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With the evolution of single molecule techniques as force-scope optical tweezers, it has become possible to
perform very accurate measurements of the elastic properties of biopolymers as e.g. DNA. Nucleic acid elasticity
is important in the interaction of these molecules with proteins and protein complexes in the living cell. Most
experimental and theoretical effort has been aimed at uncovering and understanding of the behavior of polymers
with contour lengths significantly longer than their persistence length. The well-established Worm-Like-Chain
model has been modified such that a satisfactory description of such long biopolymers is available. However,
in many single molecule experiments, such as the unfolding of RNA stem-loops1 and RNA pseudoknots,2 one
is dealing with biopolymers whose contour lengths are comparable to persistence lengths. A full understanding
of such curves requires an understanding of the physics of short biopolymers. For such cases, theories are just
beginning to emerge and there is hardly any experimental data available. We target this problem by optical
tweezers quantitative force-extension measurements on short biopolymers. The biopolymers used are primarily
double stranded DNA whose total length ( 300 nm) is comparable to their persistence length ( 50 nm). As a
control of our equipment and methods, we also stretch longer dsDNA (1100 nm), the force-extension curves of
which resemble those in literature.3 For the short DNA the force-extension curves qualitatively resemble those
predicted by WLC theories, but a reasonable fit can only be made if the persistence length is allowed to be a
fitting parameter. If made a fitting parameter, the 'apparent persistence length' is found as 8.7±4 nm, a number
which is significantly lower than the real physical value.
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