Calcium Binding to Dipeptides of Aspartate and Glutamate in Comparison with Orthophosphoserine

Aspartate binds calcium(II) better than glutamate with K-a = 7.1 +/- 0.9 L mol(-1) for Asp and K-a = 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 degrees C at pH of relevance for milk products. For the mixed peptides, the affinity seems additive with K-a = 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 K-a = 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 K-a = 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 K-a = 0.9 +/- 0.2 L mol(-1), Glu has increasing affinity for calcium in the serine dipeptide Ser-Glu with K-a = 10 +/- 3 L mol(-1), which becomes comparable to phsophorylated serine with K-a = 22 +/- 5 L mol(-1).