The dependence of the electrophoretic mobility of small organic ions on ionic strength and complex formation

Abstract

The ionic strength dependence of the electrophoretic mobility of small organic anions with valencies up to -3 is investigated in this study. Provided the anions are not too aspherical, it is argued that shape and charge distribution have little influence on mobility. To a good approximation, the electrophoretic mobility of a small particle should be equal to that of a model sphere with the same hydrodynamic radius and same net charge. For small ions, the relaxation effect (distortion of the ion atmosphere from equilibrium due to external electric and flow fields) is significant even for monovalent ions. Alternative procedures of accounting for the relaxation effect are examined. In order to account for the ionic strength dependence of a specific set of nonaromatic and aromatic anions in aqueous solution, it is necessary to include complex formation between the anion with species in the BGE. A number of possible complexes are considered. When the BGE is Tris-acetate, the most important of these involves the complex formed between anion and Tris, the principle cation in the BGE. When the BGE is sodium borate, an anion-anion (borate) complex appears to be important, at least when the organic anion is monovalent. An algorithm is developed to analyze the ionic strength dependence of the electrophoretic mobility. This algorithm is applied to two sets of organic anions from two independent research groups.

Modeling the electrophoresis and transport of peptides: The effective sphere model and complex formation

“…This increased absolute charge would result in an increase in ∣μ exp ∣ which would be consistent with experiment. Similar borate–anion complex formation has been implicated in the mobility studies of duplex DNA 3 and organic anions 4. A principle objective of the present study is to investigate more fully the possibility of complex formation between oligoglycines and borate anions at high pH.…”

“…The second way is through specific binding on one (or more) components of the BGE to the peptide. This problem has been addressed previously with regard to the mobility of small organic anions in aqueous media 4. This point shall be addressed later.…”

“…A common alternative to the extended Debye–Hückel equation is the Davies equation 36, 40, where ion size does not even enter. In a previous work on organic anions 4, we employed the Davies equation rather than the extended Debye–Hückel equation for the reason that it was simpler and the ion size did not enter. In the present study on peptides, however, we chose to use the Debye–Hückel equation setting A ± equal to 0.5 nm to be more consistent with the previous work on peptides 1, 2, 37, 38.…”

“…Before considering complex formation, however, it will be useful to consider a model that is substantially simpler than the B model for peptides discussed elsewhere 1, 5–8 and that is the “effective sphere” (ES) model 4. The ES model, in turn, can be adapted to investigate more complex phenomena like complex formation 4. Modeling complex species as “ESs” 13, 14, “effective ellipsoids” 15–17, or “effective rods” 12 has been quite successful in many regards.…”

Modeling the electrophoresis of oligoglycines

“…This increased absolute charge would result in an increase in ∣μ exp ∣, which would be consistent with experiment. Complex formation has been implicated in the mobility studies of organic anions 56, 57. In a companion study, we explore this possibility in greater depth.…”