ABSTRACT
At typical physiological concentrations (6.0 107 to 9.0 103mol=L) in plants, animals, or humans, inorganic boron is essentially present only as the monomeric species boric acid, i.e., B(OH)3, and borate, i.e., B(OH)4
.[5] Polyborate species can form near neutral physiological conditions (pH 7.4), when borate concentrations exceed 0.025mol=L,[6] an unusually high boron concentration in biological systems, but still lower than that found in the snap bean leaf (0.1mol=L).[7] Within the normal pH range of the gut and kidney, B(OH)3 would prevail as the dominant species (pH 1: 100% B(OH)3; pH 9.3: 50%; pH 11: 0%).[8] Boric acid is an exclusively monobasic acid and is not a proton donor. Rather, it accepts a hydroxyl ion (a Lewis acid) and leaves an excess of protons to form the tetrahedral anion B(OH)4
BðOHÞ3 þ 2H2O , H3Oþ þ BðOHÞ4 pKa ¼ 9:25 ð25CÞ
Biochemical Forms
Many biomolecules contain one or more hydroxyl groups, and those with suitable molecular structures can react with boron oxo compounds to form boroesters, an important class of biologically relevant boron species. Several types of boron esters exist. Boric acid reacts with suitable dihydroxy compounds to form corresponding boric acid monoesters (‘‘partial’’ esterification) (e.g., Fig. 1) that retain the trigonal-planar configuration and no charge.
