Natural cellulose (NC) is the most abundant biomacromolecule. The fact
that each pyranose ring of NC has three hydroxyl groups implies that NC should
be highly hydrophilic. The paradoxical water insolubility of NC is usually
explained by the strong tendency to form the hydrogen-bonding network by the
high content of hydroxyl groups. In this study, we present the first
experimental evidence that NC is rather hydrophobic, even if the chains are molecularly
dispersed. By single-molecule force spectroscopy, the single-chain mechanics of
NC has been studied in various liquid environments. In a common nonpolar
solvent, octane, NC shows the elastic force-extension (F-E) curve. We find that
this kind of F-E curve can be fitted well by the QM-FJC model, in which the
single-chain elasticity obtained from quantum mechanical (QM) calculations is
integrated into the freely jointed-chain (FJC) model. However, the result of NC
obtained in water is different, which shows a long plateau in the F-E curve.
Further study shows that the height of the plateau is temperature dependent.
However, the plateau disappears when an 8M urea solution is used as the liquid
environment. AFM images obtained in water show that single NC chains exist in a
compact globule conformation on the sample surface. According to the molecular
structure, methylcellulose (MC) should be more hydrophobic than NC. However, no
plateau can be observed from the MC samples in water. On the basis of all the
results above, we can draw a conclusion that both of the hydrophobic effect and
the crystallization of NC contribute to the plateau in the F-E curve obtained
in water. The experimental observation of the hydrophobic nature of NC at the
single-chain level provides new insights into the understanding of NC.
