Calcium binding to lactose, inulin and their constituting monosaccharides and perspective for calcium bioaccessibility

Abstract

Calcium ions bind to lactose, galactose, glucose and fructose in a 1:1 ratio and to low-chain inulin in a 2:1 ratio; shown by conductivity using Job's method of continuous variation. The binding constant, Ka, for aqueous solution at 25.0 °C increases along the series of simple saccharides glucose (4 L mol−1) < fructose (6 L mol−1) < galactose (8 L mol−1) < lactose (10 L mol−1) as determined from equilibrated calcium iodate solubility. Isothermal titration calorimetry showed that calcium binding was athermal and binding accordingly entropy controlled. For lactose, the binding to the α-anomer increased during 5 h equilibration, while the binding for the β-anomer decreased to reach a common equilibrium as measured electrochemically. Quantum chemical calculations confirmed that the β-anomer of lactose and glucose binds calcium stronger than the α-anomer. For inulin, binding has Ka = 35 L mol−1 for each of two calcium in a moderately endothermic process.