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

Abstract

Speciation of iron(III) (cFe=(0.5 - 40)×10−5mol 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 -0.17V was recorded, depending on the solution pH (4.5–8.6) and concentration of aspartic acid (cAsp=0.01 - 0.4mol 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=0.5mol 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.