Objective testing for the dependence of electrophoretic mobilities upon size in capillary zone electrophoresis

Summary Eighteen peptides have been modeled. From the volumetric data derived, and published mobilities, the relationship between electrophoretic mobility (μ_ep) and the hydrodynamic radius(r) has been examined. Objective testing with respect to size has been achieved by the log-log version of generalized relationship. (1) From the gradient of the plot versus log r(2.02) there is good support for the inverse square law (μ_ep α 1/r²). Equivalent calculations using molecular weight (M_r) and the number of amino acid residues (n) similarly lead to μ_ep α 1/M _r ^2/3 and μ_ep α 1/n^2/3, respectively. However, the strength of the correlation is diminished as the precision of the representation of size is degraded. (2) An examination of the effect of size at fixed charge and a statistical analysis of the charge distribution on the peptides leads to the conclusion that deviations from the averaged behaviour arise from a charge-induced volumetric effect. Taken together, (1) and (2) indicate that whilst net charge and total size can describe average electrophoretic behaviour well, these parameters are inadequate to describe the specific mobilities of individual analytes. Objective analysis of alkylpyridine data indicates μ_ep α 1/r^x where x=2.6–2.8 (depending upon the nature of the r values utilized), but is certainly ≠1 as may have been presumed. A very small range of values may be responsible for this surprising result.