Light scattering studies of the effect of Ca2+ on the structure of porcine submaxillary mucin

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Abstract
The effects of calcium ions on the solution properties of porcine submaxillary mucin (PSM) have been investigated by static and dynamic light scattering. The weight average molecular weights of PSM fractions are unaffected by the addition of up to 0.5M CaCl2: these data are within experimental error of those for solutions in 0.1M NaCl. The distribution of relaxation frequencies derived from the dynamic data shows the existence of two distinct relaxation modes. The average relaxation times have been interpreted to yield the z-average translational diffusion coefficient and the longest intramolecular relaxation time τ1. A plot of τ1 vs 〈1/Rh〉 is linear, and consistent with plots of such data recorded for PSM in 0.1M NaCl and 6M GdnHCl solutions. However, the τ1 values and the associated results for 〈R〉 in 0.5M CaCl2 are smaller than those determined in 0.1M NaCl. This suggests that the conformation of PSM in CaCl2 solution is more contracted than those in the other two solvents. These results are consistent with the compact packaging of mucin in the secretary granules that have elevated Ca2+ levels.