DYNAMIC MOBILITY OF DNA

The dynamic mobility (mu(d)) of double-stranded DNA (at 1 MHz) has bee n measured for the first time, by measurements of the electrokinetic s onic amplitude (ESA) and the hydrated density. Calfthymus DNA (average size 3000 base pairs) and herring sperm DNA (300 bp) have been used a s models for a flexible polymer and a semirigid rod, respectively. In both cases the ESA is proportional to the DNA concentration at least u p to 1.5 mg/mL, allowing mu(d) of noninteracting DNA molecules to be d etermined from the slopes. For calfthymus DNA mu(d) is very similar to literature values on the steady-state mobility, available between 50 and 4 mM NaCl. Unexpectedly, the dynamic mobility at 1 mM NaCl is lowe r than that at 4 mM, and thermal melting experiments rule out denatura tion as a cause for this nonmonotonic dependence of mu(d) on ionic str ength. Using the free-draining approximation valid in steady-state ele ctrophoresis of DNA, we evaluate the charge fraction alpha from the dy namic mobility. Above 4 mM NaCl alpha is essentially constant at 0,60 +/- 0.05, indicating that DNA can be viewed as a constant-charge cylin der also in the dynamic mobility, but at 1 mM the charge fraction drop s to 0.2. The herring sperm DNA has a considerably lower dynamic mobil ity than calf thymus DNA, which is confirmed by measurements of the ul trasonic vibration potential. Part of the lower dynamic mobility of th e herring sperm DNA can be ascribed to partial denaturation. The fact that the charge fraction is smaller than that calculated from steady-s tate mobilities of completely denatured DNA indicates, however, that t he smaller size also contributes to the low mobility of the herring sp erm DNA.