Calcium precipitation in the almost neutral environment of the intestines is a process related to weight loss management and plays an important role in the prevention of colon cancer development. This process also affects calcium bioavailability which is decreased due to decreased calcium absorption. Therefore, calcium as an essential nutrient should not be underestimated in our diet. Milk and dairy products are good sources of bioavailable calcium due to specific protein binding. Other sources of calcium, apart from a balanced and healthy diet, are calcium supplements and calcium fortified food. Therefore, an understanding of the basic chemistry of calcium binding to
low molecular weight compounds can contribute to a general knowledge about calcium bioavailability and also to product improvement.
Calcium precipitation with palmitate was described by a first-order reaction for conditions of excess calcium in neutral aqueous solutions with a stoichiometry Ca:Pal lower than 1:2. Increasing pH during aging of the precipitate and solubility product determination lead to a suggestion of an initial precipitation of calcium hydroxy palmitate as a possible precursor phase.
The binding of calcium to acidic amino acids and dipeptides was also investigated. It was found that aspartate binds calcium stronger than glutamate. This was confirmed by electrochemical determination of the association constants for aqueous solutions at ionic strength 0.20 and 1.0. The mixed dipeptides Asp/Glu showed additive affinity of calcium binding as determined by the association constants and also compared with the values predicted from the individual amino acids. In contrast, the AspAsp dipeptide showed an affinity less than additive, while the GluGlu dipeptide showed affinity of calcium binding to be more than additive. Additionally, the affinity of calcium to serine was small but was increased for the SerGlu dipeptide and the values became comparable to phosphorylated serine. An increase in ionic strength lead to decreased calcium binding of the studied free amino acids, dipeptides and phosphorylated serine as expected.
The solubility of calcium L-lactate and calcium D-lactobionate were higher compared to calcium Dgluconate as shown by the solubility products determined electrochemically for aqueous 1.0 mol·L -1 NaCl at 25 °C. The association constants for individual calcium hydroxycarboxylates and later for
their mixed solutions clearly indicated a 1:1 stoichiometry for the complex formation as determined electrochemically and also by the iodometric titration for both ionic strengths of 0.20 and 1.0 at 25 °C. Therefore, the increased solubility in the mixed solutions of calcium hydroxycarboxylates was due to complex binding.
The continuing dissolution of calcium L-lactate in already saturated aqueous solution of calcium Llactate after addition of solid sodium gluconate was found to form a homogeneous solution. This homogeneous solution became increasingly supersaturated in calcium D-gluconate, and calcium Dgluconate only slowly precipitated after a lag phase. On the other hand, the slow dissolution of calcium D-gluconate by sodium L-lactate in aqueous solution with the reverse lactate/gluconate ratio did not result in a similar solution since fast precipitation prevented formation of a homogenous solution.