Calcium binding to a calcifiable matrix: 43Ca NMR binding studies on the polypentapeptide of elastin

Calcium-43 nuclear magnetic resonance (NMR) has been used to characterize the binding of calcium ion to the polypentapeptide of elastin. The change in longitudinal relaxation time, T1, from 1000 msec to 15 msec at 38 mM CaCl2, on going from D2O to coacervate concentration at 20 degrees C, is dramatic. At 20 degrees C, the binding constant decreases with increasing calcium ion concentration over the range of 40 mM to 250 mM with apparent binding constants of Ktb congruent to 35 M-1 and Kwb congruent to 7M-1. On raising the temperature to 38 degrees C, there is a significant decrease in calcium interaction, as seen in the large increase in T1 from 15 msec at 20 degrees C to 350 msec at 38 mM CaCl2. This correlates with an inverse temperature transition for the polypentapeptide and results in a concentration dependence of the calcium interaction which shows positive cooperativity. Since the polypentapeptide has been shown to calcify massively when exposed to normal serum dialysates, these calcium interactions are of particular interest as the initiating process for calcification of this component of the elastic fiber.