Charge distribution within cell surface coats of single and interacting surfaces--a minimum free electrostatic energy approach. Conclusions for electrophoretic mobility measurements.

The fixed charge distribution in a cell surface coat model was calculated using a variational approach based on the minimum of electrostatic free energy. The influence of ionic strength on distribution was analyzed. It is shown that interaction with other coats or smooth surfaces changes the fixed charge distribution. The special effect of a possible membrane charge was calculated in addition. In this case there is a fixed, charge-free space near the membrane, depending on coat thickness, ionic strength and membrane charge/surface coat charge ratio. For several cases energies, electric potentials and fixed charge distributions are given. The general result is that fixed charge distribution is always composed of a constant space charge density over a part of or the whole surface coat and, with some exceptions, of a surface charge density at the border of the surface coat. Conclusions for electrophoretic mobility measurement interpretations are drawn.