Electrophoresis of Human Red Blood Cells and Platelets. Evidence for Depletion of Dextran

A theoretical description of polymer depletion layers near smooth and hairy surfaces is developed and used for interpretation of experiments. Electrophoretic mobility measurements of human red blood cells and platelets in aqueous electrolyte solutions were performed in the presence of dextran (M_W = 70 kD) to study the interaction of dextran with the cell surface. The electrophoretic mobility in the presence of dextran was considerably larger than expected from the viscosity. This behavior was interpreted as evidence for the existence of a polymer depletion layer. Depending on ionic strength, depletion layer thicknesses ranging from 2.9 to 4.4 nm were found. The dextran concentration at the outer border of the glycocalyx was only 10% of the bulk value. One cannot exclude the possibility that this small amount reflected adsorption. In the case of platelets, the degree of the mobility reduction depended on ionic strength. Depletion of dextran from the platelet surface apparently became smaller with decreasing ionic strength. This indicated a more complex pattern of interaction of dextran with the platelet surface than with the RBC surface. Both adsorption and polymer penetration into the glycocalyx were discussed quantitatively. It was concluded that ionic strength-dependent penetration of dextran into the hairy layer is more likely than ionic strength dependent adsorption changes. As in the case of red cells, some adsorption of dextran might be present.