  We report a study of DNA (150 bp fragments) conformations in very low added
salt $<0.05$mM, across wide DNA concentration range $0.0015\leq c \leq 8$~mM
(bp). We found an intermediate DNA conformation in the region $0.05 < c <
1$~mM, by means of fluorescence correlation spectroscopy (FCS) and
UV-absorbance measurements. FCS detected that in this region DNA has the
diffusion coefficient, $D_p$ reduced below the values for both ssDNA coils and
native dsDNA helices of similar polymerization degree $N$. Thus, this DNA
population can not be a simple mix of dsDNA and of ssDNA which results from DNA
melting. Here, melting occurs due to a reduction in screening concomitant with
DNA concentration being reduced, in already very low salt conditions. The
intermediate DNA is rationalized through the well known concept of
fluctuational openings (DNA bubbles) which we postulate to form in AT-rich
portions of the sequence, without the strands coming apart. Within the bubbles,
DNA is locally stretched, while the whole molecule remains rod-like due to very
low salt environment. Therefore, such intermediate DNA is elongated, in
comparison to dsDNA, which accounts for its reduced $D_p$.
