  The accurate knowledge of the elastic properties of single-stranded DNA
(ssDNA) is key to characterize the thermodynamics of molecular reactions that
are studied by force spectroscopy methods where DNA is mechanically unfolded.
Examples range from DNA hybridization, DNA ligand binding, DNA unwinding by
helicases, etc. To date, ssDNA elasticity has been studied with different
methods in molecules of varying sequence and contour length. A dispersion of
results has been reported and the value of the persistence length has been
found to be larger for shorter ssDNA molecules. We carried out pulling
experiments with optical tweezers to characterize the elastic response of ssDNA
over three orders of magnitude in length (60-14 k bases). By fitting the
force-extension curves (FECs) to the Worm-Like Chain model we confirmed the
above trend:the persistence length nearly doubles for the shortest molecule (60
b) with respect to the longest one (14 kb). We demonstrate that the observed
trend is due to the different force regimes fitted for long and short
molecules, which translates into two distinct elastic regimes at low and high
forces. We interpret this behavior in terms of a force-induced sugar pucker
conformational transition (C3'-endo to C2'-endo) upon pulling ssDNA.
