Do zwitterions contribute to the ionic strength of a solution?

Capillary electrophoresis has been used to determine whether zwitterions contribute to the ionic strength of a solution, by measuring the mobility of a double-stranded DNA oligomer in cacodylate-buffered solutions containing various concentrations of the ionic salt tetraethylammonium chloride (TEA(+)Cl(-)) or the zwitterion tricine(+/-). The mobility of the DNA decreased as the square root of ionic strength, as expected from the Debye-Huckel-Onsager theory of electrophoresis, when TEA+Cl- was added to the buffer. However, the mobility was independent of the concentration of added tricine(+/-). Hence, zwitterions do not contribute to the ionic strength of a solution. (C) 2007 Elsevier Inc. All rights reserved.