Gel electrophoresis measurement of counterion condensation on DNA

We used agarose gel electrophoresis to measure the effective charge neutralization of DNA by counterions of different structure and valence, including Na⁺, Mg²⁺, Co(NH₃), and sperinidine³⁺, which competed for binding with an excess of Tris acetate buffer. Linear DNA molecules ranged in size from 1 to 5 kilobases, and supercoiled plasmid pUC18 was also measured. In all cases, the results were in good agreement with theoretical predict ions from counterion condensation theory for two-counterion mixtures. © 1995 John Wiley & Sons, Inc.