Nitrogen oxyanions

Phosphorus oxyanions are entirely different from nitrogen oxyanions. First, the oxyanion species present is controlled by the pH also, phosphate oxyanions are generally not mobile in soil. However, sandy soils and soils high in phosphorus are exceptions to this rule. Any soil, though, can lose phosphate by erosion and this phosphate can cause environmental problems. Because of its unique chemistry, phosphorus will be discussed separately later. 

An additional nitrogen oxyanion is peroxynitrite, ONOO" , whose structure has recently been reported." The structure of one conformation of ONOO is shown in Figure 8-25 a twisted form with different bond angles and a different N — O distance is also found in the crystal. Peroxynitrite may play important roles in cellular defense against infection and in environmental water chemistry. ... 

X-ray crystallographic studies of serine protease complexes with transition-state analogs have shown how chymotrypsin stabilizes the tetrahedral oxyanion transition states (structures (c) and (g) in Figure 16.24) of the protease reaction. The amide nitrogens of Ser and Gly form an oxyanion hole in which the substrate carbonyl oxygen is hydrogen-bonded to the amide N-H groups. 

On the basis of the quantum-chemical studies, it has been established that it is the so called oxyanion hole of the enzyme that binds the carbonyl oxygen or nitrile nitrogen, enhancing the attack of a nucleophile, which is, in turn, activated by histidine (Scheme 5.22) [110]. 

The two important oxyanions in soil are nitrate and phosphate. Nitrate (N03 ) is the predominant oxyanion of nitrogen however, nitrite (N02 ) can also occur in the soil solution. Phosphate can exist as one of three species,... 

Many important soil components are not present as simple cations or anions but as oxyanions that include both important metals and nonmetals. The most common and important metal oxyanion is molybdate (Mo042 ). The most common and important nonmetal oxyanions are those of carbon (e.g., bicarbonate [HC03 ] and carbonate [C032-]), nitrogen (e.g., nitrate [N03 ] and nitrite [NQ2 ]), and phosphorus (e.g., monobasic phosphate [H2P04 ], dibasic... 

Of the nonmetal oxyanions, those of carbon have a different role in soil than nitrogen and phosphorus. Bicarbonate and carbonate can act as counterions to cations to keep the soil electrically neutral. They are also important because all pH changes in soil tend to involve either carbonate or bicarbonate, and thus, both are involved in soil pH and buffering. 

Compare the reactivity and movement of nitrogen and phosphate oxyanions in soil. 

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