The behavior of alkaline earth metal cations (Mg2+ and Ca2+) and transition metal cations (Zn2+ and Cu2+) interacting with lambda-DNA-HindIII fragments ranging from 2,027 to 23,130 bp in Tris-borate-EDTA buffer solutions was investigated. The divalent counterions competed with Tris+ and Na+ for binding to polyion DNA, and the competition binding situations were investigated by measuring the reduction of the DNA mobility, by pulsed- or constant-field gel electrophoresis. The interaction of Mg2+ with DNA was intensively studied over a wide range of Mg2+ concentrations. In addition, we examined the competition binding as a function of ionic strength and DNA size. To compare valence effects, we studied Co(NH3)6(3+) interaction with DNA fragments under conditions similar to that of Mg2+. At relatively low Mg2+ concentration, the normalized titration curves of DNA mobility were well fit by Manning's two-variable counterion condensation (CC) theory. The agreement between the predicted value (total charge neutralization fraction theta) from Manning's CC theory and the data based on our measured DNA electrophoretic mobility reduction was consistent under our experimental conditions. In contrast to alkaline earth metal cations (Mg2+ and Ca2+), different binding behaviors were observed for the transition metal cations (Zn2+ and Cu2+). These differences highlight the usefulness of our reduced DNA electrophoretic mobility measurement approach to describing cation interactions with polyelectrolyte DNA.

Pulse gel electrophoresis was used to measure the reduction of mobilities of λ‐DNA‐Hind III fragments ranging from 23.130 to 2.027 kilobase pairs in Tris borate buffer solutions mixed with either hexammine cobalt(III), or spermidine3+ trivalent counterions that competed with Tris+ and Na+ for binding onto polyion DNA. The normalized titration curves of mobility were well fit by the two‐variable counterion condensation theory. The agreement between measured charge fraction neutralized and counterion condensation prediction was good over a relatively wide range of trivalent cation concentrations at several solution conditions (pH, ionic strength). The effect of ionic strength, trivalent cation concentration, counterion structure, and DNA length on the binding were discussed based on the experimental measurements and the counterion condensation theory. © 1996 John Wiley & Sons, Inc.

Systematic studies were conducted to observe the binding interactions between the class of compounds including nitroaromatic munitions pollutants trinitrotoluene (TNT) and certain of its breakdown products and dissolved Aldrich humic acid (HA), which is used as a model soil matrix. Equilibrium dialysis followed by HPLC quantitation was used to determine the effect of ligand concentration, HA concentration, pH, and ionic strength on the formation kinetics and ligand binding level of the ligand-HA complex. It was found that TNT and its byproducts 2,6-diamino-4-nitrotoluene (2,6DAmNT) and 2-amino-4,6-dinitrotoluene (2AmDNT) are all able to bind to HA at different binding levels in a slow kinetic process. The HA concentration was observed to have the same inverse effect on the binding of both TNT and 2,6DAmNT, while pH had opposite effects on binding for the two compounds. Nearly a 2-fold increase in binding of TNT to HA was observed for a 5-fold increase in ionic strength of phosphate buffer. A linear binding model represented the best fit for the 2,6DAmNT isotherm data while the Langmuir model best fit the TNT isotherms. The maximum binding density of TNT for HA calculated from the Langmuir model ranged from 6 to 30 µM TNT/µM HA of average size 5000 for all conditions studied. These facts suggest that the binding mechanisms are different for the above two ligands due to their different chemical structures.

In this paper we introduce an important parameter called the iso-competition point (ICP), to characterize the competition binding to DNA in a two-cation-species system. By imposing the condition of charge neutralization fraction equivalence theta1 = ZthetaZ upon the two simultaneous equations in Manning's counterion condensation theory, the ICPs can be calculated. Each ICP, which refers to a particular multivalent concentration where the charge fraction on DNA neutralized from monovalent cations equals that from the multivalent cations, corresponds to a specific ionic strength condition. At fixed ionic strength, the total DNA charge neutralization fractions thetaICP are equal, no matter whether the higher valence cation is divalent, trivalent, or tetravalent. The ionic strength effect on ICP can be expressed by a semiquantitative equation as ICPZa/ICPZb = (Ia/Ib)Z, where Ia, Ib refers to the instance of ionic strengths and Z indicates the valence. The ICP can be used to interpret and characterize the ionic strength, valence, and DNA length effects on the counterion competition binding in a two-species system. Data from our previous investigations involving binding of Mg2+, Ca2+, and Co(NH3)63+ to lambda-DNA-HindIII fragments ranging from 2.0 to 23.1 kbp was used to investigate the applicability of ICP to describe counterion binding. It will be shown that the ICP parameter presents a prospective picture of the counterion competition binding to polyelectrolyte DNA under a specific ion environment condition.