Transient Electrophiles and Nucleophiles in Soils: Sources, Sinks, and Relevance.

Oxidations of many soil-relevant organic moieties by soil minerals such as manganese(III,IV) oxyhydroxides generate electrophiles.  Benzoquinones, for example, arise from dihydroxybenzene oxidation.  α-Keto acids arise from α-hydroxy acid oxidation.  Subsequent adduct formation can covalently sequester sulfur- and nitrogen-bearing compounds into higher molecular weight natural organic matter structures.  If electrophiles generated by oxidations are reactive enough towards oxygen-donor nucleophiles, they also hydrolyze or hydrate.  Adduct formation and hydrolysis/hydration are reactions in parallel; the predominant pathway is influenced by pH and the presence of other solutes in the soil medium.  Hydration also serves as the "ring opening" foil to "ring closing" lactonization and lactamization reactions.  Particular mineral surfaces within soils likely catalyze these interconversions.  Finally, it should be noted that reductions, e.g. by mineral-bound Fe(II), can generate nucleophiles.  Innovations in analytical instrumentation, most notably electrospray-MS and capillary electrophoresis, offer the soil community a new means of documenting these important natural organic matter reworking processes.