In order to truly unlock advanced applications of single-walled carbon nanotubes (SWNTs), one needs to separate them according to both chirality and handedness. Here we show that the chiral d-ribityl phosphate chain of flavin mononucleotide (FMN) induces a right-handed helix that enriches the left-handed SWNTs for all suspended (n,m) species. Such enantioselectivity stems from the sp3-hybridization of the N atom anchoring the sugar moiety to the flavin ring. This produces two FMN conformations (syn and anti) analogous to DNA. Electrostatic interactions between the neighboring uracil moiety and the 2′-OH group of the side chain provide greater stability to the anti-FMN conformation that leads to a right-handed FMN helix. The right-handed twist that the FMN helix imposes to the underlying nanotube similar to “Indian burn”, causes diameter dilation of only the left-handed SWNTs, whose improved intermolecular interactions with the overlaying FMN helix, impart enantioselection.

Isotopic, hydrogen-to-deuterium substitution has been an invaluable tool in the characterization of small molecules and biological nanostructures. The natural variability of most inorganic nanomaterials has hindered the use of isotopic substitution to gaining meaningful insights of their structure. The ideal helical wrapping of flavin mononucleotide (FMN) around (8,6)-SWNTs (single-walled carbon nanotubes) is presently utilized to probe isotopic-dependent intermolecular interactions. The facile proton-to-deuterium exchange of the imide group of FMN, enabled us to alter the intermolecular stability of the helix depending on the surrounding solvent (i.e. H2O vs. D2O). Our studies show that FMN-dispersed (8,6)-SWNTs exhibit greater stability in D2O as opposed to H2O. The higher complex stability in D2O was verified based on: (i) FMN helix replacement with SDBS (sodium dodecylbenzenesulfate); as well as (ii) thermal-, and (iii) pH-induced helix dissociation. This is in agreement with the previously observed stronger amide H-bonding of proteins in D2O, and to the best of our knowledge, it demonstrates the architectural fidelity of FMN-wrapped SWNTs, which is expected to further enhance the assembly repertoire of carbon nanotubes.

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