In this paper, we measure
the single chain elasticity of an oligomer ssDNA in both aqueous and
non-aqueous, apolar liquid environments by AFM-based single molecule force
spectroscopy. We find a marked deviation between the force-extension relations
recorded in the two ambients. This difference is attributed to the additional
energy required to break the H-bond directed water bridges around ssDNA chain
in aqueous solutions, which are non-existent in organic solvents. The results
obtained in8Mguanidine-HCl solution provide another evidence that water bridges around ssDNA
originate the observed deviation. Based on the results obtained by an ab-initio quantum mechanics calculation,
a parameter-free freely-rotating-chain model is proposed. We find that this
model is in a perfect agreement with the experimental force-extension curve
obtained in organic solvents, which further corroborates our assumption. Based on the experimental results, it is
suggested that the weak H-bonding between ssDNA and water molecules may be a precondition
for a stable dsDNA to exist in water.
