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A detailed study of Zn(II)-D-mannosamine and mixed ligand complex Zn(II)-D-mannosamine-glycine in aqueous solution ( θ = 25 °C, I c = 0.55 mol L −1 ) was carried out by voltammetry and potentiometry, allowing their characterization. Redox process of the [ZnManN] 2+ complex was registered at E p ∼ −1.01 V, [ZnGly] + at −1.02 V, ZnGly 2 at −1.18 V, while [ZnManNGly] + at −1.12 V (pH = 8.0). Stoichiometry and stability constants of Zn (II) complexes were determined by potentiometric titrations. Calculated stability constants were found to be: log   β ZnManN 2 + = 2.7, log   β MaNZnGly + = 7.7 and log   β MaNZnGlyH − 1 = −0.15. Voltammetric and potentiometric data were in good agreement, both indicating formation of two different ternary complexes, i.e. [ZnGlyManN] + and [ZnGlyManNH −1 ]. Calculated speciation revealed the presence of the [ZnManN] 2+ complex, as well as ternary complexes ([ZnManNGly] + ) in significant amount. This study was the first attempt to quantify complexation processes of Zn (II) with naturally occurring amino sugar and mixed complex with amino acid, important for bioavailability of zinc (II) in seawater. [ABSTRACT FROM AUTHOR]

Speciation of iron(III) ( c Fe = (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 ( c Asp = 0.01 - 0.4 mol dm −3 ). Cumulative stability constants of iron(III)-aspartate complexes: FeAsp + (log K = 13.16 ± 0.01), FeAspOH 0 (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: FeAsp 0 (log K = 4.17 ± 0.11) and Fe(Asp) 2 2− (log β = 6.53 ± 0.11), were determined at I c = 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): FeAspGly 0 (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. [ABSTRACT FROM AUTHOR]