Differentiating Left- and Right-Handed Carbon Nanotubes by DNA
Ming Zheng, Geyou Ao, Jason K. Streit, Jeffrey A. Fagan
The emergence of macromolecules of defined structures is a crucial step in the evolution of life. The mechanism by which these structures are selected from astronomical numbers of biopolymer sequences has long been a topic of controversy1,2. Experimenting structure selection from man-made macromolecular libraries may shed light to the mechanism of natural selection, and may also lead to the discovery of novel materials for technological applications. DNA-wrapped single-wall carbon nanotubes3-5 form a man-made library of enormous size due to the combinatorial diversity of nanotube structure and DNA sequence. Here, we report the use of polymer aqueous two-phase systems to select special DNA-wrapped carbon nanotubes, each of which has an ordered DNA structure bound to a nanotube of defined handedness, resembling a well-folded biomacromolecule with innate stereo-selectivity. By screening over 300 DNA sequences, we are able to achieve unprecedented nanotube enantiomer selection across the entire chiral angle range. The screening has also identified a rare DNA sequence that adopts two distinct folds on a pair of nanotube enantiomers, respectively, rendering them large differences in fluorescence intensity and chemical reactivity. This finding establishes a first example of functionally distinguishable right- and left-handed nanotubes. Mechanistic analysis, aided by structure-specific nanotube optical transitions6, suggests that sensitive dependence of hydration energy on structure variation makes conformational entropy minimization an implicit criterion in the aqueous two-phase selection, resulting in exquisite selectivity towards ordered structures. Our study introduces a new method to select well-folded bio- and bio/nano-molecules, and suggests a hydration energy driven mechanism that could be operative in the natural selection of biomacromolecules.