycerate]] is lost - arsenate is an uncoupler of glycolysis, explaining its toxicity.<ref>{{cite journal|last=Hughes|first=Michael F.|title=Arsenic toxicity and potential mechanisms of action|journal=[[Toxicology Letters]]|year=2002|issue=133|pages=4|url=http://nature.berkeley.edu/departments/nut/undergrad_class/Spr_2003_NST_120/D50.pdf}}</ref>
 
As with other arsenic compounds, arsenate can also inhibit the conversion of [[pyruvate]] into [[acetyl-CoA]], blocking the [[Krebs cycle]] and therefore resulting in further loss of ATP.<ref>{{citation|title=Arsenic Toxicity Case Study|author1=Kim Gehle|author2=Selene Chou|author3=William S. Beckett|publisher=Agency for Toxic Substances and Disease Registry|url=http://www.atsdr.cdc.gov/csem/csem.asp?csem=1&po=0|date=2009-10-01}}</ref>
Some species of [[bacteria]] obtain their energy by [[redox|oxidizing]] various fuels while [[redox|reducing]] arsenates to form [[arsenite]]s. The [[enzyme]]s involved are known as [[arsenate reductase]]s.
 
In 2008, bacteria were discovered that employ a version of [[photosynthesis]] with arsenites as [[electron donor]]s, producing arsenates (just like ordinary photosynthesis uses water as electron donor, producing molecular oxygen). The researchers conjectured that historically these photosynthesizing organisms produced the arsenates that allowed the arsenate-reducing bacteria to thrive.<ref>[http://www.rsc.org/chemistryworld/News/2008/August/15080802.asp "Arsenic-loving bacteria rewrite photosynthesis rules"], ''Chemistry World'', 15 August 2008</ref>
 
In 2010, a team at [[NASA]]'s [[NASA Astrobiology Institute|Astrobiology Institute]] cultured samples of arsenic-resistant [[GFAJ-1]] bacteria from [[Mono Lake]], using a medium high in arsenate and low in phosphate concentration. The findings suggest that the bacteria may partially incorporate arsenate in place of phosphate in some biomolecules, including DNA,<ref>[http://www.sciencemag.org/content/early/2010/12/01/science.1197258.short "A Bacterium That Can Grow by Using Arsenic Instead of Phosphorus"]. Wolfe-Simon F., Blum JSJ.S., Kulp TRT.R., Gordon GWG.W., Hoeft SES.E., Pett-RdigeRidge J., Stolz JFJ.F., Webb SMS.M., Weber PKP.K., Davies PCWP.C.W., Anbar ADA.D., Oremland RSR.S. ''Science Express'' Express. 2 December 2010.</ref><ref>[http://www.wired.com/wiredscience/2010/12/nasa-finds-arsenic-life-form/ "NASA Finds New Arsenic-Based Life Form in California"], ''Wired Science'', 2 December 2010</ref> However, these claims were immediately debated and critiqued in correspondence to the original journal of publication,<ref>Wolfe-Simon, F., Blum, J.S., Kulp, T.R., Gordon, G.W., Hoeft, S.E., Pett-Ridge, J., Stolz, J.F., Webb, S.M., Weber, P.K., Davies, P.C.W., Anbar, A.D. & Oremland, R.S. Response to Comments on "A Bacterium That Can Grow Using Arsenic Instead of Phosphorus", ''Science'', 27 May 2011, and references therein. Bibcode 2011Sci...332.1149W. doi:10.1126/science.1202098. Accessed 30 May 2011</ref> and have since come to be widely disbelieved.<ref>Drahl, C. "The Arsenic-Based-Life Aftermath. Researchers challenge a sensational claim, while others revisit arsenic biochemistry", ''Chem. Eng. News'' '''90'''(5), 42-4742–47, 30 January 2012. http://cen.acs.org/articles/90/i5/Arsenic-Based-Life-Aftermath.html; accessed 13 October 2012</ref> Reports refuting the most significant aspects of the original results have been published in the journal of the original research in 2012, including by researchers from the [[University of British Columbia]] and [[Princeton University]].<ref>''Science''. 8 July 2012. "GFAJ-1 Is an Arsenate-Resistant, Phosphate-Dependent Organism." doi: 10.1126/science.1218455. Accessed 10 July 2012.</ref><ref>''Science''. 8 July 2012. "Absence of Detectable Arsenate in DNA from Arsenate-Grown GFAJ-1 Cells."</ref> Following the publication of the articles challenging the conclusions of the original Science article first describing GFAJ-1 it was argued that the original article should be retracted because of misrepesentation of critical data.<ref>http://retractionwatch.wordpress.com/2012/07/09/despite-refutation-science-arsenic-life-paper-deserves-retraction-scientist-argues/#comments</ref>
 
==See also==
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