The hydrodynamic radii of macromolecules and their effect on red blood cell aggregation

The effects of nonionic polymers on human red blood cell (RBC) aggregation were investigated. The hydrodynamic radius ( Rh) of individual samples of dextran, polyvinylpyrrolidone, and polyoxyethylene over a range of molecular weights (1500 - 2,000,000) were calculated from their intrinsic viscosities using the Einstein viscosity relation and directly measured by quasi-elastic light scattering, and the effect of each polymer sample on RBC aggregation was studied by nephelometry and low-shear viscometry. For all three polymers, despite their different structures, samples with R-h <4 nm were found to inhibit aggregation, whereas those with R-h>4 nm enhanced aggregation. Inhibition increased with R-h and was maximal at; 3 nm; above 4 nm the pro-aggregant effect increased with R-h. For comparison, the R-h of 12 plasma proteins were calculated from literature values of intrinsic viscosity or diffusion coefficient. Each protein known to promote RBC aggregation had R-h >4 nm, whereas those with R-h<4 nm either inhibited or had no effect on aggregation. These results suggest that the influence of a nonionic polymer or plasma protein on RBC aggregation is simply a consequence of its size in an aqueous environment, and that the specific type of macromolecule is of minor importance.