Short electric‐field pulses convert DNA from “condensed” to “free” conformation

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Abstract
Electric‐field pulses of e.g. 20 kV/cm and 100 μs induce a strong decrease in the scattered light intensity of DNA condensed by spermine. Analysis of this effect demonstrates that the decrease of the scattered light intensity results from decondensation of DNA. The decondensation reaction requires an electric‐field strength exceeding a threshold value. Complete decondensation can be achieved at field strength that are only slightly higher than the threshold value. The decondensation process is strongly accelerated at high electric‐field strengths. At 30 kV/cm, the decondensation time constant is ∼8 μs, corresponding to an acceleation factor of 105 relative to the field‐free decondensation reaction. The dependence of the time constants on the electric‐field strength suggests that the field‐induced decondensation is due to a dissociation field effect. The condensation process observed after electric‐field pulses at low concentrations of DNA and spermine shows a characteristic induction period, which strongly depends on the spermine concentration. This induction period reflects the time required for the binding of spermine to DNA, until the degree of binding is sufficiently high for the condensation reaction. The fast dissociation of condensed DNA by electric‐field pulses together with a relatively long lifetime of the free DNA results in a reaction cycle resembling a hysteresis loop.