Arsenite hydrolysis

As stated in Section I, arsenate and phosphate are very similar. Hence, organisms have difficulty in assimilating phosphate without taking up arsenate, and this will uncouple their metabolism (Section II). They make use of an important difference between arsenate and phosphate to avoid this, i.e., the vastly greater ease of reduction of arsenate to arsenite than of phosphate to phosphite. By itself such a reduction would be no help, since arsenite is intensely toxic, but further processes can follow, such as alkylation to produce organoarsenic compounds (6), or extrusion of arsenite from the organism (e.g., 36, 37), driven by hydrolysis of ATP (38). Extruded arsenite may then be rendered less toxic by oxidation to arsenate by the arsenite oxidase mentioned in section III,A. 

Oxoalkylarsonic acids, exemplified by arsonoacetaldehyde and 3-arsonopyruvate, are only moderately stable they release arsenate by hydrolysis, rather than arsenite by elimination. They are presumably hydrolyzed by the mechanism of Fig. 7, but they do not need the added electron attraction of forming the imine E—-NH+=CR—CH2— As03H2 0=CR—CH2—As03H2 is fairly easily attacked by water. 

Althongh lewisite is only slightly soluble in water, 0.5 g/L (Rosenblatt et al., 1975), hydrolysis, resulting in the formation of lewisite oxide and HCl is rapid. Qi-Lewisite mnst be heated to over 40°C to react with NaOH to yield vinyl chloride, sodium arsenite, and acetylene (Rosenblatt et al., 1975). In aqneons solution, the cis isomer nndergoes a photoconversion to the trans isomer (Rosenblatt et al., 1975). Upon standing in water, the toxic trivalent arsenic of lewisite oxide is converted to the less toxic pentavalent arsenic (Epstein, 1956). 

The thiol (SH) group is introduced by reaction with potassium ethyl xanthate followed by acid hydrolysis. The phenylsulfanyl (phenylthio, SPh) group results from reaction with benzenethiolate ion. Sodium disulfide, Na2S2, yields diaryl disulfides. The arsonic acid group is introduced using Bart s reaction, in which a diazonium salt is reacted with sodium arsenite in the presence of a Cu(II) salt (Scheme 8.23). 

Another example of aqueous speciation that includes redox can be shown with the arsenic pe-pH diagram shown in Figure 1. Arsenic can exist in several oxidation states including As(-lll) as in arsine gas (ASH3), As(0) as in elemental arsenic, As(ll) as in realgar (AsS), As(lll) as in orpiment (AS2S3) and dissolved arsenite, and As(V) as in dissolved arsenate. Figure 1 shows the dominant dissolved species, arsenate and arsenite, and their hydrolysis products as a function of redox potential and pH based on the thermodynamic evaluation of Nordstrom and Archer (2003). These results show the dominance of hydrolysis for arsenate species, but it is of minor consequence for the arsenite species. 

The C-methyl derivatives (350) and (351) have been synthesized by hydrolysis and thiolysis of ethylidene bis(dibromoarsine) (349) (Equation (53)). The adamantane (350) was also synthesized simply by heating potassium arsenite with propionic acid in propionic anhydride <69ZAAC(370)3l>. The tetra-A-ethyl compound (352) was obtained from (349 Hal = Cl) by the same method as described for the synthesis of (348). 

Oxoalkylarsonic acids, exemplifled by arsonoacetaldehyde and 3-arsonopyruvate, are only moderately stable they release arsenate by hydrolysis, rather than arsenite by elimination. They are presumably hydrolyzed by the mechanism of Fig. 7, but they do not need the added electron attraction of forming the imine E—NH =CR—CH2— AsOgHg 0=CR—CH2—ASO3H2 is fairly easily attacked by water. 2-Oxoalkylphosphonic acids can be hydrolyzed in this way (119), as in the enzyme-catalyzed hydrolysis of phosphonoacetaldehyde (73, 69-71), but with much more difficulty. The lability of 2-oxoalkylarsonic acids presumably reflects the ease with which water can more easily enter the coordination shell of arsenic than that of phosphorus, because of the larger size of the atom, i.e., the same feature expressed in the lability of esters and anhydrides of arsenate. 

ArsA is a membrane-associated ATPase (see Fig. 3) (45,46) attached to the ArsB inner-membrane protein (30,47) and energizing the arsenite efflux pump by ATP hydrolysis (33,39). Such alternative energy coupling is unique among known bacterial uptake or efflux transport systems. To date, all other systems that have been studied are either membrane potential-driven or ATP-driven transporters. The ArsAB pump is the only one that can be converted from one form of energy coupling to the other by addition or removal of genes. This is a natural phenomenon (8) and also can be reconstructed in laboratory studies (33). 

Considering the extremely low persistence of lewisite and the long time period after the stoppage of its production, only transformation products may be found in soil, e.g. p-chlorovinylarsineoxide (a sufficiently stable product of lewisite s hydrolysis), arsenic oxide as well as arsenates and arsenites. Due to this, the collected samples of soil, water and sediments were first analysed for their gross arsenic content. 

Naphthylamino-6-sulfonylamide added at 60° to an aq. soln. of NH4HSO4 and Na-arsenite, heated 10 hrs. at 225-230°/26-29 atm. in an autoclave 6-sulfamoyl-2-naphthol. Y 80.3%.—Hydrolysis of the sulfamoyl group and desulfonation can be prevented by the use of the above reagents. F. e. s. W. Wojtkiewicz and Z. Jankowski, Zeszyty Nauk Politech. Lodz Chem. 13, 46 (1963). 

It has been shown that both the acid conditions used and the presence of molybdate can enhance hydrolysis of dissolved organic and condensed phosphates to give an overestimate of orthophosphate [91,92]. Similarly, colloidal phosphates in the filterable fraction may be molybdate reactive, which again will lead to an overestimation of the orthophosphate concentration [93]. Rigler observed that this overestimation of the true orthophosphate concentration may be as much as 10-100 times the true concentration of orthophosphate [94]. In attempts to avoid these hydrolytic effects, a "6-second extraction method" was developed in which phosphomolybdate formed was rapidly removed from the acidic environment [95], or excess molybdate was complexed with a citrate-arsenite reagent [96]. 

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