Calcium binding to dipeptides of aspartate and glutamate in comparison with orthophosphoserine

Aspartate binds calcium(II) better than glutamate with Ka = 7.0 ± 0.9 L mol⁻¹ for Asp and Ka = 3.0 ± 0.8 L mol⁻¹ for Glu, respectively, as determined using calcium-selective electrodes for aqueous solutions of ionic strength 0.20 at 25 °C at pH of relevance for milk products. For the mixed peptides, the affinity seems additive with Ka = 27 ± 3 L mol⁻¹ for Asp-Glu and 22.7 ± 0.1 for Glu-Asp as compared to the expected 21 L mol⁻¹. In contrast, for Asp-Asp, the affinity is less than additive with Ka = 23 ± 5 L mol⁻¹ as compared to the expected 49 L mol⁻¹, whereas for Glu-Glu, the affinity is more than additive with Ka = 26 ± 4 L mol⁻¹ as compared to the expected 9.0 L mol⁻¹, indicating specific structural effects for Glu-Glu. Ionic strength effects, 1.0 versus 0.20 studied, are similar for Asp and Glu with decreasing affinity for higher ionic strength, whereas the dipeptides with Glu as C-terminus are more sensitive to increasing ionic strength than with Asp as C-terminus. Despite little affinity of calcium to serine with Ka = 0.9 ± 0.2 L mol⁻¹, Glu has increasing affinity for calcium in the serine dipeptide Ser-Glu with Ka = 10 ± 3 L mol⁻¹, which becomes comparable to phosphorylated serine with Ka = 22 ± 5 L mol⁻¹.