Speciation of biochemically important iron complexes with amino acids: L-aspartic acid and L-aspartic acid - glycine mixture

Speciation of iron(III) (cFe = (0.5 - 40) × 10−5 mol dm−3) and indirectly of iron(II), in aqueous solution of L-aspartic acid and aspartic acid - glycine mixture was investigated by square-wave voltammetry on a static mercury drop electrode. A reversible, one-electron reduction process of iron(III)-aspartate complexes with peak potential between +0.04 and -0.17 V was recorded, depending on the solution pH (4.5–8.6) and concentration of aspartic acid (cAsp = 0.01 - 0.4 mol dm−3). Cumulative stability constants of iron(III)-aspartate complexes: FeAsp+ (log K = 13.16 ± 0.01), FeAspOH0 (log β = 20.76 ± 0.02), FeAsp(OH)2− (log β = 27.77 ± 0.12) and Fe(Asp)2− (log β = 17.62 ± 0.10); and iron(II)-aspartate complexes: FeAsp0 (log K = 4.17 ± 0.11) and Fe(Asp)22− (log β = 6.53 ± 0.11), were determined at Ic = 0.5 mol dm−3 and 25 ± 1 °C. In the solution with mixture of amino acids, aspartic acid and glycine, a formation of mixed (Fe(III)-aspartate-glycine) complexes occurred. The reduction process was also shown to be reversible, so stability constants of mixed complexes (iron(III): FeAspGly0 (log β = 17.35 ± 0.02), FeAspGly(OH)− (log β = 23.87 ± 0.33), and iron(II): FeAspGly− (log β = 6.83 ± 0.23)) were calculated, as well.