Water,
the dominant component in the physiological condition, is a complicated
solvent, which greatly affects the properties of solute molecules. Here, we utilize
atomic force microscope-based single-molecule force spectroscopy to study the
influence of water on the single-molecule elasticity of an unstructured
single-stranded RNA (poly(U)). In nonpolar solvents, RNA presents its inherent
elasticity, which is consistent with the theoretical single-chain elasticity calculated
by quantum mechanics calculations. In aqueous buffers, however, an additional
energy of 1.9 kJ/mol?base is needed for the stretching of the ssRNA chain. This
energy is consumed by bound water rearrangement (Ew) during the chain elongation. Further
experimental results indicate that the Ew value is uncorrelated to the salt
concentrations and stretching velocity. The results obtained in an 8 M guanidine?HCl
solution provide more evidence that the bound water molecules around RNA give
rise to the observed deviation between aqueous and nonaqueous environments. Compared
with synthetic water-soluble polymers, the value of Ew of RNA is much lower.
The weak interference of water is supposed to be the precondition for the RNA
secondary structure to exist in aqueous solution.
