TRIVALENT COUNTERION CONDENSATION ON DNA MEASURED BY PULSE GEL-ELECTROPHORESIS

Pulse gel electrophoresis was used to measure the reduction of mobilit ies of lambda-DNA-Hind III fragments ranging from 23.130 to 2.027 kilo base pairs in Tris borate buffer solutions mixed with either hexammine cobalt(III), or spermidine(3+) trivalent counterions that competed wi th Tris(+) and Na+ for binding onto polyion DNA. The normalized titrat ion curves of mobility were well fit by the two-variable counterion co ndensation theory. The agreement between measured ed charge fraction n eutralized and counterion condensation prediction was good over a rela tively wide range of trivalent cation concentrations at several soluti on conditions (pH, ionic strength). The effect of ionic strength, triv alent cation concentration, counterion structure, and DNA length on th e binding were discussed based on the experimental measurements and th e counterion condensation theory. (C) 1996 John Wiley & Sons, Inc.