Zinc bioavailability from whey. Enthalpy-entropy compensation in protein binding.

Zinc amino acid or peptide complexes improve zinc absorption, however, the thermodynamics behind the interaction between zinc ion and these potential ligands is not well characterized. Therefore, binding of zinc to amino acids, peptides and whey proteins were investigated by isothermal titration calorimetry to provide useful information for improving zinc bioavailability which is lowered by many food components like phytate. Zinc binding to lactoferrin and to bovine serum albumin was found exothermic with ∆H=-100kJ/mol and -30kJ/mol, respectively, in aqueous 0.16M NaCl of pH7.4 at 25°C by isothermal titration calorimetry, while binding to α-lactalbumin and β-lactoglobulin was slightly endothermic. Still the binding constant was for all four proteins found to have a value around 2×105L/mol indicating enthalpy-entropy compensation as also confirmed for zinc binding to amino acids with cysteine being enthalpically favored, while serine<histidine<glutamate<phosphorylated serine increasingly were found entropically favored. The enthalpy-entropy compensation makes whey proteins more homogeneous as zinc vehicles counteracting concentration fluctuation and precipitation of zinc.