Abrupt transition from a free, repulsive to a condensed, attractive DNA phase, induced by multivalent polyamine cations

Abstract

We have investigated the energetics of DNA condensation by multivalent polyamine cations. Solution small angle x-ray scattering was used to monitor interactions between short 25 base pair dsDNA strands in the free supernatant DNA phase that coexists with the condensed DNA phase. Interestingly, when tetravalent spermine is used, significant inter-DNA repulsion is observed in the free phase, in contrast with the presumed inter-DNA attraction in the coexisting condensed phase. DNA condensation thus appears to be a discrete, first-order-like, transition from a repulsive gaseous to an attractive condensed solid phase, in accord with the reported all-or-none condensation of giant DNA. We further quantify the electrostatic repulsive potentials in the free DNA phase and estimate the number of additional spermine cations that bind to DNA upon condensation.

FIG. 1

The [DNA] in the supernatant as a function of nominal [Spermidine] (a) or [Spermine] (b). Note that [Phosphate]=48×[DNA]. Shaded areas indicate the presence of precipitates. Variations of the first two points result from different dilutions when mixing up sample solutions.

FIG. 2

(color) Solution SAXS data of the supernatant DNA phase only in (a) Spermidine and (b) Spermine. Data (symbols) are scaled to match at high Q for easy comparison, and the lines are the fits. The lowering down of the low Q scattering intensity at highest salt concentrations (>100mM) interpreted as from lower DNA contrast due to the increased solvent electron density (particularly high [Cl⁻]).

FIG. 3

Effective charge Z_eff of the free DNA strands in solution as a function of the nominal Spermidine (a) or Spermine (b) concentrations. Dashed line is a guide to the eye. Error bars are slightly smaller than symbol sizes.

FIG. 4

Spermine series only. (a) electrostatic potential energy Φ per DNA. (b) free energy G per DNA with an offset.