The greater negative charge density of DNA in tris-borate buffers does not enhance DNA condensation by multivalent cations.

As indicated by recent measurements of the electrophoretic free solution mobility, DNA appears to have a greater helical charge density in Tris-borate-EDTA (TBE) buffers than in Tris-acetate-EDTA (TAE) buffers. Since electrostatic forces play a major role in DNA packaging processes, we have investigated the condensation of closed circular plasmid DNA using total intensity and dynamic light scattering in Tris-borate, Tris-acetate, and Tris-cacodylate buffers with cobaltic hexa-amine (III) [Co(NH(3))(3+)(6)]. We find that neither the critical concentration of Co(NH(3))(3+)(6) nor the hydrodynamic radii of the resulting condensates vary significantly in the buffer systems studied here despite the prediction that DNA condensation should occur at significantly lower Co(NH(3))(3+)(6) concentrations in Tris-borate buffers. Assuming a persistence length behavior similar to B-DNA in the presence of multivalent cations, a decrease in the attractive counterion correlation pressure decay length in Tris-borate buffers does not account for our observations. It is possible that the binding of multivalent cations to DNA may hinder borate association with the DNA double helix.