MODELING THE ELECTROPHORESIS OF RIGID POLYIONS - APPLICATION TO LYSOZYME

An algorithm is developed to determine the electrophoretic mobility of a rigid polyion modeled as a low dielectric volume element of arbitra ry shape containing an arbitrary charge distribution. The solvent is m odeled as a high dielectric continuum with salt distributed according to the linearized Poisson Boltzmann equation. Account is also taken of a Stern layer that separates the molecular surface and the surface of hydrodynamic shear, or Stern surface. Relaxation of the ion atmospher e because of the presence of the external field is ignored. The electr ostatic and hydrodynamic problems are both solved by boundary element methods, The procedure is first applied to spherical polyions containi ng monopolar, dipolar, and quadrupolar charge distributions, and calcu lated mobilities are found to be in excellent agreement with the theor y of Yoon and Kim. It is then applied to lysozyme by using models that account for the detailed shape and charge distribution of the enzyme. For reasonable choices of the molecular and Stern surfaces, calculate d and experimental mobilities are found to be in fair agreement with e ach other, However, if a pH independent Stern layer (or, equivalently, translational diffusion constant, D-t) is assumed, the calculated mob ilities exhibit a stronger pH dependence than is observed experimental ly. A small increase in D-t with increasing pH could correct this disc repancy.