  The flexibility of short DNA chains is investigated via computation of the
average correlation function between dimers which defines the persistence
length. Path integration techniques have been applied to confine the phase
space available to base pair fluctuations and derive the partition function.
The apparent persistence lengths of a set of short chains have been computed as
a function of the twist conformation both in the over-twisted and the untwisted
regimes, whereby the equilibrium twist is selected by free energy minimization.
The obtained values are significantly lower than those generally attributed to
kilo-base long DNA. This points to an intrinsic helix flexibility at short
length scales, arising from large fluctuational effects and local bending, in
line with recent experimental indications. The interplay between helical
untwisting and persistence length has been discussed for a heterogeneous
fragment by weighing the effects of the sequence specificities through the
non-linear stacking potential.
