2- Oxoalkylarsonic acids

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. 

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. 

This mechanism is supported by the fact that every 3-oxoalkylarsonic acid studied also eliminated arsenite. First, 3-hyroxypropylarsonic acid proved to be a substrate for alcohol dehydrogenase from yeast, and similarly eliminated arsenite. Further, the oxidation by periodate of the HO—CH2—CHOH—CH2—CH2—As03H2, expected to produce 0=CH—CH2—CH2—As03H2 (another 3-oxoalkylarsonic acid), also yielded arsenite, and conditions that normally transaminate amino acids, when applied to the glutamate analogue HOOC—CH(—NH2)—... 

Oxoalkylarsonic acids lose arsenite by elimination, presumably after enolization, as described earlier (Section IV,B,2,c, and Fig. 9). In contrast, 3-oxoalkylphosphonic acids are stable e.g., the phosphono-methyl analogue of glycerone phosphate is a substrate for aldolase (118). The elimination of arsenite from 3-oxoalkylarsonic acids, but not of phosphite from 3-oxoalkylphosphonic acids, may reflect the fact that arsenite is relatively stable compared with arsenate, whereas phosphite is unstable compared with phosphate. 

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