Free-Energy Landscape of the Helical Wrapping of a Carbon Nanotube by a Polysaccharide

Carbon nanotubes wrapped by polysaccharide chains like chitosan (CHTS) or its derivatives through noncovalent decoration have been shown to condense effectively and deliver DNA for gene therapy. Despite the importance of these novel nanoscale materials, the detail of the microscopic structure and underlying interaction mechanism is still fragmentary. In the present work, the complex formed by CHTS and a single-walled carbon nanotube (SWNT) has been investigated by means of atomistic molecular dynamics (MD) simulations to explore its propensity toward self-assembly, together with its structural properties in an aqueous environment. The present results reveal that CHTS can wrap spontaneously the tubular surface by regulating backbone torsional stress upon van der Waals attraction by the SWNT, resulting in a steady, right-handed helical conformation. The free-energy landscape characterizing the wrapping process of the CHTS chain from a straight conformation to a tight helical one brings to light two energetically favored helical conformations corresponding to distinct pitches. In addition, the degree of deacetylation of the polysaccharide chain, but not the pH, induces pronounced fluctuations in the geometrical properties of the helix.