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.