Density gradients prepared from colloidal silica particles coated by polyvinylpyrrolidone (Percoll)

A new gradient medium (Percoll) for density gradient centrifugation of cells and subcellular particles is described. It consists of colloidal silica particles which have been firmly coated with a layer of polyvinylpyrrolidone. The particle population is polydisperse, and the average diameter is approximately 17 nm. Both the electrophoretic mobility of the particles and the conductivity of the solution are low, indicating a low surface charge. The colloid has a high solubility and forms clear solutions. It can be used in concentrations which give solution densities between 1.00 and 1.20 g/ml. The solutions have a low osmolality and can be mixed with electrolytes of physiological pH and ionic strength. The sedimentation properties of the particles are described. Density gradients can be formed by highspeed centrifugation. The colloid has been shown to be nontoxic for a number of cells and cell organelles.