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

Eighteen peptides have been modeled. From the volumetric data derived, and published mobilities, the relationship between electrophoretic mobility (mu (ep)) and the hydrodynamic radius (r) has been examined. Objective testing with respect to size has been achieved by the log-log version of generaliz ed relationship. (1) From the gradient of the plot versus log r (2.02) ther e is good support for the inverse square law (mu (ep) proportional to 1/r(2 )). Equivalent calculations using molecular weight (M-r) and the number of amino acid residues (n) similarly lead to mu (ep) proportional to 1/M-r(2/3 ) and mu (ep) proportional to 1/(2/3)(n), respectively. However, the streng th 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 char ge and a statistical analysis of the charge distribution on the peptides le ads to the conclusion that deviations from the averaged behaviour arise fro m a charge-induced volumetric effect. Taken together, (1) and (2) indicate thai whilst net charge and total size can describe average electrophoretic behaviour well, these parameters are inadequate to describe the specific mo bilities of individual analytes. Objective analysis of alkylpyridine data indicates mu (ep) proportional to 1/r(x) where x = 2.6 - 2.8 (depending upon the nature of the r values utili zed), but is certainly not equal 1 as may have been presumed. A very small range of r values may be responsible for this surprising result.