Complexation, Stability and Behavior of L-Cysteine and L-Lysine with Different Iron Sources

Effective stability constants for cysteine and lysine with five different iron sources were evaluated along with their behavior in solution. The values obtained for ferric chloride-cysteine, ferrous sulfate-cysteine, ferric chloride-lysine, ferrous sulfate-lysine, hydrogen-reduced lysine, and electrolytic-reduced lysine were 6.81 × 10² to 2.78 × 10³, 1.33 × 10⁵ to 1.36 × 10⁵, 6.00 × 10^-4 to 7.64 × 10^-3, 6.37 ×10^-4 to 4.82× 10^-3, 9.34 × 10^-2 to 1.38 × 10^-1, and 4.18 × 10^-4 to 7.27 × 10^-4, respectively. No measurable complexation occurred with hydrogen- and electrolytic-reduced iron with cysteine nor with ferric orthophosphate and cysteine or lysine. The stability of soluble ferric cysteine over the pH range 2.0 to 7.4 indicates that this complex has the potential to be used as an iron additive in food. Approximately half of the hydrogen and electrolytic reduced iron and only 0.11% of ferric orthophosphate were soluble in acid, whereas ferric chloride and ferrous sulfate were completely soluble. Qualitative evaluation of the iron-amino acid systems over a range of pH from 2.0 to 12.0 indicated that there was a mixed valence state of free iron in most cases with low pH favoring reduction and high pH oxidation, until precipitation of iron hydroxides occurred.