Single-walled carbon nanotubes are wetted in aqueous solution of pure potato amylose and separated into homogenous suspension of nanotubes and sediment of bundled nanotubes. Similarly microcrystalline and colloidal graphite is wetted in the amylose solution whereas C60 fullerenes remained non-wetted. In the micro-Raman spectra of the amylose complexes with single-walled nanotubes, a relatively large shift of the ωD and ωG modes towards a higher wave number was observed. In such a spectra of complexes of microcrystalline and colloidal graphite corresponding shifts were much subtler. These changes in the spectra of nanotubes were accompanied by clear changes in the ratio of the integrated intensities of both the modes. No such changes could be noted in the spectra of amylose complexes with graphite. Optical, SEM and AFM microscopic observations of solidified complexes suggest that small nanotubes could be enveloped by amylose helices.
Potato amylose (PA) is a linear polymer of 1 → 4´ linked α-D-glucose units (100 – 200 kDa). In aqueous solution it resides in a randomly coiled chain, which becomes the host molecule of left-handed helical complex formed when a molecule with sufficiently long linear hydrophobic fragment (a guest molecule) is available. Such helical complexes with higher alkanols, lipids, and even I5 anion of KI5 are well known (Tomasik & Schilling, 1998). PA in such complexes has all hydroxyl groups of its glucose units directed to the space and the channel of the helix is hydrophobic. Benefits in energy resulting from non-polar interactions between PA and the hydrophobic portion of the guest is a driving force for the complex formation. Hydrogen bonds between 3-OH group and 2-O—atom belonging to adjacent glucose units [one such bond per one turn of the helix (Stein & Rundle, 1948)] and between 2-OH group and 6-O—atom of the glucose units in adjacent turns of the helix additionally stabilise helical complex. Usually, one turn of the helix employs six glucose units. The cavity diameter of such an helix is 0.54 nm. (Immel & Lichtenthaler, 2000). When the benefit from the non-polar interactions resulting from the complex formation exceeds the energy of the intra- and inter-molecular bonds stabilising the six-membered turn the helix expands even to eight-membered turn (Yamashita, 1965; Yamashita & Hirai, 1966; Yamashita & Monobe, 1971). In such a case the cavity diameter increases to 0.97 nm corresponding to that of γ-cyclodextrin (Szejtli, 1986). Because higher-membered cyclodextrins are known, helices of amylose with over eight glucose units in one turn are likely.