Single-molecule Force Spectroscopy of Biomacromolecules Comparative Studies in Aqueous Solution and Nonpolar Solvents

Biomacromolecules are the foundation of life. In the natural environment (aqueous solution), the biomacromolecules exist in the precise supramolecular structures. To date, many kinds of water soluble polymers have been synthesized. However, few of them can be self-assembled into precise supramolecular structures in aqueous solution. Compared to synthetic polymers, biomacromolecules are special and magic. Recently, the author's group utilized single-molecule force spectroscopy (SMFS) to study the interactions between biomacromolecules and water. For ssDNA, there is an evident deviation between the single-chain force extension (F-E) curves obtained respectively in the aqueous solution and nonpolar solvent, which is estimated to be 0. 58k(B)T per base in energy. Comparing to synthetic water soluble polymers, this energy is rather low. The weak competition influence of water may be a precondition for a stable dsDNA to exist in water. For dsDNA, the double-helix structure will be unwound when it is dragged across the water/nonpolar solvent interface. For a polyprotein (polyI27), the unfolding of the globules in water can be observed as saw-tooth F-E curve. However, the same sample in nonpolar solvent only presents an pure elastic behaviour similar to that of polypeptide. Similarly, a clear deviation can also be observed when comparing the single-molecule F-E curves of other biomacromolecules (such as RNA and natural cellulose) obtained respectively in the aqueous solution and nonpolar solvent. It was shown that in nonpolar solvents, the supramolecular structures of biomacromolecules will be unstable, resulting in the denaturation of the biomacromolecules. Water is the switch that regulates the supramolecular structures and functions of biomacromolecules. The concept of aqueous environment adaptability is proposed. It is speculated that biomacromolecules were selected by the aqueous environment during the early chemical evolution. The aqueous environment adaptability may be one important division between the non-biological macromolecules and the biological ones. It is anticipated that more secret of life will be unveiled with further investigation on water and biomacromolecules.